Framework for printing with daylight fluorescent inks

Transcription

1 Framework for printing with daylight fluorescent inks THÈSE N O 5636 (213) PRÉSENTÉE LE 18 JANVIER 213 À LA FACULTÉ INFORMATIQUE ET COMMUNICATIONS LABORATOIRE DE SYSTÈMES PÉRIPHÉRIQUES PROGRAMME DOCTORAL EN INFORMATIQUE, COMMUNICATIONS ET INFORMATION ÉCOLE POLYTECHNIQUE FÉDÉRALE DE LAUSANNE POUR L'OBTENTION DU GRADE DE DOCTEUR ÈS SCIENCES PAR Romain ROSSIER acceptée sur proposition du jury: Prof. M. Pauly, président du jury Prof. R. Hersch, directeur de thèse L. Choulet, rapporteur Prof. S. Süsstrunk, rapporteur Dr Ph. Urban, rapporteur Suisse 213

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3 The only laws of matter are those that our minds must fabricate and the only laws of mind are fabricated for it by matter. James Clerk Maxwell To my wife and my parents for their endless support and encouragement

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5 Acknowledgements First, I have to thank my wife Marie. I met her at the beginning of my undergraduate studies. During many years she gave me an endless support and encouragement. She always believed in me giving me the strength to reach my goals. I also thank my parents, my brothers and my sister who always encouraged me. Thank you Roger for having given me the opportunity of completing a Phd at the EPFL. You provided many challenging and interesting research topics. Thank you for your support, the weekly meetings and the discussions about my work and progress. You were sincerely involved in my different projects. Both your assistance and your guidance are invaluable. I also express my gratitude to Julien Andres for all the discussions about my questions related to chemistry. A special thank to Thomas Bugnon who was senior Phd student when I arrived at the LSP laboratory and kindly always found time to answer my questions. I also express my gratitude to Sergiu Gaman who programmed under my guidance useful applications and to Pascal Fehr who designed some images used in my thesis. During my Phd studies I had good times with friendly people. The excellent atmosphere at my working place contributed to make my studies enjoyable. Thank you Xavier Jimenez, Mathieu Hébert, Vahid Babaei, Petar Pjanic, Basile Schaeli, Florent Garcin, Mathieu Brichon, Sebastian Gerlach, Fabienne Allaire, Maria Anitua, Andrea Maesani, Jean-Marc Comby, Fabrice Rousselle, Zhe Wei, Rafik Chaabouni, Marjan Shahpaski and Sylvain Chosson. I also express my gratitude to the Swiss National Foundation for its financial support, grant n /1. Lausanne, 27 Juin 212 R. R. v

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7 Abstract During the last century technical aspects of printers such as resolution, fastness and quality of the prints have been steadily improved. It is now common to have at home desktop printers enabling quickly printing images with a good reproduction quality. However, since most of the printers use the three cyan, magenta and yellow primaries complemented with the black ink, the domain of printable color has not been significantly improved. The domain of color is limited to the color produced according to the combinations of classical cmyk inks. In this thesis, we are interested in adding an extra dimension to color prints by making use of the daylight fluorescent magenta and yellow inks. By combining these fluorescent inks with classical cmyk inks, we obtain high chroma and bright colors. We propose a new approach for expanding the gamut of classical cmyk printers by using these fluorescent inks with classical cmyk inks in a 6 ink print setup. The approach comprises first establishing a spectral prediction model dedicated to the accurate spectral prediction of halftones comprising classical and daylight fluorescent inks. This new spectral prediction model, although calibrated with a few halftone patch reflectances, is remarkably accurate for predicting spectral reflectances of both classical ink and combined classical ink and daylight fluorescent ink halftones printed with offset and inkjet printers. It also shows excellent prediction accuracies for classical halftones printed with electrophotographic printers. The approach also comprises a gamut mapping of the srgb display gamut to the fluorescent printer gamut which has colors beyond the srgb display gamut. The goal is to enhance image parts by printing them with high chroma and bright daylight fluorescent colors. We first select the image parts to be enhanced. We then apply to their colors a gamut expansion that increases both their chroma and their lightness towards the colors located at the boundary of the gamut formed by the combination of classical and daylight fluorescent inks. This expansion can be controlled by user-defined parameters. We create smooth chroma transitions between the expanded and non-expanded image parts. We also preview the printable gamut expanded image generated according to user-defined gamut expansion parameters. The resulting prototype software enables artists to create and print their own designs. Gamut expansion parameters can be set to limit printable colors to srgb gamut colors. In this situation, we are taking advantage of the larger fluorescent printer gamut without considering colors located beyond srgb gamut colors. By having both a larger printer gamut and a suitable gamut mapping from the srgb gamut to the fluorescent printer gamut, we are able to better reproduce images such as images of watches and master paintings which are known to have colors outside the gamut of classical inks. vii

8 Acknowledgements By characterizing the fluorescent printer gamut, we show that a significant part of the classical cmyk printer gamut can be reproduced by combining classical and daylight fluorescent inks. By printing parts of images with a combination of classical and daylight fluorescent inks instead of using classical inks only, we can hide security patterns within printed images. Under normal daylight, we do not see any difference between the parts printed with classical inks only and the parts printed with daylight fluorescent inks and classical inks. By changing the illumination, e.g. by viewing the printed image under a tungsten lamp or UV lamp, the daylight fluorescent inks change their color and reveal the security patterns formed by combinations of classical and daylight fluorescent inks. We also show how to hide security patterns under at the same time various natural and artificial illuminations. These security patterns are revealed under an illumination having energy only in the excitation wavelengths of the daylight fluorescent inks, such as a UV or non UV blue illumination. Keywords : color prints, color reproduction, spectral prediction models, daylight fluorescent inks, gamut mapping, gamut expansion, optical document security. viii

9 Résumé Au court du dernier siècle, les aspects techniques des imprimantes tels que la résolution, la rapidité et la qualité des imprimés ont étés régulièrement améliorés. Il est de nos jours tout à fait commun d avoir à la maison des imprimantes de bureau qui permettent d imprimer rapidement des images avec une bonne qualité de reproduction. Toutefois, comme la plupart des imprimantes utilisent les trois primaires cyan, magenta et jaune complémentées par le noir, le domaine imprimable de couleur n a pas été significativement amélioré. Ce domaine de couleur est limité aux couleurs produites par les encres classiques CMJN. Dans cette thèse, nous investiguons la possibilité d ajouter une dimension supplémentaire aux imprimés couleurs par l utilisation des encres magenta et jaunes fluorescentes à la lumière visible. En combinant ces encres fluorescentes avec les encres classiques CMJN, nous obtenons des couleurs plus lumineuses et de chroma élevé. Nous proposons une nouvelle approche d expansion de la gamme de couleur des imprimantes CMJN classiques par l utilisation d encres fluorescentes à la lumière visible avec des encres classiques dans un système d impression à 6 encres. Cette approche comprend premièrement l établissement d un modèle de prédiction spectral dédié aux prédictions précises de demi-tons comprenant des encres classiques et fluorescentes à la lumière visible. Ce nouveau modèle de prédiction spectral, bien que calibré avec peu de réflectances de demi-tons, est remarquablement précis lors de la prédiction de réflectances spectrales de demi-tons comprenant à la fois des encres classiques et la combinaison d encres classiques avec des encres fluorescentes à la lumière visible lors d impressions avec des imprimantes jet d encre et offset. Il se montre aussi très précis pour la prédiction de demi-tons classiques imprimés avec des imprimantes électrophotographiques. Cette approche comprend aussi un mappage du gamut des écrans srgb vers le gamut fluorescent de l imprimante qui a des couleurs au-delà de la gamme de couleurs des écrans srgb. Le but est ici d améliorer des parties d images en imprimant celles-ci avec le fort chroma et la puissante luminosité des couleurs fluorescentes à la lumière visible. Premièrement, nous sélectionnons les parties d images à être améliorées. Nous appliquons ensuite à leurs couleurs une expansion de la gamme des couleurs qui augmente à la fois leur chroma et leur luminosité en direction des couleurs positionnées à la frontière du gamut formé par la combinaison d encres classiques et fluorescentes à la lumière visible. Cette expansion peut être contrôlée par des paramètres définis par des utilisateurs. Nous créons des transitions de couleur douces entre les parties d images étendues et non étendues. Nous fournissons aussi un aperçu à l écran de l image étendue selon les paramètres d expansion définis par l utilisateur. Le prototype de logiciel résultant permet aux artistes de créer et d imprimer leurs propres conceptions. ix

10 Acknowledgements Les paramètres d expansion de la gamme de couleur peuvent être définis de manière à limiter les couleurs imprimables aux couleurs affichables par un écran srgb. Dans ce cas, nous tirons avantage de la large gamme de couleur fluorescente de l imprimante sans considérer les couleurs au-delà de la gamme de couleur srgb. En ayant à la fois une gamme de couleur d imprimante étendue et un mappage des couleurs approprié du gamut srgb vers le gamut fluorescent de l imprimante, nous sommes capables de mieux reproduire les images, telles que des images de montres et de peintures qui sont connues pour avoir des couleurs au-delà des couleurs reproductibles par les encres classiques CMJN. En caractérisant le gamut fluorescent de l imprimante, nous montrons qu une partie significative du gamut d imprimantes CMJN classiques peut être reproduite en combinant des encres classiques et fluorescentes à la lumière visible. En imprimant des parties d image avec une combinaison d encres classiques et d encres fluorescentes à la lumière visible à la place d utiliser des encres classiques seulement, nous pouvons cacher des motifs de sécurité dans des images imprimées. Dans des conditions normales de lumière du jour, nous ne percevons aucune différence entre les parties imprimées avec des encres classiques seulement et les parties imprimées avec des encres classiques et fluorescentes à la lumière visible. En changeant l illumination, par exemple en regardant l image imprimée sous une lampe tungstène ou une lampe UV, les encres fluorescentes à la lumière visible change leur couleur et révèlent les motifs de sécurité formés par combinaisons d encres classiques et fluorescentes à la lumière visible. Nous montrons aussi comment cacher à la fois sous plusieurs lumières artificielles et naturelles des motifs de sécurité. Ceux-ci sont révélés sous une lumière ayant de l énergie seulement dans les longueurs d onde d excitation des encres fluorescentes à la lumière visible. Mots-clefs : imprimés en couleur, reproduction couleur, modèles de prédictions couleur, encres fluorescentes à la lumière visible, expansion de gamut, sécurité optique de documents. x

11 Contents Acknowledgements Abstract (English/Français) List of figures List of tables v vii xii xv 1 Introduction Motivations Challenges Characteristics of daylight fluorescent colorants Dissertation outline Contributions Review of the prior art Prediction model for classical and daylight fluorescent inks Introduction Limitation of the Yule-Nielsen model for predicting fluorescent ink halftones The ink spreading enhanced cellular Yule-Nielsen model Ink spreading extension of the Cellular Yule-Nielsen model Characterizing ink spreading with sensor responses Prediction accuracies for classical ink halftones Prediction accuracies for fluorescent ink halftones Summary Framework for printing with combined classical and daylight fluorescent inks Introduction Comparison of fluorescent and non-fluorescent ink gamuts Comparison of gamut volumes Comparison of inkjet and offset fluorescent ink gamuts Summary xi

12 Contents 4 Gamut mapping expansion and reduction for color reproduction with daylight fluorescent inks Introduction Mapping the lightness range of the srgb gamut into the ink destination gamut Mapping the lightness adapted srgb gamut onto the printable fluorescent gamut User driven gamut expansion Display preview Halftoning and printing Summary of application user-defined parameters Summary Gamut expanded images Introduction Preview and corresponding print of a gamut expanded image Advertising images Better reproduction of input srgb image colors Artistic images Summary Hiding patterns with daylight fluorescent inks Introduction Hiding security patterns by printing colors either with or without daylight fluorescent inks Illustrations of hidden security patterns Hiding a variable intensity security image Hiding security patterns under multiple illuminations Summary Conclusion 73 Bibliography 8 A Hue planes of the fluorescent gamuts of both an offset and an inkjet printer 81 B Curriculum Vitæ 85 xii

13 List of Figures 1.1 Spectral power distribution of the F7, D65 and A illuminants Reflectance factors of inkjet daylight fluorescent colorants Comparison between inkjet and offset daylight fluorescent colorants Fading of the inkjet and offset daylight fluorescent m f and y f colorants Illustration of one of the 8 subdomains of the cellular Yule-Nielsen model Cyan dot gain curve for a cmy laser print Cellular Yule-Nielsen model accounting for ink spreading Comparison between the 3D G cmyk, G f and lightness adapted G srgb gamuts Color gamuts of the non-linearly lightness adapted srgb space, the classical cmyk 4 ink print setup and the 6 ink print setup combining the cmyk inks with the m f and y f inks under the D65 illuminant Comparison of the non-linearly lightness adapted srgb gamut and the fluorescent 6 ink gamut under the D65, F7 and A illuminants Comparison of the inkjet and offset fluorescent G f gamuts under the D65 illuminant Linear and non-linear mapping of srgb lightnesses Comparison between the linearly and non-linearly lightness adapted display gamut and the fluorescent ink gamut Mutiple foci gamut mapping approach for both gamut expansion and reduction Constant hue planes for an inkjet printer showing both constant and nonconstant lightness mapping lines at hue angles of the magenta and green fluo colorants Constant hue planes for an offset printer showing both constant and nonconstant lightness mapping lines at hue angles of the yellow and red fluo colorants Spatial interpolation map for an arbitrary selection generated with a limitation factor κ = Luminance gamma curves γ lum for both the Dell U2212 HM and the Eizo Color- Graphic CG245W displays Photographs of previewed prints of a gamut expanded lizard image and corresponding both classical cmyk and fluorescent prints xiii

14 List of Figures 5.2 Photographs of printed both non-gamut expanded and gamut expanded Rolex Yachtmaster images Fluorescent and non-fluorescent ink color separation layers of the Yatchamster image shown in Figure Photographs of printed both non-gamut expanded and gamut expanded AMG car images Photographs of printed both non-gamut expanded and gamut expanded lipstick advertisement images Photographs of a printed advertisement Hublot pink gold watch, printed with the ECI ISO Coated V2 profile and the fluorescent ink gamut G f Photographs of a printed advertisement Rolex yellow gold watch, printed with the ECI ISO Coated V2 profile and the fluorescent ink gamut G f Photographs of a Claude Monet master painting printed with the ECI ISO Coated V2 profile and the fluorescent ink gamut G f Photographs of a part of a J. M. W. Turner master painting printed with the ECI ISO Coated V2 profile and the fluorescent ink gamut G f Photographs of a Paul Gauguin master painting printed with the ECI ISO Coated V2 profile and the fluorescent ink gamut G f Photographs of a flower image printed with the ECI ISO Coated V2 profile and both non-gamut expanded and non-linearly gamut expanded and printed with the fluorescent ink gamut G f Photographs of a designed flaming girl image printed with the ECI ISO Coated V2 profile and of the same image linearly gamut expanded and printed with the fluorescent offset gamut G f Photographs of a fluorescent mushroom image printed with the classical inkjet G cmyk gamut and linearly gamut expanded and printed with the fluorescent ink gamut G f Color gamut G cmyk of the classical cmyk inkjet ink set and the strictly fluorescent inkjet gamut G s f Color gamut G cmyk of the classical cmyk offset ink set and the strictly fluorescent offset gamut G s f Example of an image design incorporating the hidden "VALID" security text Photographs of a printed Japanese girl image incorporating the repetitive "VALID" pattern viewed under both daylight and UV illuminations Photographs of a printed Iceland landscape incorporating the repetitive "VALID" pattern viewed under normal daylight, under UV illumination and under A illumination Halftoned variable intensity tiger image that is to be incorporated within a security image Photographs of a printed grayscale girl image incorporating a variable intensity tiger image viewed under both normal daylight and under a UV illumination.. 68 xiv

15 List of Figures 6.8 Character L of the "VALID" mask message to be hidden under different illuminants at 2% gray level intensity together with an enlargement of a small region of its corresponding blue noise halftone Photographs of an offset printed Iceland landscape incorporating the repetitive "VALID" pattern viewed under normal daylight, under illumination A, under fluorescent tube illumination F7 and under a blue low consumption Swiss light classic 55 non UV lamp Photographs of an inkjet printed Iceland landscape incorporating the repetitive "VALID" pattern viewed under both normal daylight and UV illuminations with different printed maximal daylight fluorescent cmy f ink surface coverages A.1 Comparison of constant hue planes between the fluorescent gamut of an inkjet printer and the non-linearly lightness adapted srgb gamut A.2 Comparison of constant hue planes between the fluorescent gamut of an offset printer and the non-linearly lightness adapted srgb gamut xv

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17 List of Tables 1.1 E 94 color difference between original offset and inkjet m f and y f measures and after specific period of time under daylight behind a window glass Prediction accuracies of the IS-YNSN model for 125 cm f y f test samples printed with an inkjet Epson P5 printer and an offset printer Prediction accuracies of both the IS-YNSN model and the IS-CYNSN models for classical cmy inkjet and electrophotgraphic prints Prediction accuracies for both the IS-YNSN model and the IS-CYNSN models when characterizing ink spreading using simulated RGB sensors Prediction accuracies of the CYNSN, IS-YNSN and IS-CYNSN models for the fluorescent and non-fluorescent inkjet ink sets used to establish the fluorescent G f printer gamut Prediction accuracies of the CYNSN, IS-YNSN and IS-CYNSN models for the fluorescent and non-fluorescent offset ink sets used to establish the fluorescent G f printer gamut Prediction accuracies of the IS-CYNSN model for 625 cmyk and 125 cm f y f test samples printed with a Canon Pro 95 inkjet printer and measured under both the A and F7 illuminants Comparison of gamut volumes for the non-linearly lightness adapted srgb gamut Comparison of gamut volumes for the linearly lightness adapted srgb gamut Prediction accuracies for both the Dell U2212 HM and the Eizo ColorGraphic CG245W display characterizations xvii

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19 1 Introduction 1.1 Motivations In a society where images take an important place, they must satisfy high quality criteria. Thus, they must be pleasant, faithful and their quality must be well controlled. Traditional printing technologies, such as offset printing, electrophotgraphy printing and inkjet printing use classical cmyk inks. In order to reproduce specific colors, e.g. brand colors, offset printers can also use additional Pentone colors. However, most printing systems only perform color separation with the classical cmyk inks. Daylight fluorescent colorants were introduced in the middle of the last century, for devices which should capture the attention, such as road markers, safety jackets and warning signs. They were also used for graphic arts, mainly for painting and decoration. At present, they are used in many products, such as highlighting markers, toys and optical brighteners in tissues and papers. The main characteristic of daylight fluorescent colorants is that their colors are brighter and more saturated than the corresponding classical colors. By combining these colorants with classical colorants, one may increase the overall reproducible gamut. In this thesis, we propose to create a color management framework by combining classical cmyk inks with the daylight fluorescent magenta and yellow inks in a 6 ink printing system. We would like to explore the possibilities offered by the high chroma and bright daylight fluorescent inks. These daylight fluorescent inks enable improving the reproduction of color images, especially in respect to bright and saturated colors. Since the daylight fluorescent colorants are much brighter and more saturated than the classical colorants it is also possible to highlight image regions of special interest. We also would like to create specific tools for exploiting the capabilities of the fluorescent 6 ink printing system. These tools may enable selecting image regions, applying to specific regions different gamut expansions reinforcing chroma and brightness in these regions, displaying a preview of the enhanced image having colors located beyond srgb gamut colors and printing the resulting gamut expanded image. Such an application offers new means to designers 1

20 Chapter 1. Introduction working in fields such as photography, advertisement and magazine production. Another application of fluorescence is the authentication of security documents. By printing parts of images with a combination of classical and daylight fluorescent inks instead of using classical inks only, we can hide security patterns within printed images. We are interested in applying a metameric color match under a specific illumination, e.g. the D65 illuminant, between the printed parts with combinations of classical inks and daylight fluorescent inks and the printed part with classical inks only. By changing the illumination, e.g. by viewing the image under a tungsten lamp or under a UV lamp, the daylight fluorescent inks change their colors and reveal the security patterns formed by combinations of classical and daylight fluorescent inks. 1.2 Challenges In order to propose a comprehensive and complete solution to print images on a 6 ink printing system using the classical cyan, magenta, yellow, black inks and the two additional daylight fluorescent yellow and magenta inks, we need to characterize this printing system. Characterizing a printer incorporating daylight fluorescent inks induces many difficulties. First, classical spectral prediction models, such as the Yule-Nielsen modified spectral Neugebauer model (Viggiano 199) and the Clapper-Yule model (Clapper and Yule 1953) do not predict well fluorescent ink halftones. In this situation, we have to develop a spectral prediction model dedicated for predicting the spectral reflectance of halftones comprising daylight fluorescent inks. Accurate spectral predicitons are needed for establishing the printable destination fluorescent gamut as well as for creating a correspondence between CIELAB colors and corresponding fluorescent ink dot surface coverages. In addition, when hiding security patterns, an exact relationship between CIELAB colors and ink surface coverages enable printing perfectly metameric colors between image parts printed either with combinations of classical and daylight fluorescent inks or with classical inks only. The patterns will be therefore perfectly hidden under a specific illumination, i.e. the illumination used for calibrating the spectral prediction model. We also would like a spectral prediction model calibrated with a few halftone patch reflectances. In addition, daylight fluorescent inks strongly change in appearance when changing the illumination, i.e. the total reflectance is strongly illuminant dependent. Depending on the nature of the fluorescent inks, some fading effects may come up, when putting them under the exposure of normal daylight (Connors-Rowe et al. 25). The problem of printing with custom inks raises similar problems as printing with combined classical and fluorescent inks. There is a need for selecting specific subsets of inks from many possible ink subsets, for mapping the input gamut into the gamut achievable with the multiink halftones and for allowing the ink layers to be printed without inducing undesired moiré layer superposition effects. 2

21 1.3. Characteristics of daylight fluorescent colorants Producing gamut expanded images raises several challenges. We have to determine expansion factors increasing the chroma of input srgb colors to colors beyond the srgb gamut and possibly modify their lightnesses. The goal is to enhance given image parts with higher chroma and brighter colors. We also have to ensure the continuity of colors at the boundary between highlighted and non-highlighted image regions. In addition, we have to generate halftoned images comprising at different locations different gamut mappings between the input image and the destination image. In order to preview the printable expanded images having colors beyond the srgb gamut, we have to simulate a lower quality display for classical image parts and render the extended srgb colors by making use of the full capabilities of the display. Such a preview enables visualizing the differences between the color expanded and non-expanded image parts. 1.3 Characteristics of daylight fluorescent colorants Daylight fluorescent inks contain organic molecules (Streitel 29) that fluoresce by absorbing light within one wavelength range and remitting light at a longer wavelength range. Classical daylight fluorescent inks, such as the daylight fluorescent yellow ink and the daylight fluorescent magenta ink are mainly excited in the visible region between 4 and 55 nm. In addition, they have an ultra-violet narrow excitation band between 35 and 4 nm (Connors-Rowe et al. 25). The emission peak is located in the visible spectrum at wavelengths corresponding to the desired color, i.e. for the daylight fluorescent magenta ink, the two emission peaks yield a very bright and strongly saturated magenta color. These daylight fluorescent inks therefore do not behave like classical inks where part of the incident light is absorbed by the inks. They behave additively, i.e. the fluorescent emission behaves as a color light source. The total reflectance factor R total (λ) of a fluorescent ink patch is the light reflected by that ink patch plus the light emitted by fluorescence divided by the light reflected by a perfect Lambertian white reflector (Grum 198). Both the UV and the visible range of an illuminant have an impact on the energy that is available for fluorescent emission. In order to illustrate the strong impact in color appearance of daylight fluorescent colorants when changing the illuminant, we consider the A, D65 and F7 illuminants. Figure 1.1 shows the spectral power distribution of these illuminants. They have been measured with a Maya 2 Pro spectrophotometer calibrated from the known spectral power distribution of an Ocean Optics DHL-2-BAL lamp. The spectral power distribution of illuminant F7 has been measured from a Just Normlicht mini 5 light table. Both the A and the D65 illuminant emulations are measured light sources of a SpectroEye Xrite spectrophotometer. The color appearance of daylight fluorescent colorants is characterized by the total spectral reflectance factor R total (λ) under a given illuminant. From now on, we use the term reflectance as a short denomination of total reflectance factor. Figure 1.2 provides the measured spectral reflectance of four daylight fluorescent colorants, the daylight fluorescent m f magenta ink (Farbel Castel ink ref ), the daylight fluorescent y f yellow ink (Farbel Castel ink ref. 3

22 Chapter 1. Introduction 4 F7 relative spectral power distribution A D wavelengths [nm] Figure 1.1: Spectral power distribution of the Just Normlicht mini 5 light table emulating an F7 light source (solid lines) and the emulations of the A (dashed lines) and D65 (dotted lines) illuminants of a SpectroEye Xrite spectrophotometer ), the daylight fluorescent red colorant (m f superposed with y f ) and the daylight fluorescent green colorant (cyan superposed with y f ) printed with a Canon Pro 95 inkjet printer on a paper containing optical brighteners (Canon MP-11) under the considered illuminants as well as the reflectances of the corresponding classical non-fluorescent colorants. In Figure 1.2a we observe in the m f spectral reflectance two fluorescent peaks. The first peak is located between 42 and 45 nm. Since the D65 illuminant has the highest energy in the UV range (see Figure 1.1, dotted lines), it is responsible for the strongest peak at a reflectance factor The second peak is located between 59 and 61 nm. Since the D65 and F7 illuminants have more energy in the second part of the fluorescent excitation range (5-56 nm) than the A illuminant, they produce the largest peak. Regarding the daylight fluorescent yellow ink y f (Figure 1.2b), since the F7 illuminant has the highest energy in the excitation range (Figure 1.1, solid lines), the strongest peak is observed under this illuminant with a reflectance factor of 1.62 at 52 nm. We observe similar behaviors with the daylight fluorescent green (Figure 1.2d) and the daylight fluorescent red (Figure 1.2c) colorants. Note that the peak of the fluorescent red at 59 nm is higher than the corresponding peak of the fluorescent magenta, due to the additional energy absorbed due to the peak of the daylight fluorescent yellow, i.e. at 53 nm. Finally, we observe that all daylight fluorescent colorants are more saturated and brighter than the corresponding classical colorants. For instance, the daylight fluorescent green colorant (Figure 1.2d) has a narrow fluorescent peak between 5 and 54 nm with a maximal reflectance factor of 1.15 at 53 nm and almost no reflectance at the other wavelengths. In comparison with the classical green colorant which has a maximal reflectance factor of.51, the daylight fluorescent green colorant is much more saturated and brighter. Ink manufacturers have also developed daylight fluorescent inks for offset printers. Test 4

23 1.3. Characteristics of daylight fluorescent colorants Reflectance factors m f m Reflectance factors y f y F7 D65 A Classical inks wavelengths [nm] (a) m f y f c y f wavelengths [nm] (b) Reflectance factors 1.5 my Reflectance factors 1.5 cy paper white (D65) wavelengths [nm] (c) wavelengths [nm] (d) Figure 1.2: Reflectance factors of (a) the daylight fluo magenta m f colorant, (b) the daylight fluo yellow y f colorant, (c) the daylight fluo red colorant (m f superposed with y f ) and (d) the daylight fluo green colorant (cyan superposed with y f ) under the F7 (solid lines), D65 (dotted lines) and A (dashed lines) illuminants, together with the classical colorant reflectances under the D65 illuminant (pointed lines). have been conducted for demonstrating the feasibility of combining classical offset inks with daylight fluorescent offset inks. We combined classical cmyk inks with two daylight fluorescent inks, the daylight fluorescent magenta ink (AMRA AG, ink ref ) and the daylight fluorescent yellow ink (AMRA AG, ink ref ). The print has been carried out on an Heidelberg 6 ink offset printer at printing company Jean Genoud SA in Lausanne, Switzerland. With this print setup, the classical inks have to be printed before the daylight fluorescent inks. Otherwise, due to the surface characteristics of the daylight fluorescent inks, the classical inks do not adhere to the print. These offset daylight fluorescent inks differ from the inkjet daylight fluorescent inks. Compared to the inkjet m f colorant (Figure 1.3a, pointed line), the offset m f colorant (Figure 1.3a, solid line) has not a strong fluorescent emission peak between 42 and 45 nm. The offset m f ink seems to more absorb the paper fluorescent emission (Figure 1.3a, dashed line) at these wavelengths. We also observe that the second emission peak located near 6 nm is slightly higher for the offset daylight fluorescent magenta ink with a reflectance factor of 1.4. The offset y f colorant is less saturated than the inkjet y f colorant (Figure 1.3b) with two smaller 5

24 Chapter 1. Introduction fluorescent emission peaks of.92 and 1.18 at respectively 52 and 59 nm. When superposing the m f ink with the y f ink (Figure 1.3c), we obtain for the offset print a strongly saturated red colorant with a fluorescent emission peak at a reflectance factor of 1.39 at 62 nm and for the inkjet print a strong orange colorant with two peaks at respectively 51 and 58 nm. 1.5 m f offset paper white 1.5 y f inkjet y f Reflectance factors 1.5 inkjet m f offset m f Reflectance factors 1.5 offset y f simulated offset y f wavelengths [nm] (a) 1.5 m f y f wavelengths [nm] (b) Reflectance factors 1.5 inkjet m f y f offset m f y f wavelengths [nm] (c) Figure 1.3: Reflectance factors of (a) the daylight fluo m f colorant, (b) the daylight fluo yellow y f colorant, (c) the daylight fluo red colorant (m f superposed with y f ) measured on an offset print (solid lines) and on an inkjet print (pointed lines) under the D65 illuminant. The dashed lines in (b) represent the simulated offset y f colorant obtained with the inkjet y f and m f inks. We can demonstrate that the offset y f colorant has been created by mixing at different concentrations the same fluorescent compounds that are used in the y f ink and in the m f inkjet ink. We calibrate a spectral prediction model with the inkjet y f and m f colorant spectral reflectances (see Chapter 2). We then invert the spectral prediction model in order to get the y f and m f ink surface coverages that approximate the spectral reflectance of the offset y f colorant. Spectral prediction model inversion is performed with a gradient descent by minimizing the root mean square difference (RMS) between the predicted and measured spectral reflectance of the offset y f colorant. Figure 1.3b (dashed line) shows the simulated y f offset colorant spectral reflectance obtained by printing with a Canon Pro 95 inkjet printer an halftone at 1 % y f and 36% m f surface coverages. The simulated and measured spectral reflectance of the y f offset colorant are nearly identical with a RMS difference of.56, showing that the offset y f ink uses the same fluorescent compounds found in the inkjet m f 6

25 1.3. Characteristics of daylight fluorescent colorants and y f inks. The small differences are possibly due to the different properties of the inkjet and offset inks, such as the viscosity, the opacity, the density and the different papers used in the two different prints. When putting daylight fluorescent colorants under daylight illumination fading effects may come up (Connors-Rowe et al. 25), yielding a change of their color appearance. The fading effect expressing the change in color is generally provided by the fluorescent pigment and/or ink manufacturer. This effect is calculated for a specific exposure such a daylight or a fluorescent lamp exposure. It can be calculated with an accelerated procedure by exposing the sample for a short period of time under a Xenon lamp filtered for simulating the exposure. Some testing procedures expose the sample during a long period of time and the change in color appearance is reported (AST 211). We perform our own experiment in order to evaluate the fading of these fluorescent colorants. For this purpose, we let both an offset and inkjet printed sample behind a window at a location where direct sunlight cannot illuminates directly the samples and we measure at respective intervals during one month the total spectral reflectances of the y f and m f colorants. Figure 1.4 shows the decrease of the fluorescent emission peaks of both the inkjet and offset y f and m f colorants. In case of the offset colorants, we do not see much difference between the original measures (blue solid lines) and the measures performed after four weeks (blue dashed lines), with small E 94 differences of 1.13 for the m f colorant and of.9 for the y f colorant (Table 1.1). In case of the y f inkjet colorant, it already shows after one week a significant decrease of its fluorescent emission peak from a reflectance factor of 1.39 to a reflectance factor of 1.2 at 52nm, yielding a strong change in color appearance with a E 94 difference of 3.93 (Table 1.1). After four weeks, we observe a huge fading effect for the inkjet y f colorant with a E 94 difference of The offset m f and y f colorants have a good lightfastness, while the lightfastness of the inkjet m f colorant is poor and the one of the inkjet y f colorant is very poor. Fading of the daylight fluorescent inks can be limited by printing thick layers and/or by coating the printed layers with a UV-absorbing coating (Streitel 29). Ink manufacturers are developing fading resistant daylight fluorescent pigments. Table 1.1: E 94 color difference between original offset and inkjet m f and y f measures and after specific period of time under daylight behind a window glass. E 94 1 week 2 weeks 3 weeks 4 weeks offset m f colorant offset y f colorant inkjet m f colorant inkjet y f colorant

26 Chapter 1. Introduction Reflectance factors m f 2-5 inkjet offset Reflectance factors y f offset inkjet wavelengths [nm] (a) wavelengths [nm] (b) Figure 1.4: Reflectance factors of (a) the daylight fluorescent m f colorant and (b) the daylight fluorescent y f colorant under the D65 illuminant. Black lines show the inkjet colorants and blue lines show the offset colorant, with original measured spectral reflectances as solid lines and after four weeks as dashed lines. 1.4 Dissertation outline Chapter 2 starts by establishing a spectral prediction model dedicated for predicting the spectral reflectances of halftones combining classical and daylight fluorescent inks. The accuracy of this model is compared to the accuracy of classical spectral prediction models such as the cellular Yule-Nielsen model (CYNSN) and the ink spreading enhanced Yule-Nielsen model (IS-YNSN). Accuracy is computed as E 94 prediction error for uniformly distributed halftones printed with inkjet, electrophotographic and offset printers. In Chapter 3 we then compare for an inkjet printer the gamuts of a classical cmyk ink print setup and of a fluorescent 6 ink print setup comprising the classical cmyk inks and the daylight fluorescent magenta and yellow inks. The gamuts are compared under the A, D65 and F7 illuminants as CIELAB constant lightness planes and by calculating the additional volume of the srgb display gamut covered by the 6 ink fluorescent print setup. We also compare as CIELAB constant lightness planes the inkjet and offset fluorescent 6 ink gamuts. In Chapter 4 we describe gamut expansion and reduction algorithms used to map input srgb images to destination fluorescent printable images. We describe the advantages and the disadvantages of different possible projections. In addition, we show how to control gamut mapping expansion and reduction from the display srgb gamut to the printable fluorescent ink gamut. The mapping is controlled by user-defined parameters such as the type of function used to map input lightnesses to destination lightnesses, a non-linear mapping chroma reinforcement factor controlling how fast the chroma expansion is applied, a gamut expansion limitation factor that limits the maximal effective gamut expansion, the focal points giving the direction of mapping lines and means of creating smooth chroma transitions between expanded and non-expanded image parts. We also show how to display preview the destination gamut expanded image. 8

27 1.5. Contributions In Chapter 5 we show results of the gamut expansion and reduction of srgb images by comparing photographs of both offset and inkjet printed images either with classical cmyk inks only or with the fluorescent 6 ink print setup. In Chapter 6 we show how to use daylight fluorescent magenta and yellow inks in order to hide under a specific illuminant security patterns or variable intensity images within printed images and to reveal them under an illuminant different from the reference illuminant. We also show how to hide the security patterns under various common natural and artificial illuminations. We draw the conclusions in Chapter Contributions The list of contributions of this thesis are described as follow: - We propose an extension of the cellular Yule-Nielsen spectral Neugebauer model by accounting for ink spreading of each ink within each subdomain. Ink spreading is characterized by jointly fitting within each subdomain the ink spreading functions on a single halftone patch located at the center of the considered subdomain. In case of three ink halftones, compared to the cellular Yule-Nielsen model, this extension requires only 8 additional spectral reflectance measurements. - We show that this new ink spreading extension of the cellular Yule-Nielsen model is remarkable accurate for predicting the spectral reflectances of classical halftones printed with inkjet, electrophotographic and offset printers and for predicting combined classical ink and daylight fluorescent ink hafltones printed with inkjet and offset printers. - We show that ink spreading can be characterized with tri-stimulus sensor responses instead of using full spectral measurements without reduction of prediction accuracy. - We propose a complete solution for printing with a 6 ink print setup combining classical and daylight fluorescent inks. The solution comprises a spectral prediction model dedicated for the accurate spectral prediction of halftones combining classical and daylight fluorescent inks, a gamut mapping from the srgb display to the partly fluorescent printer gamut, and a generation of the 6 ink color separation layers. - We propose a user-driven gamut expansion method for mapping srgb images to printable fluorescent ink gamut images. The expansion ensures continuity of colors of the input image and the gamut expansion parameters allow controlling the chroma expansion of image parts as well as creating smooth chroma transitions between expanded and non-expanded image parts. - We propose a solution for previewing on a standard srgb display destination gamut expanded images having colors beyond the srgb display gamut. 9

28 Chapter 1. Introduction - We show how to hide under a specific illumination security patterns within printed images with the 6 ink print setup combining classical and daylight fluorescent inks. These patterns are revealed under an illumination different from the reference illumination used to hide the security patterns. - We show how to hide security variable intensity images within printed images. These security images are revealed under an illumination different from the reference illumination. - We show how to hide security patterns under various common natural and artificial illuminations. These security patterns are revealed under an illuminant having energy only in the excitation wavelengths of the daylight fluorescent inks. 1.6 Review of the prior art A printer is characterized by the relationship between the printer s input in terms of nominal surface coverages of the inks and the resulting output colors. This relationship is often obtained by printing hundreds of color halftones at different combinations of ink surface coverages. Each sample is measured by a spectrophotometer under the reference illumination and converted to a color value. One may then interpolate between these color values to create the mapping between desired color and surface coverages of the inks, see (Bala 23a). Another approach consists in modeling the interaction of the light and the print according to a spectral prediction model. At the present time, among the existing spectral reflection prediction models, mainly the well-known Yule-Nielsen modified spectral Neugebauer model (YNSN) (Yule and Nielsen 1951), (Viggiano 199) is used for predicting reflection spectra (Bala 1999), (Wyble and Berns 2), (Ogasahara 24). Due to the printing process, the deposited ink dot surface coverage is generally larger than the nominal surface coverage, yielding a "physical" dot gain responsible for the ink spreading phenomenon (Bala 1999). In order to make accurate spectral prediction, the YNSN model needs to take into account ink spreading of the ink halftone dots. Most of the time, ink spreading is accounted for each ink separately through a single tone reproduction curve (Pobboravsky and Pearson 1972). However, it has been shown by Hersch et al. that dot gain also depends on the ink superposition conditions. They proposed an ink spreading extension of the Yule-Nielsen model, the ink spreading enhanced Yule-Nielsen modified spectral Neugebauer model (IS-YNSN) (Hersch and Crété 25) by establishing for each ink in the different superposition conditions, i.e. alone on paper, in superposition with one ink, in superposition with two or more inks, ink spreading curves mapping nominal to effective surface coverages. In order to provide a higher prediction accuracy, Heuberger et al. (1992) proposed the Cellular Neugebauer Model. Bala (1999) has shown that the cellular subdivision is also applicable to the Yule-Nielsen spectral Neugebauer model (CYNSN). In prior work, the cellular Yule-Nielsen model was further improved along the following lines: 1

29 1.6. Review of the prior art (a) Optimization of the Neugebauer primary reflectances according to the color halftone patches forming the learning set (Bala 1999). (b) Octtree like hierarchical subdivision of the surface coverage cube and subcubes until the desired prediction accuracy is reached (Agar and Allebach 1998). (c) Introducing for each ink a single function relying on single ink halftone ramps mapping nominal to effective coverages (Bala 1999) (Chen et al. 24). (d) Optimization of the positions for the non-uniform cellular subdivisions of the surface coverage unit cube (Chen et al. 24). In the past, there was an attempt to print with classical and daylight fluorescent inks. Guyler (21) compared the gamuts of classical and combined classical and daylight fluorescent inks for offset prints by relying on Neugebauer primaries and on printed color patch measurements. However, no attempt to create a full color management framework combining classical and daylight fluorescent inks was proposed. Printing with combined classical and daylight fluorescent custom inks faces similar problems as printing with custom inks. There is a need to select a specific subset of inks from many possible subsets and to map the input gamut into the gamut achievable with the multi-ink hafltones. Stollnitz, Ostromoukhov, and Salesin (1998) modeled the gamut of printable custom colorants by a modified Neugebauer model accounting for trapping, dot gain and multiple internal reflectances. With this model, they optimized the selection of custom inks in order to obtain a given color. Tzeng and Berns (2) used cyan, magenta, yellow, black, orange and green inks and developed an algorithm for selecting a subset of 4 inks among the 6 inks to reproduce a given reflection spectrum as accurately as possible, by minimizing metamerism. For generating gamut expanded images there is a need of mapping an input gamut, e.g. the display srgb gamut, to the fluorescent printable destination gamut. Traditional gamut mapping techniques are presented by Morovic and Luo (21). Kang et al. (23) performed a user study which in addition to gamut compression also dealt with users performing interactive gamut expansion from print gamut to display gamut. For most colors, besides memory colors, the users tried to extend the chroma of the images. Hirokawa et al. (27) showed that, compared with the linear expansion, non-linear chroma expansion of srgb images displayed on a wide gamut Adobe RGB monitor was preferred by users. Another application of fluorescence is the authentication of security documents (Van Renessse 25). Bala et al. (27) used the fluorescence of paper incorporating fluorescent brighteners in order to create images embedding security information that is invisible under normal daylight and revealed under UV illumination. Security features relying on single invisible fluorescent inks are widely used in passports, bank notes and credit cards (Van Renessse 25). The hidden patterns are generally printed with a single invisible fluorescent ink, for example the yellow "VISA" appearing on Visa credit cards under a UV light source. Hersch et al. (27) 11

30 Chapter 1. Introduction proposed to enhance the security provided by invisible fluorescent inks by creating full color images viewable under UV light with three inks having their fluorescent emission in different parts of the visible wavelength range. Coyle and Smith (24) propose to create fluorescent color images with red, green and blue emitting fluorescent inks, which are invisible under daylight. Narita and Eto (22) teach how to form an image with color gradations using fluorescent red, green and blue colorants, colorless under normal daylight and emitting fluorescence under UV illumination. Brehm and Erbar (21) also describe an additive fluorescent ink mixing process capable of creating continuous tone halftone images. Jones II et al. (22) describe a method of marking products by tags formed by luminescent inks having specific emission wavelength ranges and specific decay times. Auslander and Cordery (23) propose a print head system with a first ink having a first color under normal daylight and a second fluorescent ink having the same color as the first ink under normal daylight but discernible from the first ink when subjected to fluorescentexciting radiation. This second ink is visible only under an exciting radiation enables creating covert markings. Auslander et al. (22) teach a method for printing a security marking with an ink absorbing light under daylight (dark patterns) and emitting light under an excitation illumination. This security marking is viewed both under daylight and by fluorescence under fluorescent excitation illumination. However, these security features rely on invisible fluorescent ink which do not absorb in the visible wavelength range. In contrast to these methods, we propose to hide security patterns by printing image parts with combinations of classical and daylight fluorescent inks while the rest of the image is printed with combinations of classical inks only. In addition, we apply a metameric color match between the image part printed with and without daylight fluorescent inks. 12

31 2 Prediction model for classical and daylight fluorescent inks 2.1 Introduction The total reflectances shown in Chapter 1 Section 1.3 indicate that new saturated and bright colorants can be obtained either by combining classical and daylight fluorescent inks or by combining two daylight fluorescent inks. The current chapter describes how to accurately predict the spectral reflectances of halftones comprising daylight fluorescent inks with an ink spreading extension of the cellular Yule-Nielsen spectral prediction model (IS-CYNSN). Accurate spectral predictions are needed for establishing a mapping between CIELAB colors and corresponding ink dot surface coverages and for establishing the exact printer gamut. We show that although this new ink spreading extension of the cellular Yule-Nielsen model is calibrated with a few halftone patch reflectances, i.e. only 35 halftone patch reflectances for predicting 3 ink halftones, this model is remarkably accurate for predicting spectral reflectances of both halftones comprising classical inks only printed with inkjet, offset and electrophotographic printers (Section 2.5) and halftones combining classical and daylight fluorescent inks printed with inkjet and offset printers (Section 2.6). 2.2 Limitation of the Yule-Nielsen model for predicting fluorescent ink halftones Within halftones comprising classical and daylight fluorescent inks, light propagates from one fluorescent colorant to a non-fluorescent colorant and vice versa. The emitted part of fluorescent ink halftone element depends on the absorption of its neighbouring halftone elements. In this situation, the Yule-Nielsen modified Neugebauer model (Yule and Nielsen 1951) (Viggiano 199) is not accurate for predicting the total spectral reflectances of halftones comprising daylight fluorescent inks. To illustrate the low prediction accuracy of the Yule- Nielsen model, we printed all combinations of the cm f y f inks by varying the nominal ink surface coverages by steps of 25%, yielding 5 3 = 125 halftones. Halftones were printed with an inkjet Epson P5 printer and with an offset printer. We calibrate an ink spreading enhanced 13

32 Chapter 2. Prediction model for classical and daylight fluorescent inks Table 2.1: Prediction accuracies of the IS-YNSN model under the D65 illuminant for 125 cm f y f test samples printed with an inkjet Epson P5 printer and an offset printer. Test set E 94 Avg 95% Max inkjet cm f y f offset cm f y f Yule-Nielsen spectral Neugebauer model (IS-YNSN) (Hersch and Crété 25) by measuring 44 uniform halftone samples. We then run a spectral prediction for all the 125 halftones. The prediction accuracies for the offset and inkjet printed fluorescent halfltones are listed in Table 2.1. We obtain under the D65 illuminant for the inkjet and offset prints the respective CIELAB mean E 94 prediction errors of 2.47 and 3.37 and respective 95% quantile prediction errors of 3.8 and We also observe for the offset print a huge maximal E 94 prediction error of These prediction accuracies are not sufficient in order to create precise colors by deriving from the model the surface coverages of the inks. 2.3 The ink spreading enhanced cellular Yule-Nielsen model As an alternative, we established an ink spreading extension of the cellular Yule-Nielsen model (named: IS-CYNSN) (Rossier et al. 21). The IS-CYNSN model has a finer ink surface coverage space subdivision and therefore better accounts for the fluorescent emission. In addition, the IS-CYNSN model accounts for the ink spreading phenomenon, yielding accurate spectral predictions. Finally, the IS-CYNSN model needs to be calibrated with only a limited number of spectral reflectance measurements. In the case of halftones formed by 3 inks, we only need 35 spectral reflectance measurements and in the case of halftones formed by 4 inks, we only need 97 spectral reflectance measurements Ink spreading extension of the Cellular Yule-Nielsen model The Yule-Nielsen modified Neugebauer spectral model (Equation 2.1) is used to predict the spectral reflectance R(λ) of a color halftone as a weighted sum of Neugebauer primary reflectances R i (λ), where a i is the area coverage of the i th primary, R i (λ) its reflection spectrum and n the Yule-Nielsen value accounting for the lateral propagation of light (in general, 1 < n < 1). ( R(λ) = i a i R i (λ) 1/n ) n (2.1) 14

33 2.3. The ink spreading enhanced cellular Yule-Nielsen model Hereinafter, we focus on the spectral reflectance prediction models for three inks. However, the models can be extended to 4 inks (Bugnon et al. 28). With three inks, we have 2 3 = 8 primaries corresponding to all combinations of % and 1% ink surface coverages. Assuming independently printed cyan, magenta and yellow inks, the area coverages a i of the primaries white (a i ), cyan (a c ), magenta (a m ), yellow (a y ), red (a r ) (superposition of cyan and yellow), blue (a b ) (superposition of cyan and magenta) and black (a k ) (superposition of cyan, magenta and yellow) are calculated according to the Demichel s equations (Wyble and Berns 2) expressed by Eqs. 2.2 a w = (1 c) (1 m) (1 y) a c = c (1 m) (1 y) a m = (1 c) m (1 y) a y = (1 c) (1 m) y a r = (1 c) m y a g = c (1 m) y a b = c m (1 y) a k = c m y (2.2) where c, m, y represent respectively the cyan, magenta and yellow ink surface coverages. These 8 area coverages are identical to the 8 coefficients used for tri-linear interpolation between known cube vertex values. yellow (,.5,.5) (.5,.5,.5) (,,.5) magenta (,.5,) (.5,,.5) (.5,.5,) (,,) (.5,,) cyan Figure 2.1: Illustration of the cellular Yule-Nielsen model where the illustrated cube represents one of the 8 subdomains produced by all combinations of %, 5% and 1% surface coverages of the three inks. At the vertices of the cube, subdomain primary reflectances R c,m,y (λ) have been measured. Thanks to the cellular Yule-Nielsen extension of the Neugebauer model (Bala 1999), we improve the prediction accuracy by dividing the CMY ink surface coverage space into 8 subdomains. As Neugebauer primaries, we not only consider reflectances of printed haltones at % and 1% surface coverages, but also printed halftones (called subdomain primaries) at all combinations of %, 5% and 1% surface coverages (2 3 = 27 combinations). Figure 2.1 illustrates a subdomain where the cyan, magenta and yellow ink surface coverages vary from to.5. For ink surface coverages within that subdomain, we first normalize the subdomain coverages. With c, m, y ink surface coverages of cyan, magenta and yellow between and.5, 15

34 Chapter 2. Prediction model for classical and daylight fluorescent inks the normalized coverages c, m and y are c = c.5 m = m.5 y = y.5 (2.3) The areas of subdomain primaries are calculated from the normalized coverages c, m and y with coefficients expressed by the Demichel s equations (Eqs. 2.2). The spectral prediction is carried out by tri-linear interpolation, i.e. by weighting the subdomain primary reflectances with the corresponding areas of subdomain primaries according to the Yule-Nielsen equation (Equation 2.1). More precisely, for an arbitrary cellular subdivision and with cyan, magenta and yellow ink surface coverages c, m, y within the subdomain delimited by c [c l,c h ], m [m l,m h ] and y [y l, y h ], the normalized c,m and y ink coverages are c = c c l c h c l m = m m l m h m l y = y y l y h y l (2.4) The predicted reflectance R(λ) of a halftone of surface coverages c [c l,c h ], m [m l,m h ], y [y l, y h ] is obtained by tri-linear interpolation of cube vertex reflectances R(λ) = ( (1 c ) (1 m ) (1 y ) R cl,ml,yl (λ) 1/n + c (1 m ) (1 y ) R ch,ml,yl (λ) 1/n + (1 c ) m (1 y ) R cl,mh,yl (λ) 1/n + (1 c ) (1 m ) y R cl,ml,yh (λ) 1/n + (1 c ) m y R cl,mh,yh (λ) 1/n + c (1 m ) y R ch,ml,yh (λ) 1/n + c m (1 y ) R ch,mh,yl (λ) 1/n + c m y R ch,mh,yh (λ) 1/n) n (2.5) where R c,m,y (λ) represents the measured spectral reflectance at surface coverages (c,m, y) of the cyan, magenta and yellow inks. The prediction accuracy of the cellular Yule-Nielsen model can be improved by a finer subdivision or by multiple levels of subdivisions. For instance, we can increase the number of subdomains by choosing for the subdomain primaries the combinations of %, 25%, 5%, 75% and 1% nominal ink surface coverages. In this case, the number of required subdomain primary spectral measurements increases significantly (in the present case: 5 3 = 125 spectral measurements). In order to improve the prediction accuracy, as an alternative to the increase of subdomains, we propose an ink spreading extension of the cellular Yule-Nielsen model where ink spreading is accounted for within each subdomain. We create within each subdomain ink spreading curves expressing the ink spreading behavior of the ink hafltone dots. Since the dot gain of 16

35 2.3. The ink spreading enhanced cellular Yule-Nielsen model one ink within a subdomain does not depend strongly on the other ink surface coverages, we only consider the ink spreading of each ink for a single ink superposition condition. This yields, for each ink i within each subdomain j, an ink spreading curve f i,j (u ) mapping the i,j normalized ink coverage u i,j to a normalized effective ink coverage u. The ink spreading i,j,eff curves are obtained by printing halftones in one ink superposition condition, i.e. with one ink at a nominal surface coverage corresponding to the mid-range of the considered subdomain and the other inks at their lower bounds. For instance, the ink spreading curve for the cyan ink (i = c) within the subdomain j delimited by its low (l) and high (h) bounds u i=c,j [c j l,c j h ], u i=m,j [m j l,m j h ], and u i=y,j [y j l, y j h ] is established by printing a halftone at cyan midrange, magenta low bound and yellow low bound ink nominal surface coverages, i.e. a halftone at cyan u i=c,j = (c j l +c j h )/2 at magenta u i=m,j = m j l and at yellow u i=y,j = y j l. Then, we fit the mid-range cyan normalized effective surface coverage q i=c,j of subdomain node reflectances by minimizing the sum of square differences between measured halftone reflection spectrum ( Ri=c,j (λ) ) and the corresponding predicted reflectance spectrum ( ˆR i=c,j (λ) ) R i=c,j (λ) = R (c j l +c j h )/2,m j l,y j l (λ) ˆR i=c,j (λ) = ( q i=c,j R c j h,m j l,y j l (λ) 1/n + (1 q i=c,j ) R c j l,m j l,y j l (λ) 1/n) 1/n q i=c,j = argmin [ k Ri=c,j (λ k ) ˆR i=c,j (λ k ) ] 2 (2.6) The minimization can be carried out with a computer executable procedure implementing Powells function minimization (Press et al. 1998). The two other q i=m,j and q i=y,j ink normalized effective surface coverages of subdomain node reflectances are obtained by replacing the cyan mid-range by the corresponding u i,j mid-range, the cyan higher bound c j h by the u i,j higher bound and keeping in Equation 2.6 the other ink coverages at their lower bound. The fitted ink normalized effective surface coverage q i,j indicates the amount of ink spreading of ink i within the subdomain j. The ink spreading curves u i,j,eff = f i,j (u ) within the subdomain j are obtained by quadratic interpolation between the points (,), (.5, q i,j ) and i,j (1,1), with u i,j,eff = (2 4 q i,j ) u i,j 2 + (4 q i,j 1) u i,j. Computing the normalized effective surface coverages q i=c,j, q i=m,j and q i=y,j with Equation 2.6 requires for each subdomain j three spectral reflectance measurements, i.e. we have to use for each ink i an halftone at nominal surface coverage of the ink i corresponding to the mid-range of the considered subdomain and the two other inks at their lower subdomain bound. As an alternative, in order to decrease the number of reflectance measurements to one per subdomain, we propose to jointly fit the normalized effective surface coverages on a 17

36 Chapter 2. Prediction model for classical and daylight fluorescent inks single halftone located at the center of the considered subdomain j (Equation 2.7) R i=center,j (λ) = R (c j l +c j h )/2,(m j l +m j h )/2),(y j l +y j h )/2(λ) ˆR i=center,j (λ) = ( (1 q i=c,j ) (1 q i=m,j ) (1 q i=y,j ) R c j l,m j l,y j l (λ) 1/n + q i=c,j (1 q i=m,j ) (1 q i=y,j ) R c j h,m j l,y j l (λ) 1/n + (1 q i=c,j ) q i=m,j (1 q i=y,j ) R c j l,m j h,y j l (λ) 1/n + (1 q i=c,j ) (1 q i=m,j ) q i=y,j R c j l,m j l,y j h (λ) 1/n + (1 q i=c,j ) q i=m,j q i=y,j R c j l,m j h,y j h (λ) 1/n + q i=c,j (1 q i=m,j ) q i=y,j R c j h,m j l,y j h (λ) 1/n + q i=c,j q i=m,j (1 q i=y,j ) R c j h,m j h,y j l (λ) 1/n + q i=c,j q i=m,j q i=y,j R c j h,m j h,y j h (λ) 1/n) n { } [ qi=c,j, q i=m,j, q i=y,j = argmin k Ri=center,j (λ k ) ˆR i=center,j (λ k ) ] 2 (2.7) Figure 2.2 illustrates a cyan dot gain curve for a cmy laser print, where the normalized dot gain is defined as d i,j (u i,j ) = f i,j (u i,j ) u, within the subdomain j = 1 delimited by c [,.5], i,j m [,.5] and y [,.5]. The computed cyan normalized effective surface coverage q i=c,1 of subdomain node reflectances for an optimal n-value = 14 calculated with Equation 2.6 is equal to.61. It represents a normalized dot gain of.11 in the range [,1] and therefore a real dot gain of.55 in the range [,.5]. The cellular Yule-Nielsen model prediction error for the considered uniform simple halftone without taking into account the dot gain is E 94 = 3.6. Introducing the dot gain obtained by the fitted cyan normalized effective surface coverage q i=c,j of subdomain node reflectances decreases for this halftone the prediction error to E 94 =.22. Since for the present print configuration, the ink dot gain within each subdomain j is at least.1, accounting for ink spreading considerably increases the spectral prediciton accuracy. dot gain d i=c,j=1 qi=c,j= normalized nominal surface coverages Figure 2.2: Cyan dot gain curve corresponding to the cyan ink spreading curve within the subdomain c, m, y [,.5], for a cmy laser print (Brother 4-HL) at a screen frequency of 12lpi and using an optimal n-value of 14. When computing the normalized effective surface coverages q i,j of all subdomains j according to Equation 2.7 instead of Equation 2.6, the coefficients are similar, i.e. the dot gains do not 18

37 2.4. Characterizing ink spreading with sensor responses deviate by more than 1%. We therefore obtain the same prediction accuracy improvements by jointly fitting the normalized effective surface coverages on a single center subdomain halftone relfectance as when fitting them on the three spectral reflectance measurements required by Equation 2.6. Note that the optimal n-value is found by predicting for successive n-values with the full model all mid-range reflectances. The n-value yielding the minimal average prediction error is kept as the optimal n-value for the considered setup of printer, inks and paper. Since we already measured the spectral reflectances of the mid-range for computing the normalized effective surface coverages at model calibration, the n-value can be fitted without measuring additional haftone patch reflectances. The cellular Yule-Nielsen model accounting for ink spreading (IS separately -CYNSN and IS single - CYNSN) is illustrated in Figure 2.3. At model calibration, the subdomain ink spreading curves f i,j (u ) are established either by separately fitting the normalized effective surface coverages i,j with Equation 2.6 (IS separately -CYNSN) or by jointly fitting the normalized effective surface coverages with Equation 2.7 (IS single -CYNSN). At run time, nominal ink surface coverages of the considered halftone are normalized according to Equation 2.4, the normalized effective ink surface coverages are deduced by making use of the corresponding ink spreading curves, the normalized effective areas of the subdomain primary reflectances are calculated according to Equation 2.2 and the halftone reflection spectrum is predicted according to Equation 2.1. c m y Normalization within sub-domain j c j m j y j f i=m,j (m j ) f i=c,j (c j ) f i=y,j (y j ) c j,eff m j,eff y j,eff Calculation of eff. primary coverages a jw a jc a jm a jy a jr a jg a jb a jk Spectral prediction sub-domain primary reflectances Predicted reflection spectrum Figure 2.3: Cellular Yule-Nielsen model accounting for ink spreading. 2.4 Characterizing ink spreading with sensor responses The three ink non-cellular ink spreading Yule-Nielsen modified spectral Neugebauer model (IS-YNSN) is calibrated with 8 Neugebauer primaries (Hersch and Crété 25). In addition, in order to account for ink spreading in all ink superposition conditions, 12 ink spreading curves are established mapping nominal surface coverages to effective surface coverages. Establishing the ink spreading curves requires measuring hafltones of each ink in its 4 possible 19

38 Chapter 2. Prediction model for classical and daylight fluorescent inks ink superposition conditions, i.e. alone on paper, superposed with one solid ink, superposed with the other solid ink and superposed with the two others solid inks. In this setup, the model requires = 2 spectral measurements. In the case of the ink spreading cellular Yule- Nielsen models, with one level of subdivision we obtain 27 subdomain primaries (subdomain primaries are considered by taking all combinations of %, 5% and 1% surface coverages yielding 2 3 = 27 combinations). Ink spreading is modeled by establishing 3 ink spreading curves within each of the 8 subdomains (Section 2.3.1). Thus, in case of separately fitted coefficients, the IS separately -CYNSN model requires = 51 measurements and in case of jointly fitted normalized effective surface coverages, the IS single -CYNSN model requires = 35 measurements. However, since establishing the ink spreading curves requires fitting only one scalar variable (IS separately -CYNSN) or three scalars (IS single -CYNSN) at a time, it is possible to use for example Red, Green and Blue sensor responses for the fitting process (Garg et al. 28). The ink spreading characterization of the IS-YNSN, the IS separately -CYNSN and the IS single -CYNSN is performed by minimizing the sum of square differences between predicted and measured sensor response values. Fitting with sensors considerably reduces the number of required spectral measurements. Only the spectral measurements of Neugebauer primaries are necessary, 8 for the IS-YNSN model and 27 for a single level subdivision IS separately -CYNSN and IS single -CYNSN models. In order to demonstrate the feasibility of using sensor responses instead of spectral measurements, we simulate the RGB sensor devices by the DIN standard RGB sensitivities for densitometric measurements (DIN 1995). Samples are illuminated with a standard CIE D65 illuminant. Reflected light generates the sensor responses C i C i = k S i (λ k ) R(λ k ) I (λ k ) /( I (λ k ) S(λ k ) ) (2.8) where C i represents the i th sensor response values, S i the spectral sensitivity of the i th sensor, I (λ) the illuminant and the R(λ) the spectral reflectance (Bala 23b). We can fit the normalized effective surface coverages q i,j of subdomain node reflectances by replacing in Equation 2.6, respectively in Equation 2.7 predicted ˆR i,j and measured R i,j reflectances by their corresponding Ĉ i,j and C i,j sensor responses, according to Equation 2.8. Let us consider as example the cmy laser print. The three dot gain curves within the subdomain shown in Figure 2.1 are similar one to another. The normalized effective surface coverages q i=c,j =1, q i=m,j =1 and q i=y,j =1 of node reflectances are either respectively equal to.65,.5989 and.5912 when fitted with the spectral reflectance metric or respectively equal to.669,.6 and.5945 when fitted with the simulated RGB sensor response metric. The normalized effective surface coverages of subdomain node reflectances q i,j in all subdomain j do not deviate by more than 1.5% when comparing these two metrics. Therefore, the prediction accuracy remains the same when characterizing ink spreading for the IS separately - 2

39 2.5. Prediction accuracies for classical ink halftones CYNSN, the IS-YNSN and the IS single -CYNSN models with three sensor responses instead of spectral measurements. 2.5 Prediction accuracies for classical ink halftones We performed spectral predictions with the cellular Yule-Nielsen model, the ink spreading enhanced cellular Yule-Nielsen models and the ink spreading enhanced Yule-Nielsen model (Table 2.2). In order to compare the resulting prediction accuracies with prior work, we also consider for each ink a single global ink spreading function, which is fitted with the YNSN model at 25%, 5%, 75% nominal ink surface coverages. We also performed spectral predictions by characterizing ink spreading with simulated RGB sensors for both the IS-CYNSN and the IS-YNSN models (Table 2.3). The experiment were performed on an inkjet printer (Canon Pixma Pro 95 at 6 dpi) with standard cyan, magenta and yellow inks printed on Canon MP-11 paper at a screen frequency of 12 lpi. In addition, test samples were printed with a laser printer (Brother 4-HL at 6 dpi) with standard cyan, magenta and yellow toners on Canon MP-11 paper at a screen frequency of 12 lpi. The test samples were printed at all combinations of nominal ink surface coverages,.25,.5,.75 and 1 (5 3 = 125 test patches) with classical rotated screen. Relfectances were measured with a GretagMacBeth Color i7 spectrophotometer with geometry (d : 8 ) under a D65 illuminant. In Tables 2.2 and 2.3, we give the mean prediction error in terms of E 94 values, the maximal prediction error, the 95% quantile prediction error, the number of spectral primary reflectance measurements (p) and the number of ink spreading measurements (i ). The n-value yielding the best prediction accuracies for all models and test sets is 14. The spectral prediction based on the proposed ink spreading extension for the cellular Yule- Nielsen model (IS separately -CYNSN) provides a significantly higher prediction accuracy compared to the stand-alone cellular Yule-Nielsen model (CYNSN). The E 94 mean prediction error decreases from 2.29 to 1.6 for a laser print (Table 2.2, Brother 4-HL test set) and from.92 to.54 for a classical CMY inkjet print (Table 2.2, Canon pro 95 test set). When printing with a laser printer (Brother 4-HL), we observe a strong dot gain within all surface coverage subdomains. Therefore, for this printer, there is a large difference in prediction accuracy between the IS separately -CYNSN model that accounts for ink spreading and the CYNSN model that does not account for ink spreading. In addition, considering ink spreading within each subdomain also offers a higher prediction accuracy than when using within the CYNSN model for each ink a single global function mapping nominal to effective surface coverages (IS global -CYNSN), i.e. the E 94 mean prediction error decreases from 1.3 to 1.6 for the laser print and from.7 to.54 for the inkjet print. Introducing ink spreading within each subdomain by jointly fitting the normalized effective surface coverages provides higher prediction accuracies than by separately fitting the normalized effective surface coverages. For instance, in case of the Canon Pro 95 print, with the IS separately -CYNSN model, we obtain a E 94 mean prediction error of.54 and 95% 21

40 Chapter 2. Prediction model for classical and daylight fluorescent inks quantile prediction error of 1.49 and with the IS single -CYNSN model these prediction errors decrease respectively to.38 and.99. This can be explained by the fact that the center of each subdomain contains the most useful information in respect to the ink spreading phenomenon. The IS single -CYNSN represents therefore an excellent tradeoff between number of measurements and prediction accuracy. With only 35 spectral reflectance measurements we obtain remarkable prediction accuracies. Similar test have been conducted on an offset print, a proofing device (Kodak Approval) and other inkjet prints. In all cases, mean prediction E 94 error around.4 have been obtained. In this situation, for the rest of this thesis, we consider ink spreading for the cellular Yule-Nielsen by jointly fitting the normalized effective surface coverages only. The IS-CYNSN model therefore will always refer to the IS single -CYNSN model. We also remarked that the ink spreading enhanced Yule-Nielsen model that accounts for ink spreading without cellular subdivision is more accurate than the cellular Yule-Nielsen model. This can be explained by the fact that the ink spreading behavior of multi-ink halftones is well captured by the Yule-Nielsen spectral Neugebauer model enhanced to account for ink spreading in all superposition conditions and that ink spreading has a strong impact on the resulting printed color. The prediction accuracies obtained when characterizing ink spreading with RGB sensors for the IS separately -CYNSN, the IS single -CYNSN, the IS global -CYNSN and the IS-YNSN models (Table 2.3) are nearly identical with the ones obtained with ink spreading characterized by spectral measurements (Table 2.2). This shows that ink spreading characterization can be performed by making use of RGB sensors instead of using spectral reflectance measurements. The cost of including RGB sensors within printers is much lower compared with the cost of including a spectrophotometer. Therefore, the ink spreading enhancement of the non-cellular as well as the cellular Yule-Nielsen models offers the potential of characterizing printers at run time at a moderate cost. 2.6 Prediction accuracies for fluorescent ink halftones In order to create a 6 ink print setup using the classical cyan ink c, magenta ink m, yellow ink y, black ink k and the two daylight fluorescent magenta m f and yellow y f inks, we define the total printer gamut G f as the conjunction of the four cmyk, cm f y f, cm f y and cmy f ink set sub-gamuts. In order to establish these exact sub-gamuts, we need a spectral prediction model that is accurate for predicting colors combining the inks present in the different sub-gamut ink sets. In order to test the prediction accuracies of the IS-CYNSN, the IS-YNSN and the CYNSN models for the considered fluorescent 6 ink print setup, we print for each ink set all the ink combinations by varying the ink nominal surface coverages by steps of 25%, yielding 5 3 = 125 halftones for the sets comprising three inks (cm f y f, cm f y and cmy f ) and 5 4 = 625 halftones for the set comprising 4 inks (cmyk). The halftones were printed both with an offset printer on a HEAVEN 42 SOFTMATT coated paper and with an Epson P5 inkjet printer on a Canon MP-11 paper. There were measured under the D65 illuminant with a SpectroEye 22

41 2.6. Prediction accuracies for fluorescent ink halftones Table 2.2: Prediction accuracies for cyan, magenta, yellow test samples printed with a Canon Pro 95 inkjet printer and for cyan, magenta and yellow test samples printed with a Brother 4-HL laser printer. Test sets # measurements E 94 Model p + i Avg 95% Max Brother 4-HL Prior art: CYNSN 27 + = IS global -CYNSN = IS-YNSN = new: IS separately -CYNSN = IS single -CYNSN = Canon Pro 95 Prior art: CYNSN 27 + = IS global -CYNSN = IS-YNSN = new: IS separately -CYNSN = IS single -CYNSN = Table 2.3: Prediction accuracies for both the IS-YNSN model and the IS-CYNSN models when characterizing ink spreading using simulated RGB sensors. Test sets # measurements E 94 Model p + i Avg 95% Max Brother 4-HL IS global -CYNSN = IS-YNSN = IS separately -CYNSN = IS single -CYNSN = Canon Pro 95 IS global -CYNSN = IS-YNSN = IS separately -CYNSN = IS single -CYNSN =

42 Chapter 2. Prediction model for classical and daylight fluorescent inks Table 2.4: Prediction accuracies of the CYNSN, IS-YNSN and IS-CYNSN models for the fluorescent and non-fluorescent inkjet ink sets used to establish the fluorescent G f printer gamut. Table 2.5: Prediction accuracies of the CYNSN, IS-YNSN and IS-CYNSN models for the fluorescent and non-fluorescent offset ink sets used to establish the fluorescent G f printer gamut. Test sets (inkjet) E 94 Model Avg 95% Max cm f y f CYNSN IS-YNSN IS-CYNSN cm f y CYNSN IS-YNSN IS-CYNSN cmy f CYNSN IS-YNSN IS-CYNSN cmyk CYNSN IS-YNSN IS-CYNSN Test sets (offset) E 94 Model Avg 95% Max cm f y f CYNSN IS-YNSN IS-CYNSN cm f y CYNSN IS-YNSN IS-CYNSN cmy f CYNSN IS-YNSN IS-CYNSN cmyk CYNSN IS-YNSN IS-CYNSN Xrite spectrophotometer with geometry (45 : ). Tables 2.4 and 2.5 give for respectively the inkjet and offset prints the mean prediction error in terms of E 94 values, the maximal prediction error and the 95% quantile prediction error. The Yule-Nielsen n-value yielding the best prediction accuracies for the two printers and for all test sets is 14. In case of the inkjet and offset prints (Tables 2.4 and 2.5) both the CYNSN and IS-YNSN models are not accurate for predicting combinations of classical and daylight fluorescent inks with E 94 mean prediction errors varying between 1.34 and 3.44, with 95% quantile prediction errors varying between 2.82 and 9.37, and with a maximal prediction error of In contrast, the IS-CYNSN spectral prediction model is remarkably accurate for predicting combinations of classical and daylight fluorescent inks. In case of the inkjet prints, the E 94 prediction errors vary between.55 and.86 with a maximal 95% quantile prediction error of 2.32 for the cm f y f ink set and in case of the offset prints, the E 94 prediction errors vary between.65 and.97 with a maximal 95% quantile prediction error of 2.71 for the cm f y f ink set. The IS-CYNSN model also accurately predicts halftones made of classical cmyk inks with a E 94 mean prediction error of.44 for the inkjet print and of.49 for the offset print. From these prediction accuracies, we conclude that in order to establish the exact fluorescent sub-gamuts and therefore the fluorescent print gamut G f, we shall use the specially developed 24

43 2.7. Summary IS-CYNSN spectral prediction model that is remarkably accurate for predicting combinations of classical and daylight fluorescent inks. In addition, this model is calibrated with only a few halftone patch reflectance measurements. 2.7 Summary We proposed an extension of the cellular Yule-Nielsen spectral Neugebauer model by accounting for ink spreading separately within each subdomain. The ink spreading characterization of the cellular Yule-Nielsen model can be established by jointly fitting the ink spreading interpolation coefficients on a single halftone centered within each subdomain. We obtain excellent spectral prediction accuracies for predicting halftones combining classical inks only for inkjet, offset and electrophotography prints. Prediction accuracies are also remarkable for predicting halftones combining classical and daylight fluorescent inks for both offset and inkjet prints. Compared with the original cellular Yule-Nielsen model and the ink spreading enhanced Yule-Nielsen models, prediction accuracies are significantly improved. By jointly accounting for ink spreading on a single haltone located at the center of the considered cellular subdomain, we can characterize fluorescent ink halftones with a small number of calibration measurements. For this reason, in the rest of the thesis, the ink spreading cellular Yule-Nielsen model (IS-CYNSN) will always refers to this ink spreading characterization. In addition, we show that ink spreading characterization can be performed with RGB sensors without reduction of prediction accuracy. This offers the potential of characterizing printers at run time at moderate cost. 25

44

45 3 Framework for printing with combined classical and daylight fluorescent inks 3.1 Introduction In the previous chapter, we have shown that the IS-CYNSN spectral prediction model accurately predicts the spectral reflectances of hafltones comprising daylight fluorescent inks. This enables the exact computation of the gamut G f enclosing all colors printable with the 6 ink print setup. The exact computation of the fluorescent G f gamut is required for establishing a mapping between the srgb display gamut G srgb and the 6 ink printer gamut. In order to be able to print the displayed colors, we have to establish a mapping between the srgb display gamut and the printer gamut (Morovic and Luo 21). In order to drive the printer, we also need to establish a relationship between the gamut mapped CIELAB colors and the corresponding ink surface coverages. For mapping the srgb gamut into the ink gamut, we need in a first step to map the lightness range of the srgb gamut into the lightness range of the ink gamut by lightness adaptation and to establish the lightness adapted srgb gamut and the joint fluorescent gamut of the fluorescent and non-fluorescent ink sets, i.e the conjunction of the sub-gamuts G cm f y f, G cm f y, G cmy f, G cmyk. Detailed of the possible lightness adaptation functions are given in Chapter 4. In the current section, we focus on gamut comparison only. We are interested to see what is the color domain extent by adding the m f and y f inks to the classical cmyk inks. In order to show the colors available by printing combinations of classical and daylight fluorescent inks, we compare for an inkjet printer the G cmyk and G f gamuts with the srgb display gamut G srgb (Section 3.2). The comparison is done under the A, D65 and F7 illuminants by calculating the additional volume of the srgb gamut covered by the 6 ink fluorescent setup, by calculating the volume of colors offered by the G f gamut and by showing these gamut boundaries in constant CIELAB lightness planes. We also compare the different color domain extensions offered by the fluorescent G f gamuts for both the inkjet and offset printers by showing their gamut boundaries in constant CIELAB 27

46 Chapter 3. Framework for printing with combined classical and daylight fluorescent inks lightness planes (Section 3.3). 3.2 Comparison of fluorescent and non-fluorescent ink gamuts A color device gamut in the CIELAB space is a 3D volume whose surface can be described by triangles. Every color point inside this surface belongs to the gamut. In order to compute the lightness adapted srgb gamut, we generate CIELAB points by varying the srgb Red, Green and Blue values by steps of.3, convert them to CIE-XYZ tri-stimulus values according to known phosphor tri-stimulus values, to CIELAB colors and then apply an adaptation of the lightness L values according to a specific lightness adaptation function, yielding 34 3 = 3934 CIELAB color points. For the gamut comparisons, we consider only a non-linear Bézier lightness adaptation function for mapping the srgb lightness range to the ink lightness range. This non-linear function as well as a detailed analysis of different lightness adaptation functions are given in Chapter 4. In order to obtain the gamut of the 4 considered ink sets, we predict with the IS-CYNSN spectral prediction model the total reflectance factors of halftones by varying the nominal ink surface coverages by steps of.5 for the 4 ink set cmyk and by steps of.3 for the 3 ink sets cm f y f, cm f y, cmy f, convert them to CIE-XYZ tri-stimulus values according to the spectral power distribution of the considered illuminant and compute CIELAB colors, yielding = CIELAB color points. The non-convex gamut boundary is obtained by performing a Delaunay triangulation of the set of CIELAB color points and by computing with the ball-pivoting technique (Bernardini et al. 1999) the set of surface triangles defining the concave boundary. The srgb white is mapped to the paper white by taking as CIELAB white reference the display white for converting srgb values to CIELAB colors and the paper white for converting tri-stimulus values of print samples from CIE-XYZ to CIELAB colors. In order to show the new colors available by printing combinations of classical and daylight fluorescent inks, we establish for the inkjet Canon Pro 95 printer the non-fluorescent G cmyk gamut comprising only the colors generated with the classical cmyk inks, the joint fluorescent G f gamut comprising colors generated with the classical inks and the daylight fluorescent magenta and yellow inks, and the non-linearly lightness adapted srgb gamut G srgb. Under the D65 illuminant, Figure 3.1a shows in 3D the G cmyk and the G f gamuts. At high lightness values (L > 6), we observe a gamut extension in the yellow, magenta, red and green parts of the gamut due to the fluorescence of the y f and m f inks. Figure 3.1b illustrates a comparison between the joint fluorescent G f and the non-linearly lightness adapted G srgb gamuts. A significant part of the G f gamut is outside the G srgb gamut. In Chapter 4, we propose a suitable mapping from the G srgb gamut to the G f gamut that enables expanding the input G srgb gamut colors into the G f printable gamut. Let us compare the gamuts within constant lightness planes. Figure 3.2 illustrates a comparison between the non-linearly lightness adapted G srgb gamut, the G cmyk gamut and the fluorescent gamut G f under the D65 illuminant. Gamut boundaries are shown within constant 28

47 3.2. Comparison of fluorescent and non-fluorescent ink gamuts G f G f 1 8 L * L * 6 G' srgb 4 G cmyk a * (a) -4 4 b * b a * * (b) Figure 3.1: (a) Comparison between the gamut G cmyk (colored solid) and the joint fluorescent ink gamut G f (mesh grid) and (b) comparison between the joint fluorescent gamut G f (colored solid) and the display non-linearly lightness adapted gamut G srgb (mesh grid). lightness planes from L = 5 to L = 95. At a lightness between L = 55 and L = 65, there are not many differences between the classical ink gamut G cmyk and the joint fluorescent gamut G f. For lightnesses lower than L = 5, there is strictly no difference between the G cmyk and the G f gamuts. For lightnesses between L = 7 and L = 8, the G srgb gamut is significantly better covered by the joint fluorescent gamut G f in the green, orange, magenta, yellow and blue parts of the gamut than by the classical ink gamut G cmyk. This is due to the high saturation offered by the daylight fluorescent inks, as become apparent in the spectral reflectances of the m f and y f inks (Figures 1.2a and 1.2b, pointed lines). For lightnesses varying between L = 85 and L = 95, the classical inks fill only a small part of the srgb gamut while the joint fluorescent gamut covers most of the srgb gamut. At L > 95, only the G f gamut fills the yellow and green parts of the G srgb gamut. For establishing the G f gamut under the F7 illuminant, fluorescent and non-fluorescent ink set halftones were measured under the F7 illuminant by making use of the Just Normlicht mini 5 light table as light source, by building a custom measurement setup with geometry ( : 45 ) comprising an optical fiber capturing the reflected light, connected with a Maya Pro 2 spectrophotometer. In case of establishing the G f gamut under the A illuminant, the halftones were measured with the A illuminant of a SpectroEye Xrite spectrophotometer with geometry (45 : ). CIELAB color points defining the gamut are predicted with the IS-CYNSN spectral prediction model. Accuracies of the IS-CYNSN model for the fluorescent and non-fluorescent ink sets under these two illuminants are listed in Table 3.1. For measurements performed under the A illuminant, we obtain for the cmyk and cm f y f test sets remarkable prediction accuracies, with respective mean E 94 prediction error of.34 and.49 and respective quantile 95% prediction error of.8 and 1.1. In the case of measurements performed with the Just Normlich light table (illuminant F7), we obtain slightly less accurate predictions with 29

48 Chapter 3. Framework for printing with combined classical and daylight fluorescent inks L * = 55 L * = 65 L * = 75 G' srgb 5 G cmyk 5 5 b * b * b * -5 G f a * L * = a * a * L * = 9 L * = b * b * b * a * -5 5 a * -5 5 a * Figure 3.2: Color gamuts of the non-linearly lightness adapted srgb space (solid lines), the 4 ink print gamut using the classical cyan, magenta and yellow inks (dashed lines) and the joint fluorescent gamut using the classical cyan, magenta, yellow, black and the two additional daylight fluorescent magenta and yellow inks (dotted lines) under the D65 illuminant. a mean E 94 prediction error of 1.1 when predicting the 625 classical cmyk test samples and of.71 when predicting the 125 cm f y f test samples. Since the test sets containing one daylight fluorescent ink (cm f y and cmy f ) show the same prediction accuracies as the set containing two daylight fluorescent ink (cm f y f ) they are not listed in Table 3.1. These prediction accuracies enable establishing the exact G f gamuts under both the A and D65 illuminants. Figure 3.3 compares the joint fluorescent G f gamuts under the D65, A and F illuminants and the non-linearly lightness adapted G srgb gamut. Due to its low energy within the UV and blue excitation wavelength ranges, the A illuminant induces less fluorescence (Figure 1.2, dashed lines) and therefore provides the smaller gamut (G A dashed lines). The D65 illuminant provides the largest fluorescent ink gamut included within the G srgb gamut, especially in the green, magenta and yellow parts. The F7 illuminant provides also a strong fluorescent emission of the daylight fluorescent colorants, but achieve less coverage of the G srgb gamut, compared with the D65 illuminant. 3

49 3.2. Comparison of fluorescent and non-fluorescent ink gamuts Table 3.1: Prediction accuracies of the IS-CYNSN model for 625 cmyk and 125 cm f y f test samples printed with a Canon Pro 95 inkjet printer and measured under the A and F7 illuminants. Illuminant E 94 Test set Avg 95% Max Xrite A cmyk cm f y f Just Norm. F7 cmyk cm f y f L * = 55 L * = 65 L * = 75 G' srgb G F7 b * b * b * -5 G A G D a * L * = a * L * = 9 L * = a * b * b * b * a * -5 5 a * -5 5 a * Figure 3.3: Comparison of the non-linearly lightness adapted srgb gamut G srgb (thick solid lines) and the fluorescent 6 ink gamuts under the D65 (G D65, solid lines), F7 (G F7, dotted lines) anda(g A, dashed lines) illuminants Comparison of gamut volumes In this section, we compare the gamut of the classical cmyk print setup with the joint fluorescent ink gamut by showing the percentage of the srgb gamut that can be reproduced. This can be done by computing the volume of the intersection of the G srgb gamut with the 31

50 Chapter 3. Framework for printing with combined classical and daylight fluorescent inks print gamuts. Bala and Dalal (1997) proposed to compute the volume of a printer gamut by summing the volume of tetrahedra formed by the vertices of each surface triangle defining its boundary and a point inside the gamut. With the volume of the non-linearly lightness adapted srgb gamut V (G srgb ), the volume of the joint fluorescent gamut V (G f ) and the volume of the conjunction of the G f and G srgb gamuts, we can compute the volume of the intersection of the G f gamut with the G srgb gamut with the following equation V (G f G srgb ) = V (G f ) +V (G srgb ) V (G f G srgb ) (3.1) In an analog manner, we can compute the intersection of the G cmyk and G f gamut with the G srgb gamut. Table 3.2 shows the volumes of the classical G cmyk and joint fluorescent G f ink gamuts, and the volumes formed by overlap between their gamuts and the G srgb gamut in thousands L a b volume units under the A, D65 and F7 illuminants. It also shows the gain in overlapped volume offered by the joint fluorescent 6 ink gamut, in respect to the traditional cmyk gamut. It further shows the gain g sr relative to the volume of the non-linearly lightness adapted srgb gamut G srgb, as defined by Equation 3.2: g sr = V (G f G srgb ) V (G cmyk G srgb ) V (G srgb ) (3.2) Table 3.2: Comparison of gamut volumes for the G srgb gamut. Illuminant D65 A F7 Volume G cmyk Volume G f Volume G cmyk G srgb Volume G f G srgb gain 31% 14% 29% g sr gain relative to srgb volume 13% 5% 11% (V of G srgb = 693) Since the A illuminant has less energy in the fluorescent excitation range, the A illuminant yields the smallest gamut volume. By comparing the classical 4 ink gamut with the joint fluorescent 6 ink gamut, we observe a gamut volume increase of 31% and 29% for respectively the D65 and F7 illuminants. When considering the part of the G srgb gamut covered by respectively the joint fluorescent and classical ink gamuts, under the D65 illuminant, 54% of the G srgb gamut are covered instead of 41% and under the F7 illuminant 47% of the G srgb gamut are covered instead of 36%. Since the CIELAB color space is not perceptually constant 32

51 3.3. Comparison of inkjet and offset fluorescent ink gamuts for large color differences, these computed volumes are approximate. A better approximation of these volumes may be obtained with a more perceptually constant color space, such for instance the LAB2HL color space (Lissner and Urban 212). L * = 55 L * = 65 L * = b * b * b * a * a * L * = 85 L * = 9 L * = a * b * b * b * a * -5 5 a * -5 5 a * Figure 3.4: Comparison of the inkjet (solid lines) and offset (dashed lines) fluorescent G f gamuts under the D65 illuminant. 3.3 Comparison of inkjet and offset fluorescent ink gamuts In this section, we compare the fluorescent printer gamuts obtained by printing combinations of classical and daylight fluorescent inks either with inkjet inks or with offset inks. Figure 3.4 illustrates this comparison as CIELAB constant lightness planes from L = 55 to L = 95. For lightnesses between L = 55 and L = 75, the offset G f gamut shows larger boundaries in the red and orange parts of the gamut. These larger boundaries are due to the more saturated red colorant obtained by superposing the offset m f and y f inks than by superposing the inkjet m f and y f inks (Figure 1.3c). Since the offset m f colorant has a larger fluorescent emission peak near 6 nm with a reflectance factor of 1.31 than the inkjet m f colorant with a reflectance factor of 1.2 (Figure 1.3a), we also observe for the offset G f gamut larger boundaries in the magenta part of the gamut. The inkjet G f gamut shows for these lightnesses larger boundaries in the green regions of the gamut. Since the y f inkjet colorant is more saturated than the y f offset colorant (Figure 1.3b), we therefore obtain a more saturated fluorescent green colorant 33

52 Chapter 3. Framework for printing with combined classical and daylight fluorescent inks by superposing the inkjet cyan and y f inks than by superposing the offset cyan and y f inks. For lightnesses more than L = 75, while the inkjet G f gamut shows larger boundaries in the green region of the gamut, the offset G f gamut shows larger boundaries in the orange and yellow parts of the gamut. Since within the green visible wavelength range near 52 nm, the inkjet y f colorant has a significant higher fluorescent emission peak than the offset y f colorant with respective reflectance factors of 1.41 and.92 (Figure 1.3b), the inkjet domain extension of colors for high lightnesses is mainly visible in the green region. For the offset y f colorant, the two fluorescent peaks are located in both the green and red wavelength range, yielding strong fluorescent yellow colors. 3.4 Summary Thanks to the fluorescent ink halftones accurate spectral predictions of the IS-CYNSN spectral prediction model, we establish for the inkjet printer the fluorescent gamut G f combining the classical cyan, magenta, yellow and black inks together with the two additional daylight fluorescent yellow and magenta inks. We compute the classical and fluorescent gamut volumes and compare their gamut volumes in respect to the srgb gamut, under the D65, fluorescent F7 and A illuminants. The large inkjet fluorescent gamut is present for light source having much energy in the UV and blue wavelength range, such as the D65 and F7 illuminants. By comparing the boundaries of the fluorescent gamut with the boundaries of the lightness adapted srgb display gamut, under the D65 and F7 illuminants the fluorescent gamut covers at least 11% more the srgb display gamut, compared with the classical cmyk gamut. From the computation of the gamut volumes and the comparison of gamut boundaries, we conclude that the two additional inkjet daylight fluorescent yellow and magenta inks considerably expand the gamut of classical printers for images that are viewed under normal daylight and fluorescent tubes. The fluorescent gamuts extend the domain of colors mainly at high lightness from L = 55 to L = 1. We also compare the fluorescent gamuts G f obtained with offset and inkjet printers. The offset printer shows a color domain extension larger in the magenta, orange and yellow parts of the gamut than the one obtained with the inkjet printer, while the inkjet printer shows a color domain extension larger in the green part of the gamuts than the one obtained with the offset printer. In the next part part of this thesis, we use these new fluorescent colors to both better reproduce input srgb colors and to highlight image regions of special interest by making use of the new available bright and high chroma fluorescent green, yellow, orange, red and magenta colors. 34

53 4 Gamut mapping expansion and reduction for color reproduction with daylight fluorescent inks 4.1 Introduction In this chapter, we show how to map input srgb images to printable destination fluorescent images by gamut expansion and reduction mapping algorithms (Section 4.3). Gamut mapping contraction and expansion techniques are not new (Morovic and Luo 21), (Kang et al. 23). However, we adapt these techniques in order to accommodate the destination fluorescent ink gamut G f. We also have the goal of enhancing specific image parts by printing them with high chroma and bright colors. For this purpose, we developed a software that enables controlling the gamut expansion. We first select the image parts to be enhanced. We then apply to their colors a user-controlled gamut expansion that increases both their chroma and lightness towards colors located at the boundary of the destination fluorescent ink gamut G f. We also create smooth chroma transitions between the expanded and non-expanded image parts (Section 4.4). We then preview the resulting printable gamut expanded image (Section 4.5). Finally, we show how to perform the color separation into its 6 ink color separation layers (Section 4.6). The resulting prototype software enables artists to create and print their own designs. 4.2 Mapping the lightness range of the srgb gamut into the ink destination gamut Mapping the srgb gamut into the ink gamut requires in a first step to map the lightness range of the srgb gamut into the lightness range of the ink gamut (Morovic and Luo 21). In order to map the G srgb lightness range into the destination gamut lightness range, we first determine the minimal lightness L of the inks, i.e. the lightness of the solid pure black inksmin ink. We may then either apply a linear mapping that better preserve lightness differences of the input srgb image space but raises all srgb lightnesses, apply a partly non-linear mapping 35

54 Chapter 4. Gamut mapping expansion and reduction for color reproduction with daylight fluorescent inks L * inks L * inksmin 2 Linear P 1 P L * srgb Figure 4.1: Linear (dashed line) and non-linear mapping of srgb lightnesses. For the nonlinear mapping, the input srgb lightness values are either non-linearly mapped between L = and L = 5 and preserved beyond L = 5 (solid line) or preserved between L = 5 and L = 6 and shifted to higher values beyond L = 6 (pointed line). that preserves high lightness values but maps low lightness values into a smaller lightness range or apply an s-shape like non-linear mapping. These lightness mappings can be defined with a cubic Bézier function B(t) = (1 t) 3 P + 3 (1 t) 2 t P (1 t) t 2 P 2 + t 3 P 3 (4.1) where P, P 4 are the points (,L ), (1,1), and where parameter t is varied between inksmin zero and one. Control points P 1 and P 2 are user-defined according to the type of mapping. Figure 4.1 illustrates a typical case of a srgb lightness adaptation for an inkjet Epson P5 printer which has a minimal L inksmin at 23. In case of a linear lightness mapping, the control points P 1 and P 2 are respectively set to (,L ) and (1,1) (Figure 4.1, solid line). In case of a partly inksmin non-linear mapping, P 1 and P 2 are both set to (L inksmin,l ). This preserves lightnesses inksmin for L > 5 (Figure 4.1, dashed line). In case of a s-shaped non-linear mapping, lightnesses between L = 5 and L = 6 are preserved, but raised at lightnesses L > 6. Control points P 1 and P 2 can for instance be set to (2 L inksmin,l ) and (8,1) (Figure 4.1, pointed inksmin line). By comparing in 3D the non-linearly lightness adapted srgb gamut where control points P 1 and P 2 are both set to (L inksmin,l ) and the linearly lightness adapted srgb gamut where inksmin control points P 1 and P 2 are respectively set to (,L inksmin ) and (1,1) with the G f gamut (Figure 4.1), we observe that the linearly adapted srgb gamut G srgb achieves more overlap with the fluorescent ink gamut G f for lightnesses L > 7. However, for lower lightnesses it achieves less overlap than the non-linearly lightness adapted gamut G srgb, especially in the 36

55 4.2. Mapping the lightness range of the srgb gamut into the ink destination gamut G f L * L * G f G'sRGB G'' 1 srgb b a * * (a) b a * (b) Figure 4.2: (a) Comparison between the fluorescent ink gamut G f (colored solid) and the display linearly lightness adapted gamut G srgb (mesh grid) and (b) comparison between the fluorescent ink gamut G f (colored solid) and the non-linearly lightness adapted display gamut G srgb (mesh grid). Table 4.1: Comparison of gamut volumes for the G srgb gamut. Illuminant D65 A F7 Volume G cmyk G srgb Volume G f G srgb gain 36% 17% 31% g sr gain relative to srgb volume 16% 7% 13% (V of G srgb = 598) blue part of the gamuts. This linearly lightness adapted G srgb gamut has colors shifted to slightly higher lightness values. The G f gamut therefore covers a larger fraction of the G srgb gamut. Under the D65 and F7 illuminants, respectively 62% and 54% of the G srgb gamut are covered (See Table 4.1) instead of 54% and 47% of the G srgb gamut (See Table 3.2, Chapter 3, Section 3.2.1). This Bézier lightness adaptation function can be user-defined depending on the darkest color achievable by a given set of inks and printer and also on the input image content. For bright images, mapping a large range of dark colors into a small range of dark colors enables preserving the lightnesses of the original image without loss of details within the printed image. On the contrary, for dark images, it is preferable to linearly map the lightnesses with the goal of preserving the input image details in dark tones. 37

56 Chapter 4. Gamut mapping expansion and reduction for color reproduction with daylight fluorescent inks L * L * =l h G dest α C sourcemax I C source II C dest C sourcemax C destmax L * L * =l h G dest α C destmax II C destmax C dest I C source L * =l l G source L * =l l C G sourcemax source (a) C * (b) C * Figure 4.3: Multiple foci gamut mapping approach for (a) gamut expansion of point C source to C dest and (b) gamut reduction of point C source to C dest. 4.3 Mapping the lightness adapted srgb gamut onto the printable fluorescent gamut In order to map the srgb gamut onto the printable destination gamut, we first establish the lightness adapted srgb gamut G srgb by varying the srgb Red, Green and Blue values in small steps, convert them to CIE-XYZ values and then to CIELAB. We then apply an adaptation of the lightness L values according to the desired lightness range mapping function with Equation 4.1 and establish the concave gamut by Delaunay triangulation and ball-pivoting (Bernardini et al. 1999). The fluorescent gamut is established by predicting with the IS-CYNSN spectral prediction model the total reflectance factors of halftones by varying nominal ink surface coverages by small steps for the 4 ink sets cmyk, cm f y f, cm f y, cmy f, by converting them to CIE-XYZ tri-stimulus values according to the D65 illuminant, by computing corresponding CIELAB colors and by deriving with ball-pivoting the set of surface triangles defining the concave boundary. The srgb white is mapped to the paper white by taking as CIELAB white reference the display white for converting srgb values to CIELAB colors and the paper white for converting tri-stimulus values of print samples from CIE-XYZ to CIELAB colors. The lightness adapted gamut G srgb is mapped to the printable destination fluorescent gamut according to a multiple foci approach (Morovic and Luo 21), as shown in Figure 4.3. We define an upper lightness bound l h and a lower lightness bound l l. For a color point C source of the source gamut G source, we apply a mapping from point C source to C dest on a line passing through C source and through the focal point on the black and white axis for C source color points that have a lightness either L source > l h or L source < l l or on a constant lightness line passing through C source when l l L source l h (Figure 4.3, pointed lines). The mapping line intersects the source and destination gamut boundaries at respective intersection points C sourcemax and C destmax. 38

57 4.3. Mapping the lightness adapted srgb gamut onto the printable fluorescent gamut In case of gamut expansion, i.e. C destmax > C sourcemax (Figure 4.3a), we apply a chroma expansion to chroma values C dest by mapping the interval (I): [α C sourcemax,c sourcemax ] into the interval (II): [α C sourcemax,c destmax ] according to the following equation ( ) Csource α C γ sourcemax C dest = α C sourcemax + (C destmax α C sourcemax) (4.2) C sourcemax α C sourcemax where factor γ expresses a possible non-linearity of the chroma mapping. With γ = 1, the mapping is linear and with < γ < 1, chroma is non-linearly expanded. Factor α between and 1, defines the internal part of the source gamut where chroma values do not change, i.e. within the interval α C sourcemax we have C dest = C source. This prevents the chroma increase of low chroma colors. Figure 4.4 illustrates the need of a multiple foci approach for expanding color points. In case of chroma expansion towards the color of a daylight fluorescent colorant, we should avoid mapping a small range of colors into a large range of colors. For the considered inkjet printable fluorescent gamut, we set the upper bound to l h = 76. In the hue plane of the inkjet m f colorant, compared with constant lightness mapping (Figure 4.4a, upper dashed line) the non-constant lightness mapping line passing through the inkjet m f ink color (Figure 4.4a, upper pointed line) maps a significantly larger range of input gamut colors. The non-constant lightness mapping is also required in order to map input colors into the color of the inkjet y f colorant whose maximal lightness is L = 18, i.e. higher than the maximal source gamut lightness at L = 1. For mapping inkjet green colors, e.g. at a hue angle of 147, applying a constant lightness mapping in the lightness range l l L l h = 76 is appropriate since the source range of color is large enough in comparison with the destination gamut (Figure 4.4b, dashed line). In addition, the upper lightness focal point also controls the lightness shifts when expanding source colors. For instance, by decreasing the lightness of the focal point l h, colors with lightness higher than l h are expanded towards higher lightnesses. Other hue planes, from a hue angle of degree to 3 degrees by steps of 6 degrees are shown in Appendix A. Figure 4.5 illustrates the need of a multiple foci approach for expanding color points for the considered offset printer. We set the upper lightness bound l h to 85. When expanding colors towards the color of the offset fluo y f colorant, the non-constant mapping line passing through the offset y f colorant maps a significant large range of colors (Figure 4.5a, upper pointed line). In case of mapping colors towards the red fluo colorant, constant mapping is appropriate since the destination color range is large enough in comparison with the source gamut (Figure 4.5b, dashed line). We also show a comparison between the non-linearly lightness adapted G srgb gamut and the offset fluorescent gamut G f as constant hue planes from degree to 3 degrees in Appendix A. In case of gamut reduction, i.e. C sourcemax C destmax (Figure 4.4b), we apply a chroma reduction to chroma values C dest by mapping the interval (I): [α C destmax,c destmax ] according to the 39

58 Chapter 4. Gamut mapping expansion and reduction for color reproduction with daylight fluorescent inks L * 1 hue angle of 352 Gf L * 1 hue angle of 146 G' srgb m f 8 l h 8 l h g f 6 6 G f G' srgb l 4 l l 4 l 2 c 1 c (a) C * (b) C * Figure 4.4: Constant hue planes for an inkjet Epson P5 printer (a) at a hue angle of 352 showing both a constant (upper dashed line) and a non-constant (upper pointed line) expansion lightness mapping line in direction of the magenta fluo solid ink (m f ), and both a constant (lower dashed line) and non-constant (lower pointed line) reduction lightness mapping line for a low lightness color (c 1 ) and (b) at a hue angle of 146 showing a constant expansion lightness mapping line (dashed line) in direction of the green fluo colorant (g f ) and non-constant reduction lightness mapping line (pointed line) for a low lightness color (c 2 ). following equation ( ) Csource α C destmax C dest = α C destmax + (C destmax α C destmax ) (4.3) C sourcemax α C destmax The lower lightness bound l l used for reducing chroma prevents a too strong chroma reduction in dark tones and therefore better preserves the original image colors. In case of the inkjet printer, we set l l to 4. When reducing chroma of a low lightness color c 1 located in a hue plane at 352, the non-constant lightness mapping line passing through c 1 and a single point in the black and white axis at a lightness L = 4 (Figure 4.4a, lower pointed line) reduces less chroma than a constant lightness mapping line (Figure 4.4a, lower dashed line). The non-constant lightness mapping line for reducing chroma in dark tones, e.g. a dark c 2 color (Figure 4.4b) is not required in a hue plane at 146 since the destination gamut G f boundary is as large as the source gamut G srgb. However, in order to ensure color continuity while reducing chroma, we use the same low lightness l l bound for all hue planes. For the considered offset printer, we set the lower lightness bound l l to 3. In this situation, the chroma of a low lightness color c 3 located in a hue plane at an angle of 98 is not reduced too much (Figure 4.5a, lower pointed line). 4

59 4.4. User driven gamut expansion L * hue angle of 98 1 G f L * hue angle of 41 1 y f l h 8 l h 8 G f r f l l G' srgb 4 G' srgb 2 13 c (a) 1 C * C * (b) Figure 4.5: Constant hue planes for an offset Heidelberg printer (a) at a hue angle of 98 showing a non-constant (upper pointed line) expansion lightness mapping line in direction of the yellow fluo solid ink (y f ), and a non-constant (lower pointed line) reduction lightness mapping line for a low lightness color (c 3 ) and (b) at a hue angle of 41 showing a constant lightness mapping line (dashed line) in direction of the red fluo colorant (r f ). 4.4 User driven gamut expansion In this section, we define user parameters that enable controlling the gamut expansion. We would like to control the chroma enhancement within image parts selected by users. Within image parts, we apply a gamut expansion of the input image srgb colors by mapping the input chroma interval either linearly or non-linearly onto the printable output chroma interval, by considering the maximal chroma or a part of the maximal chroma achievable by the printable fluorescent gamut. In addition, since within the select image part we may strongly increase both chroma and brightness of the colors, it is possible to see strong color differences at the boundary between the expanded and non-expanded image parts. In order to reduce these strong color differences, we create smooth chroma transitions from the center of the selected image part reproduced with high chroma colors to the border of the selection reproduced with lower chroma colors. Input srgb image colors are mapped according to the equations presented in Section 4.3 with additional user-defined parameters that are described below. Outside the selected image parts, no srgb gamut expansion is performed. An input C source color is mapped according to Equation 4.3 when C destmax C sourcemax or when C destmax > C sourcemax is kept as it is, i.e. C dest = C source. Within the selected image parts, the lightness adapted srgb chroma can be enhanced. We distinguish two cases. The first case is when the destination fluorescent gamut is greater than the source gamut along the mapping line (C destmax > C sourcemax ). In this case, the chroma can be expanded according to Equation 4.2 and possibly with the non-linear chroma reinforcement factor γ varying within < γ < 1. This chroma expansion can be limited according to a user-defined chroma expansion limitation factor δ. This chroma expansion limitation factor limits the effective maximal 41

60 Chapter 4. Gamut mapping expansion and reduction for color reproduction with daylight fluorescent inks chroma expansion C destmaximalexp to values between C destmax and C sourcemax. The effective maximal chroma expansion then becomes C destmaximalexp = C sourcemax δ +C destmax (1 δ) (4.4) and replaces C destmax in Equation 4.2, for δ 1. Parts of images reproduced with the maximal expansion limitation factor δ = 1 do not contain chroma expanded colors. The second case is when the source gamut is greater than the destination gamut along the mapping line (C sourcemax > C destmax ). In this case, the chroma of a C source color can be reinforced by non-linearly increasing the source C source chroma towards the chroma C sourcemax with the non-linear chroma reinforcement factor γ as follows ( ) Csource α C γ destmax C sourceexp = α C destmax + (C sourcemax α C destmax) (4.5) C sourcemax α C destmax and then performing the gamut reduction C sourceexp to C dest according to Equation 4.3 by replacing C source with C sourceexp. This yields a reinforced chroma color C dest within the printable fluorescent gamut. By using the same chroma reinforcement factor γ in both the chroma expandable and the non-expandable parts of the input gamut, we ensure the continuity of the mapped colors. The two user parameters δ and γ respectively limit the maximal gamut expansion and provide a non-linear increase of the chroma. In order to suppress strong chroma differences at the boundaries between selected and nonselected image parts, we create smooth chroma transitions at the proximity of the boundaries of the selected image parts. For this purpose, we establish a spatial interpolation map with values varying between 1 and. The final colors are obtained by interpolation between the gamut mapped colors C destexp with user-defined γ and δ parameters and the non-expanded destination colors C destnonexp located outside the selected image parts. C dest = C destexp (x, y) +C destnonexp (1 (x, y) ) (4.6) where the values are given by the spatially laid out interpolation map. With = 1, C dest represents the used-defined gamut expanded colors and with =, C represents the nonexpanded colors located outside the selected image parts. The spatial interpolation map is dest created with the distance transform algorithm (Rosenfeld and Pfaltz 1968). Pixels outside the user selected image part are set to black and inside the selection to white. We then apply the distance transform to obtain for each white pixel its distance to the nearest black pixel. This 42

61 4.5. Display preview Figure 4.6: Spatial interpolation map for an arbitrary selection (red line) generated with a distance limitation factor κ = 2.5, where white represents 1 and black represents. distance map is normalized by dividing its values by its maximal value. In order to limit the distance from the boundary where the interpolation is performed, we multiply the map with a distance limitation factor κ (1 < κ). Values of the map greater than one are set to 1. Figure 4.6 shows the generated spatially laid out interpolation map for an arbitrary selection (red line) when using a distance limitation factor κ = 2.5. By spatially interpolating colors between non-expanded colors C destnonexp and expanded colors C destexp, we create smooth chroma transitions along the boundaries of the selected image parts. 4.5 Display preview We developed a tool for designers enabling selecting image parts, applying to these selections the user-defined gamut expansion parameters described in the previous section and previewing the printable destination gamut expanded image. In order to display accurate colors we have to characterize the display device. Display characterization is performed by computing its gamma exponent γ lum correction and by computing the 9 coefficient matrix used to convert CIE-XYZ tri-stimulus values to linear RGB phosphor values (Brainard et al. 22). In order to display the chroma expanded image parts, we need to show on a srgb display colors located beyond the original display gamut. With the goal of both preserving the overall appearance of the destination image and observing the differences between the expanded and non expanded image colors, we simulate on a standard srgb display a display having lower tri-stimulus phosphor values. This simulated lower luminance display renders the non-expanded colors as well as the expanded colors. We characterized both the Dell U2212 HM and the Eizo ColorGraphic CG245W displays. Their gamma exponent γ lum correction has been obtained by displaying 11 gray patches, i.e. by using for the three Red, Green and Blue channels 11 evenly distributed values between and 1. Here we assume that srgb Red, Green and Blue values vary between and 1. The emitted irrandiances of these 11 gray patches have been measured with a Maya Pro 2 43

62 Chapter 4. Gamut mapping expansion and reduction for color reproduction with daylight fluorescent inks Output luminance Y 1 5 γ lum = Input luminance Y (a) Output luminance Y 1 5 γ lum = Input luminance Y (b) Figure 4.7: Luminance gamma curves γ lum for (a) the Dell U2212 HM display and (b) for the Eizo ColorGraphic CG245W display. Solid lines show the gamma curves γ lum that approximate the measured CIE-XYZ luminance Y channels (black circles). spectrophotometer and then converted to CIE-XYZ tri-stimulus values. We then fitted the gamma curve by minimizing the differences between the measured CIE-XYZ luminance Y channels with the predicted CIE-XYZ luminance Y channels. Since the gamma curves for the three Red, Green and Blue phosphors are identical for these two displays, we fit only one gamma curve. We obtained for the Dell U2212 HM display a luminance gamma curve with γ lum = 1.91 and for the Eizo ColorGraphic CG245W a luminance gamma curve with γ lum = Note that the Eizo ColorGraphic CG245W display is a quality monitor calibrated from the factory. It therefore shows an ideal gamma curve γ lum close to 2.2. Figure 4.7 shows the γ lum curves (black lines) that approximates the measured CIE-XYZ Y luminance channel (black circles) for (a) the Dell U2212 HM display and for (b) the Eizo ColorGraphic CG245W display. For computing the 9 coefficients a 1 to a 9 of the matrix converting CIE-XYZ tri-stimulus values to RGB linear values [R l G l B l ], we measured the emitted irrandiances of the displayed maximal Red, Green and Blue srgb phosophor values, i.e the srgb [R s G s B s ] respective component values [1 ], [ 1 ], [ 1]. The nine a 1 to a 9 unknowns are found by solving the following equation system for these three maximal Red, Green and Blue srgb component values, i.e. each srgb value yields 3 equations. [R l G l B l ] = [R s G s B s ] γ lum [R l G l B l ] = [X Y Z ] M a 1 a 3 where M =..... a 7 a 9 (4.7) 44

63 4.5. Display preview In order to test the characterization of these two displays, we displayed 125 uniformly distributed patches by varying the srgb Red R s, Green G s and Blue B s phosphor values by step of 25% (5 3 = 125). The CIE-XYZ tri-stimulus values of these displayed patches are either measured or predicted according to the following equation derived from Equation 4.7 [X Y Z ] = [R s G s B s ] γ lum ( M) 1 (4.8) We finally converted both the predicted and measured CIE-XYZ tri-stimulus values to CIELAB colors, with the display white acting as reference white. The average E 94 prediction error, the quantile 95% prediction error and the maximal prediction error for both the two considered displays are listed in Table 4.2. We obtain excellent prediction accuracies with a mean E 94 prediction error of 1.16 for the Dell display and of.66 for the Eizo display. The Eizo display is remarkable stable yielding a quantile 95% prediction error of E 94 = Table 4.2: Prediction accuracies for both the Dell U2212 HM and the Eizo ColorGraphic CG245W display characterizations. Display E 94 Avg 95% Max Dell U2212 HM Eizo ColorGraphic CG245W Once the display is characterized, printable gamut mapped CIELAB values that are to be previewed are transformed into CIE-XYZ tri-stimulus values X Y Z printable, using the measured CIE-XYZ tri-stimulus display white as reference. We then apply to the X Y Z printable values a multiplicative tri-stimulus reduction factor ɛ ( ɛ 1) and obtain the simulated X Y Z sim values. The simulated X Y Z sim values are then transformed to display srgb values according to Equation 4.8 with the display characterized by its gamma value γ lum and by the nine coefficient matrix. The system (or the user) can modify the tri-stimulus reduction factor ɛ until no displayed values saturates the srgb display Red, Green and Blue channels. By assuming that the eye adapts on the simulated ɛ X n Y n Z n white reference, it becomes possible to visualize the selected gamut expanded image parts. Since the maximal lightness, respectively maximal chroma obtainable with the 6 ink print setup (offset or inkjet printers) is L = 18, respectively C = 123, values of the tri-stimulus reduction factor ɛ are always above.7. 45

64 Chapter 4. Gamut mapping expansion and reduction for color reproduction with daylight fluorescent inks 4.6 Halftoning and printing For printing an input srgb image mapped according to user-defined gamut mapping parameters, we generate the 6 ink separation layers containing the ink surface coverages of the classical cmyk inks and the additional daylight fluorescent magenta and yellow inks. These separation layers are obtained by establishing a relationship between ink surface coverages and gamut mapped CIELAB values. In a first step, we create a uniform CIELAB grid within the destination fluorescent gamut and fit the surface coverages. Then, for each input srgb gamut mapped color that is to be printed, we apply within the CIELAB grid a 3 dimensional lookup table base interpolation (Kang 26) between computed surface coverages. The uniform 3D CIELAB grid is established by varying CIELAB values L, a, b by small steps s p, e.g. s p = 2, from the minimal to the maximal CIELAB values of the destination fluorescent gamut G f. Some points of the CIELAB grid may not be present within the G f gamut. A point C grid of the uniform CIELAB grid can be removed by creating a line passing through it, a single point C b/w in the black and white axis and intersecting this line with the destination gamut boundary at C destmax. If the distance from C b/w to C grid is strictly larger than the distance from C b/w to C destmax, this point does not belong to the G f gamut and can therefore be removed from the uniform CIELAB grid. At the boundary of the destination gamut, in order to prevent removing one of the 8 necessary CIELAB cube vertices used to perform the 3D interpolation between mapped CIELAB values and corresponding ink surface coverages, we reduce the step size s p to 1 and we keep an additional point beyond the destination gamut. For each CIELAB values of the so established CIELAB grid, we compute the corresponding ink surface coverages by performing a gradient descent on the IS-CYNSN model, i.e. by minimizing the E 94 differences between predicted spectral reflectances converted to CIELAB and CIELAB grid values. The minimization is done for each ink set. We store in 4 lookup tables the mapping between CIELAB grid values and cmyk, cm f y f, cm f y and cmy f fitted surface coverages as well as corresponding E 94 prediction differences. The minimizations are carried out with the fmincon Matlab operator. In addition, in order to ensure the grey component replacement (GCR), we build the cmyk lookup table by constraining the cmyk separation on the lightness, as is usual in most cmyk printing systems. Finally, the ink separation layers are generated by performing a tri-linear interpolation between mapped input image CIELAB points and fitted surface coverages of the 8 surrounding vertices of the CIELAB grid. We also perform a tri-linear interpolation in order to obtain interpolated prediction differences. We then test if the mapped color can be reproduced with one of the ink set. This is the case when one of the interpolated prediction differences shows a negligible E 94 difference. In order to minimize the amount of fluorescent ink, we test the ink sets in the order cmyk, cm f y, cmy f and cm f y f. Since we have performed the gamut mapping for the union G f of the gamuts of the ink sets, we ensure that at least one ink set is able to reproduce the given color. 46

65 4.7. Summary of application user-defined parameters 4.7 Summary of application user-defined parameters In order to print gamut expanded images, we developed a prototype software. This software maps input srgb images according to the many different user-defined parameters presented in this chapter. In this section, we summarize and discuss these parameters in the list that follows: - The δ chroma expansion limitation factor limits the maximal effective chroma expansion achievable by the fluorescent gamut, where δ 1. With δ = 1, the limitation is maximal and no color beyond srgb gamut colors is printed. In order to take advantage of the large fluorescent gamut with the goal of having a better reproduction of input image srgb colors, we set this parameter to one. With δ =, we use the maximal expansion achievable with the fluorescent gamut G f. - The γ non-linear chroma reinforcement factor increases non-linearly chroma and brightness towards high chroma and bright fluorescent colors ( < γ 1). With γ = 1, we do not reinforce colors. We decrease its values depending on how much we want to reinforce colors of image regions of special interest. - The l h high lightness focal point controls the positive lightness shift when expanding source colors. By decreasing its value, we map input srgb color towards higher lightnesses. This parameter depends on the destination gamut boundary shape. For the considered inkjet fluorescent gamut we set l h = 76 and for the considered offset fluorescent gamut we set l h = The l l low lightness focal point controls the negative lightness shift when reducing source colors. This parameter is used to avoid reducing too much chroma in dark tones. This parameter depends on the destination gamut boundary shape. For the considered inkjet fluorescent gamut we set l l = 4 and for the offset fluorescent gamut we set l l = 3. - The ɛ display tri-stimulus reduction factor simulates a display with lower tri-stimulus phosphor values. The user or the application (in automatic mode) set this parameters until no mapped color saturates the srgb display Red, Green and Blue channels. This parameter enables visualizing the difference between the expanded and non-expanded image parts of the previewed destination gamut mapped image. - The κ distance limitation factor creates smooth chroma transitions along the selected region boundary. - The P 1, P 2 lightness adaptation function parameters control the lightness from the input srgb space to the selected printable output gamut. Since linear lightness mapping better preserves lightness differences present in the original input image, we use it for the results presented in the next chapter. 47

66 Chapter 4. Gamut mapping expansion and reduction for color reproduction with daylight fluorescent inks 4.8 Summary We propose a framework for expanding the colors of srgb images towards printable high chroma and bright colors located beyond display srgb gamut colors. High chroma and bright colors are obtained by performing a gamut expansion of the original image gamut onto the gamut covered by the combined cmyk and the magenta and yellow fluorescent inks. Fluorescent ink halftones add a new dimension to color prints. They enable either better reproduction of srgb colors or enable highlighting image parts to attract the attention of the observer. Applications include the design of posters and images for advertisement, as well as improved reproductions such as watch images or art paintings. The proposed color reproduction framework enables users to choose (a) the image regions to be enhanced, (b) how far the chroma should be expanded and (c) the possible non-linearity of the chroma expansion. User can display a preview of the gamut expanded image print. This is useful for designers working in fields such as photography, advertisement and production of catalogues and magazines as well as for artists who want to create new design effects. 48

67 5 Gamut expanded images 5.1 Introduction In this chapter, we show gamut expanded images generated with the developed prototype software. Gamut expansion of these images is controlled according to the gamut expansion parameters presented in the previous chapter. In a first step, we validates our preview software by showing a preview of a gamut expanded image together with its corresponding print (Section 5.2). Then, we show printed gamut expanded images that illustrate the new possibilities offered by the fluorescent gamut. These images comprise designs for advertisement (Section 5.3), images of watches and master painting whose colors are better reproduced with the fluorescent gamut than by using a classical cmyk gamut (Section 5.4) and images of artistic designs (Section 5.5). The previewed images where displayed on an calibrated Eizo ColorGraphic CG245W monitor and corresponding prints are printed either on an Canon MP-11 paper with an inkjet Epson P5 printer with original cmyk inks and the inkjet daylight fluorescent magenta and yellow inks or on a HEAVEN 42 SOFTMATT coated paper with an offset 6 ink Heidelberg printer with classical cmyk inks and the Pentone offset daylight fluorescent magenta and yellow inks. All figures contain photographs of the prints as well as the previewed prints taken by a Canon PowerShot S95 camera under normal daylight conditions. 5.2 Preview and corresponding print of a gamut expanded image In this section, we show an example of gamut expanded images previewed on a display and printed. Figure 5.1a shows a photograph of the print preview of a lizard where the selection comprising the animal and its boundaries has been gamut expanded with a chroma non-linear reinforcement factor γ =.3 without smooth chroma transition between the gamut expanded and non-gamut expanded part. Figure 5.1b shows the same display preview with smooth chroma 49

68 Chapter 5. Gamut expanded images transitions along the boundary formed by the gamut expanded and non-gamut expanded image part. The corresponding spatial interpolation map using a distance limitation factor κ = 2.5 is shown in Figure 4.6. In addition, in order to render out of srgb printable gamut colors, a tri-stimulus display reduction factor of ɛ =.88 has been applied. Figure 5.1c shows a photograph of the inkjet printed image previewed in Figure 5.1b. Figure 5.1d shows a photograph of the same lizard image printed without chroma expansion with classical cmyk inkjet inks only. (a) (b) (c) (d) Figure 5.1: Photographs of (a) the display preview of a lizard image where a selection comprising the lizard as well as its boundary pixels has been gamut expanded and (b) the same display preview where smooth chroma transitions have been created between the inner and outer parts of the selection, (c) the image with smooth chroma transitions printed with fluorescent and classical inks and (d) the classical cmyk print of the lizard image. Please observe the images on the electronic version of the thesis. In Figure 5.1a the rectangle regions show along the selection boundaries strong color artefacts due to the high differences in chroma and in brightness between the gamut expanded and non-gamut expanded parts of that image. For instance, below the mouth of the animal, some bright colors have been strongly gamut expanded while neighboring darker colors have not 5

69 5.3. Advertising images being gamut expanded. This yields in this rectangular area strong color differences between neighboring pixels. By creating smooth chroma transitions with a spatial interpolation map, these color artefacts disappear as it is apparent within the rectangular regions of Figure 5.1b. By comparing the printed gamut expanded image (Figure 5.1c) with its corresponding display preview (Figure 5.1b), we observe that the non-expanded part of the image have similar colors both in the preview and in the print. However, the gamut expanded part within the head of the lizard have different colors, i.e. in the photograph, the print colors appear brighter and less saturated. This mainly due to the fact that colors of the print within the selected image part are beyond srgb colors and can therefore not be rendered with a srgb image captured by a digital camera. Finally, by comparing the printed gamut expanded image with a classical cmyk print of that image (Figure 5.1d), we observe that within the selected image part (lizard) colors are brighter and have a higher chroma. In Figure 5.1c, since the G f gamut covers a larger part of the G srgb gamut compared with G cmyk gamut coverage, the image parts outside the selection where no chroma expansion has been applied are also better reproduced. For example, in the printed fluorescent ink image, the stone floor colors match the original srgb image while the stone colors in the cmyk printed image (Figure 5.1d) do not match the original srgb stone colors. 5.3 Advertising images In this section, we show examples of printed gamut expanded images that could be used for advertisement. These images have been produced by selecting image parts and by increasing chroma and brightness of their colors. The goal is to either reinforce the observer attention to a specific region of the image, for example to attract its attention on the product that is to be sold or to highlight some image parts. Figure 5.2 shows photographs of the Rolex Yachtmaster watch advertising image produced with the considered inkjet printer (a) non-gamut expanded and printed with classical cmyk inks and (b) the same image with the watch being gamut expanded with a non-linear chroma reinforcement factor of γ =.3. By expanding the chroma of the watch colors, the attention of the observer will be directed towards the watch. By comparing the man in both prints, we observe that even without being gamut expanded, the face and hair are better rendered and have a higher contrast when reproduced with the G f gamut (Figure 5.2b). They better preserve the original Yachtmaster advertising image. For the watch shown in Figure 5.2b, Figure 5.3 represents the y, y f, m and m f ink layers in grayscale. The darkness at each location represents the ink surface coverage. This yields the brighter and higher chroma colors of the watch shown in Figure 5.2b. Figures 5.4 shows photographs of a printed Mercedes AMG car both gamut expanded with the offset fluorescent gamut and non-gamut expanded with the classical cmyk offset gamut. In order to compare printed gamut expanded images with a standard industrial print of that image, the color separation layers of the classical cmyk print of that image have been generated 51

70 Chapter 5. Gamut expanded images (a) (b) Figure 5.2: Photographs of a printed (a) non-gamut expanded Rolex Yachtmaster image printed according to the classical G cmyk gamut and (b) of the same image printed according to the G f gamut, where the selected watch is gamut expanded with a non-linear chroma reinforcement factor γ =.3. Please observe these images on the electronic version of the thesis. (a) (b) (c) (d) Figure 5.3: The yellow (a), daylight fluorescent yellow (b), magenta (c) and daylight fluorescent magenta (d) ink layers of the gamut expanded Rolex Yachtmaster watch image. Layers are shown in grayscale with the darkness at each location representing the ink surface coverage. with the standard ECI ISO Coated V2 profile. In Figures 5.4b and 5.4c, we linearly and nonlinearly expanded colors of the car by using respective chroma reinforcement factors γ = 1 and γ =.5. By comparing the standard print of that image (Figure 5.4a) with its corresponding linearly expanded print (Figure 5.4b), we observe that colors of the car are more bright and 52

71 5.3. Advertising images saturated in the gamut expanded image. This reinforce light reflects in the front parts and in the doors of the car. These reflects are even more pronounced with the image produced with a non-linear chroma reinforcement factor (Figure 5.4c). We also observe that the background of the image is better reproduced with the G f gamut than with the G cmyk gamut. For instance, lights of the dome are significantly better reproduced by printing them with combinations of classical cmyk inks and daylight fluorescent inks (Figures 5.4 b and c) than by printing them with classical cmyk inks only (Figure 5.4a). (a) (b) (c) Figure 5.4: Photographs of a printed (a) non-gamut expanded AMG car image with the classical offset G cmyk gamut, produced with the standard ECI ISO Coated V2 profile and (b) of the same image printed with the offset G f gamut, where a selection comprising the car is linearly gamut expanded and (c) non-linearly gamut expanded with a non-linear chroma reinforcement factor γ =.5. Please observe these images on the electronic version of the thesis. The last example shows a lipstick advertisement. Lipstick are used to reinforce colors of the lips by reflecting strong saturated and bright colors. By using the fluorescent gamut instead of the classical cmyk gamut, it is possible to better reproduce these colors as well as to create the desired attractive effect on the lips. Figure 5.5 shows both gamut expanded and non-gamut expanded offset prints of a home made design of a lipstick advertisement. Figure 5.5a shows the classical cmyk print of that design using the standard ECI ISO Coated V2 profile and Figure 5.5b shows its corresponding gamut expanded print where a selection comprising the lips of the girl as well as the lipstick has been linearly gamut expanded. Since by superposing the m f 53

72 Chapter 5. Gamut expanded images (a) (b) Figure 5.5: Photographs of a printed (a) non-gamut expanded advertisement lipstick image with the classical G cmyk gamut, produced with the standard ECI ISO Coated V2 profile and (b) of the same image printed with the G f gamut, where a selection comprising the lips as well as the lipstick is linearly gamut expanded. Please observe these images on the electronic version of the thesis. ink with the y f ink we obtain a higher chroma and brighter red color than by superposing the m ink with y ink (see Chapter 1, Figure 1.3), red colors of the lips and the lipstick are reinforced in the gamut expanded print. In addition, since the G f gamut is larger than the G cmyk gamut, the face of the girl is better reproduced in the gamut expanded image. 5.4 Better reproduction of input srgb image colors In this section, we show that input srgb colors are significantly better reproduced by using the fluorescent G f gamut than by using a classical cmyk gamut. Better reproduction of input srgb image colors is obtained by limiting the maximal expansion achievable with the fluorescent G f gamut to the srgb gamut and by not reinforcing input chroma, i.e. by using a maximal effective expansion factor δ = 1 (no beyond srgb colors are printed) and a non-linear chroma reinforcement factor γ = 1 (no chroma is reinforced). Examples show offset printed images of watches and master paintings. As a comparison, we also show classical industrial cmyk prints whose color separation layers have been generated with the standard ECI ISO Coated V2 profile. The first example shows photographs of offset prints of a pink gold Hublot watch printed with the ECI ISO Coated V2 profile (Figure 5.6a) and the same image printed with the G f gamut (Figure 5.6b). Since the G f gamut is larger than the G cmyk gamut in both the bright magenta and red regions of the gamut (See Chapter 3), pink gold colors are better reproduced in the fluorescent print. Appropriate low ink surface coverages of the m f ink (Figure 5.6c) enable achieving improved printed pink gold colors. In addition, since the prototype software is calibrated for the specific printer, paper, inks and machine parameters, we are able to achieve a better black than by using a standard profile that is used for various papers and inks. With 54

73 5.4. Better reproduction of input srgb image colors 1%, 73%, 4% and 1% respective cmyk ink surface coverages for the fluorescent print, we obtain a black at CIELAB L = 15, a =.9 and b =.8 values while the black obtained with the ECI ISO Coated V2 profile gives CIELAB values at L = 2, b = 1 and a = 4 with 87%, 78%, 65% and 93% respective cmyk ink surface coverages. By using less printed inks in the fluorescent print (total surface coverage = 277%) than in the classical cmyk print (total surface coverage = 323%), we obtain a deeper black that is less colored. Compared to the classical cmyk print of the Hublot watch, the fluorescent print is of higher contrast and reflects more vivid colors. (a) (b) (c) Figure 5.6: Photographs of a printed (a) advertisement Hublot pink gold watch with the classical G cmyk gamut, with color separation layers produced with the standard ECI ISO Coated V2 profile and (b) of the same image printed with the G f gamut. In addition, image (c) shows the m f ink layer of the image (b) where the darkness at each location representing the ink surface coverage. Please observe these images on the electronic version of the thesis. Figure 5.7 shows photographs of offset prints of a Rolex watch (a) printed with the standard cmyk ECI ISO Coated V2 profile and (b) printed with the fluorescent gamut. By comparing these two images, we observe that yellow gold colors are better reproduced with the G f gamut. Gold colors of the Rolex watch printed with the G cmyk gamut are reproduced with the yellow ink. Since the offset yellow ink does not reflected enough light in the red visible wavelengths, the reproduced gold color are greenish. By adding a little amount of m f ink (Figure 5.7c) to the yellow ink, we increase the reflected light in the red wavelengths, thereby obtaining a color closer to the yellow gold color. With a deeper black, improved red rubis and gold colors, the fluorescent print of the Rolex watch has a higher contrast and is more colorful. Note that these two watch images have been directly reproduced from a watch photograph without pre-press retouching work. Images printed on catalogues are modified until customer requirements are achieved. The images shown here can therefore not be compared with the images printed in official Rolex and Hublot catalogues. Master painting are known to have colors which are beyond classical cmyk printer gamut 55

74 Chapter 5. Gamut expanded images boundaries. Impressionist painters, such as for instance Claude Monet, Joseph Mallord William Turner and Paul Gaugin have produced master paintings with high chroma yellow, red, magenta and orange colors. These paintings can therefore be better reproduced with a fluorescent gamut having larger boundaries in these color regions than with a classical cmyk gamut. (a) (b) (c) (d) Figure 5.7: Photographs of a printed (a) advertisement Rolex yellow gold watch printed with the classical G cmyk gamut, with color separation layers produced with the standard ECI ISO Coated V2 profile and (b) of the same image printed with the G f gamut. In addition, image (c) and (d) shows the respective m f and y f ink layers of the image (b) with the darkness at each location representing the ink surface coverage. Please observe these images on the electronic version of the thesis. Figure 5.8 shows photographs of the master painting "San Giorgio Maggiore" by Claude Monet printed according to (a) the offset classical G cmyk and (b) the offset fluorescent G f gamut. By comparing these two prints, we observe that strongly saturated red, orange, yellow and blue colors of this master painting are significantly better reproduced in the fluorescent print. J. M. W. Turner is an english artist who painted sunset and sunrise landscapes. In "The Fighting Temeraire", the half right part of this master painting shows a chip in a esturary at sun set. Sun light goes through the clouds and the flaming red of the clouds is reflected by the river. These flaming colors have been better reproduced with the offset fluoresent gamut G f (Figure 5.9b) than with the classical cmyk gamut (Figure 5.9a). In addition, since the G f gamut is larger than the classical cmyk gamut, we observe more details in the fluorescent print of this master painting. For instance, we observe more details in the water as well as in the painted blue regions. 56

75 5.4. Better reproduction of input srgb image colors (a) (b) Figure 5.8: Photographs of the master painting "San Giorgio Maggiore" by Claude Monet (a) printed with the classical offset G cmyk gamut, according to the standard ECI ISO Coated V2 profile and (b) of the same image printed with the offset G f gamut. Please observe these images on the electronic version of the thesis. (a) (b) Figure 5.9: Photographs of a part of the master painting "The Fighting Temeraire" by J. M. W. Turner (a) printed with the classical offset G cmyk gamut, according to the standard ECI ISO Coated V2 profile and (b) of the same image printed with the offset G f gamut. Please observe these images on the electronic version of the thesis. The last example of this section shows offset prints of the master painting "Women of Andavadoaka" by Paul Gaugin. By comparing the classical cmyk print of that master painting (Figure 5.1a) with its corresponding fluorescent print (Figure 5.1b), we observe that the fluorescent gamut better renders brown skin colors as well as yellow and orange background colors. We also observe that green colors of the trees and the clothes of one of the painted girl are better rendered in the fluorescent print. 57

76 Chapter 5. Gamut expanded images (a) (b) Figure 5.1: Photographs of the master painting "Women of Andavadoaka" by Paul Gauguin (a) printed with the classical offset G cmyk gamut, according to the standard ECI ISO Coated V2 profile and (b) of the same image printed with the offset G f gamut. Please observe these images on the electronic version of the thesis. 5.5 Artistic images In this section, we show examples of artistic prints obtained by increasing chroma and brightness of input srgb image colors. The resulting printed images have vivid colors beyond srgb gamut colors. Figure 5.11 shows a flower image (a) printed with the offset G cmyk gamut, (b) printed with the offset G f gamut and non-gamut expanded with a chroma reinforcement factor γ = 1 and a chroma expansion limitation factor δ = 1 (no beyond srgb colors are printed), and (c) printed with the offset G f gamut and non-linearly gamut expanded with a chroma reinforcement factor γ =.8. By comparing the non-fluorescent print with the non-gamut expanded fluorescent print, we observe that colors of the input srgb image are better reproduced in the fluorescent print. By applying to the input image a non-linear chroma reinforcement factor (Figure 5.11c), we rapidly increase the chroma of input srgb colors towards strongly bright and high chroma daylight fluorescent colors. The next example shows printed reproductions of an image specially designed for offset fluorescent prints. A graphical designer used our prototype proofing software to design a magenta flaming girl image with the goal of obtaining interesting printed fluorescent colors. Figure 6.7a shows the classical cmyk offset print of that image and Figure 6.7b shows the linearly gamut expanded fluorescent print of that image. By using combinations of classical inks with daylight fluorescent inks, we are able to create a strong flaming effect around the girl. Flames in the fluorescent print appear to be strongly yellow and of high chroma magenta colors. In the classical cmyk print, flames appear to be bright and of a lower chroma. In the fluorescent print, both the background black color is deeper and colors have a higher chroma. 58

77 5.5. Artistic images (a) (b) (c) Figure 5.11: Photographs a of flower image (a) printed with the classical offset G cmyk gamut, according to the standard ECI ISO Coated V2 profile, (b) of the same image non-gamut expanded and printed with the offset G f gamut, and (c) non-linearly gamut expanded and printed with the fluorescent offset G f gamut. Please observe these images on the electronic version of the thesis. These intense colors cannot be obtained with classical cmyk inks. Figure 5.13 shows of a fluorescent mushroom (a) printed with the classical inkjet G cmyk gamut and (b) linearly gamut expanded and printed with the inkjet fluorescent G f gamut. Superposing the inkjet cyan and daylight fluorescent yellow ink yields the high chroma and bright green colors, enabling creating the fluorescent effect of the mushroom. This is effect is not achievable with classical inkjet cyan and yellow inks, as shown in Figure 5.13a. (a) (b) Figure 5.12: Photographs of a designed flaming girl (a) printed with the classical offset G cmyk gamut, according to the standard ECI ISO Coated V2 profile and (b) of the same image linearly gamut expanded and printed with the fluorescent offset G f gamut. Please observe these images on the electronic version of the thesis. 59

78 Chapter 5. Gamut expanded images (a) (b) Figure 5.13: Photographs of a fluorescent mushroom image (a) printed with the classical inkjet G cmyk gamut and (b) of the same image linearly gamut expanded and printed with the fluorescent inkjet G f gamut. Please observe these images on the electronic version of the thesis. 5.6 Summary The printed gamut expanded images shown in this chapter illustrate the new possibilities offered by establishing a 6 ink print setup combining the classical cmyk inks complemented with the daylight fluorescent magenta and yellow inks. These fluorescent inks add a new dimension to classical cmyk prints. They enable printing image regions with high chroma and bright colors. This can be used for advertising by highlighting within the destination fluorescent image the product that is to be sold or by reinforcing the attraction of the observer on a specific product, e.g. the lips and the lipstick of the lipstick advertisement image. Since the fluorescent gamut is significantly larger than the classical cmyk gamut, it also enables better reproducing input srgb image colors. We showed that compared to classical cmyk prints, fluorescent prints significantly improved the printer reproduction capabilities for watches and also for master paintings which are known to have color outside the gamut of classical inks. The developed prototype software enables controlling the gamut expansion and displays a preview of the printable destination gamut expanded image. A graphic designer showed that is is possible to design images conceived to make use of the fluorescent gamut. He created both an advertising lipstick image and a flaming girl artistic image with interesting vivid colors that cannot be obtained with classical cmyk printers. 6

79 6 Hiding patterns with daylight fluorescent inks 6.1 Introduction In this chapter, we propose a method for hiding security patterns within printed images by making use of classical and of the two daylight fluorescent magenta and yellow inks. Under the D65 illuminant, we establish in the CIELAB space the gamut of a classical cmyk printer and the gamut of the same printer using a combination of classical inks with daylight fluorescent inks. For the two categories of previously considered offset and inkjet printers, these gamuts show that a significant part of the classical ink gamut can be reproduced by combining classical inks with daylight fluorescent inks. Parts of images are either printed with classical inks (with the ink set cmyk) or printed with combinations of classical inks with one or two daylight fluorescent inks (ink sets cm f y f, cm f y, cmy f ). By applying a metameric color match under the D65 illuminant between the ink set comprising no daylight fluorescent ink and the ink sets comprising daylight fluorescent inks, we create images which look the same under normal daylight. By changing the illumination, for example by observing the image under a tungsten, a colored blue or a UV illumination, we reveal the security patterns formed by the parts of the image printed with daylight fluorescent inks (Section 6.2). By spatially interpolating between the parts of the image printed with a fluorescent ink set, e.g the cmy f ink set and the parts of the image printed with the nonfluorescent ink set cmyk, we are also able to hide a variable intensity (grayscale) image within a printed full color image (Section 6.3). In the last section of this chapter, we also propose a method for hiding security patterns at the same time under the most common illuminations, such as for instance a tungsten, a fluorescent tube F7, and a daylight illumination. These patterns are revealed under an illumination having energy only in the excitation wavelength range of the daylight fluorescent inks (Section 6.4). They can be typically revealed under a colored blue or a UV illumination. Security features relying on fluorescent inks are not new. For instance, single invisible fluorescent inks are widely used in passports, bank notes and credit cards (Van Renessse 25). The 61

80 Chapter 6. Hiding patterns with daylight fluorescent inks hidden patterns are generally printed with a single invisible fluorescent ink, such as the yellow "VISA" text appearing under a UV light source. Other security features relying on fluorescent inks are described in Chapter 1 under the section reviewing the prior art (Section 1.6). Daylight fluorescent inks offer a better protection compared with invisible fluorescent inks. In order to print colors of the original image there is a need of establishing a gamut mapping from the original image color space, e.g. the srgb display gamut to the ink destination gamut. In addition, there is also a need of finding an exact relationship between gamut mapped original image colors and corresponding fluorescent and non-fluorescent ink dot surface coverages. This can be achieved only with a spectral prediction model dedicated for predicting spectral reflectances of halftones comprising daylight fluorescent inks. Finally, hidden patterns printed with invisible fluorescent inks are revealed only under a UV illumination. Hidden patterns printed with daylight fluorescent inks are not only revealed under a UV illumination but also under a visible illumination different from the reference illumination such as a filtered blue illumination. Finally, this method for hiding security patterns is fully compatible with the gamut expanded image framework presented in Chapter 4. Once a print company has integrated this framework with its printing workflow, this company can propose to its customers this security feature without additional effort. Compared to security features relying on invisible fluorescent inks, this security feature does not need any invisible security UV inks whose diffusion is restricted to companies active in the field of security. The protection of the proposed security feature with daylight fluorescent inks is embedded into the way of how these fluorescent inks are printed and not into the fluorescent emission characteristic of the fluorescent inks. 6.2 Hiding security patterns by printing colors either with or without daylight fluorescent inks In Chapter 1 Section 1.3 we have shown that is possible to create interesting colorants by superposing classical inks with daylight fluorescent inks or by superposing several daylight fluorescent inks. With these fluorescent colorants, we establish a strictly fluorescent gamut G s f that comprises all colorants combining classical inks with a least one daylight fluorescent inks. More precisely, the strictly fluorescent gamut G s f is the conjunction of the three cm f y f, cm f y, cmy f ink set fluorescent sub-gamuts, i.e. one color of the strictly fluorescent gamut G s f is associated with at least one of these sub-gamuts and is printed with its corresponding 3 inks. By comparing the G s f gamut with the classical ink G cmyk gamut under the D65 illuminant, we determine the colors of the G cmyk gamut which are metameric to the colors of the G s f gamut under normal daylight conditions. We print the hidden patterns whose colors are located within the G s f gamut with a fluorescent ink set so as to have an exact metameric match with the gamut mapped original color printed with the classical ink G cmyk gamut viewed under the D65 illuminant. These patterns will therefore be hidden under normal daylight. Figure 6.1 illustrates in the CIELAB space a comparison between the inkjet G cmyk and G s f 62

81 6.2. Hiding security patterns by printing colors either with or without daylight fluorescent inks 8 L * = 45 8 L * = 5 8 L * = 55 4 inkjet G cmyk 4 G cmyk 4 G cmyk b * b * G sf b * G sf G sf a * 5 L * = a * 5 L * = a * 5 L * = 7 4 G sf 4 G sf 4 G sf b * G cmyk b * G cmyk b * G cmyk a * 5 L * = a * 5 L * = a * 5 L * = b * G sf G cmyk b * G sf G cmyk b * G sf G cmyk a * 5-5 a * 5-5 a * 5 Figure 6.1: Color gamut G cmyk of the classical cmyk inkjet ink set (solid lines) and the strictly fluorescent inkjet gamut G sf (dotted lines). gamut boundaries under the D65 illuminant. For lightnesses less than L = 55, we observe that the G sf gamut is smaller than the G cmyk gamut. This can be explained by the fact that the daylight fluorescent colorants are brighter than the corresponding classical colorants (see Chapter 1, Figure 1.2). At a lightness between L = 55 and L = 65, there are not many differences between the classical ink gamut (G cmyk ) and the restrictive fluorescent gamut (G sf ), i.e. only a small part of the G cmyk gamut is outside of the G sf gamut. For lightnesses higher than L = 65, the G cmyk gamut is included within the G f gamut. We therefore observe that for bright CIELAB colors (having a lightness L > 55), we are able to reproduce most of the classical inkjet cmyk colors by combining classical and daylight fluorescent inks. Figure 6.2 illustrates in the CIELAB space a comparison between the offset G cmyk and G sf gamut boundaries under the D65 illuminant. For lightnesses less than L = 35, we observe that the G sf gamut is smaller than the G cmyk gamut. For lightnesses between L = 35 and L = 55, only a small region of the G cmyk gamut has larger boundaries than the G sf gamut. For lightnesses higher than L = 55, the G cmyk gamut is strictly included within the G sf gamut. In this situation, for mid-bright colors having a lightness higher than L = 35 and less than 63

82 Chapter 6. Hiding patterns with daylight fluorescent inks L * = 35 L * = 45 L * = 55 5 offset G cmyk 5 G cmyk 5 G cmyk b * b * b * -5 G sf G sf a * -5 5 a * L * = 6 G sf L * = 65 G sf a * G sf L * = 7 G sf b * b * b * G cmyk G cmyk G cmyk a * L * = a * L * = a * 5 L * = b * G cmyk b * G cmyk b * G cmyk a * G sf G sf G sf a * a * Figure 6.2: Color gamut G cmyk of the classical cmyk offset ink set (solid lines) and the strictly fluorescent offset gamut G sf (dotted lines). L = 55, we are able to reproduce most of the classical offset cmyk colors by combining offset classical and daylight fluorescent inks. For bright colors having a lightness higher than L = 55, we are able to reproduce all offset cmyk colors by combining classical offset cmyk inks with at least one daylight fluorescent ink. For hiding security patterns within an image, we define a mask. The mask can represent any patterns such as for instance the security "VALID" text shown in Figure 6.3. While generating a specific image, we print outside the mask the colors of the image with the G cmyk gamut, i.e. with classical inks only. Inside the mask, if colors of the G cmyk gamut are reproducible by colors of the G sf gamut, we print them with fluorescent colorants, i.e. combinations of classical and daylight fluorescent inks. In the contrary case, we use classical inks only. The next challenge consists in establishing an exact relationship between the CIELAB colors and the ink surface coverages of the inks defining either the G cmyk gamut or the G sf gamut. This relationship must be exact in order to print perfectly metameric colors. This exact relationship is established thanks to the IS-CYNSN spectral prediction model that is remarkably 64

83 6.2. Hiding security patterns by printing colors either with or without daylight fluorescent inks VALID Gsf or Gcmyk G cmyk Figure 6.3: Example of an image design incorporating the hidden "VALID" security text. Outside the "VALID" mask, colors are printed with classical cmyk inks only (with G cmyk ). Inside the "VALID" mask, colors are printed either with classical cmyk inks only (with G cmyk ) or with combinations of classical cmyk and daylight fluorescent inks (with G s f ). accurate for predicting total spectral reflectances of halftones comprising classical inks only or combining classical and daylight fluorescent inks (See Chapter 2, Sections 2.5 and 2.6). In order to obtain a relationship between the ink surface coverages and the srgb values of the image that is to be reproduced, we map all the srgb CIELAB values by steps of 3% R, G and B into the G cmyk gamut. This is achieved by the standard multiple foci gamut mapping approach as it is explained in Chapter 4. We obtain the ink surface coverages corresponding to the color mapped into the printer non-fluorescent G cmyk gamut with the IS-CYNSN model by minimizing the E 94 differences between the predicted color and the desired color. The minimization is carried out for each ink set. We store the fitted ink surface coverages plus the corresponding E 94 differences between desired and predicted colors. This yields four lookup tables mapping srgb values to cmyk, cm f y f, cm f y and cmy f ink surface coverages with corresponding E 94 differences. The minimizations are carried out with the Matlab fmincon operator. Note that the IS-CYNSN models are calibrated for the D65 illuminant by measuring the calibration patches with that illuminant. For generating an image incorporating a hidden pattern we test if the mapped srgb colors within the mask can be reproduced by one of the fluorescent ink sets. This is the case when the corresponding entry in one of the cm f y f, cm f y and cmy f lookup table shows a negligible E 94 difference between desired gamut mapped color and the color predicted with the fitted ink surface coverages. In order to maximize the amount of fluorescent ink, we test the ink set in the order cm f y f, cm f y, cmy f. If no fluorescent ink set provides the desired color, it is printed with the classical cmyk ink set. Gamut mapped colors outside the mask are printed with the classical cmyk ink set Illustrations of hidden security patterns The printed images shown in this section embed the repetitive hidden "VALID" text pattern. Lookup tables mapping the srgb values to the ink surface coverages have been generated for the D65 illuminant. Thus, these patterns are hidden under a normal daylight viewing 65

84 Chapter 6. Hiding patterns with daylight fluorescent inks condition but revealed under both the A or the UV illuminations. Images were printed with the EPSON P5 printer with native EPSON cmyk inks and with the considered inkjet daylight fluorescent magenta and yellow inks. Pictures of the prints have been taken with a Canon PowerShot S95 camera under normal daylight conditions, under UV-A black light and under a tungsten lamp (A illuminant). Figure 6.4 illustrates a printed Japanese girl image embedding the repetitive "VALID" hidden pattern, photographed both under normal daylight (left image) and under UV light (right image). Under normal daylight conditions it not possible to distinguish the text "VALID" formed by combinations of classical and daylight fluorescent inks. This is due to the fact that we have a perfect metameric match between the inner and outer part of the "VALID" mask. Under UV illumination, the text "VALID" is visible in almost all parts of the image, except in the hair. Since the hair is dark, it is not possible to reproduce it with daylight fluorescent colorants. Figure 6.5 illustrates a printed Iceland landscape embedding the repetitive "VALID" hidden pattern. While under normal daylight it is not possible to distinguish the hidden pattern, under both A and UV illuminations, it is revealed. Since the A illuminant has less energy than the D65 illuminant in the excitation range of the daylight fluorescent inks, there is less fluorescent emission and therefore the "VALID" mask content appears darker than when seen under the D65 illuminant, see Figure 6.5c. (a) (b) Figure 6.4: Photographs of a printed Japanese girl image incorporating the repetitive "VALID" pattern, (a) viewed under normal daylight and (b) viewed under UV illumination. 6.3 Hiding a variable intensity security image In this section, we show how to embed a hidden variable intensity image within a printed image. The hidden variable intensity image comprises halftones enabling to spatially interpolate between a fluorescent ink set and the non-fluorescent classical cmyk ink set. When seen under 66

85 6.3. Hiding a variable intensity security image (a) (b) (c) Figure 6.5: Photographs of a printed Iceland landscape incorporating the repetitive "VALID" pattern viewed under (a) normal daylight, (b) under UV illumination and (c) under A illumination. Please observe the images in the electronic version of the thesis. a UV illumination, parts printed with a fluorescent ink emit a colored light whose intensity is proportional to the amount of printed fluorescent ink, i.e. proportional to the intensity level of the hidden image that is revealed. In order to hide a variable intensity image within a printed image, we first locate a spatial region of the gamut mapped destination image whose colors are located at the intersection of the strictly fluorescent gamut G s f and the classical cmyk gamut G cmyk. Intersected G cmyk and G s f gamut color regions are shown for the considered inkjet and offset gamuts respectively in Figures 6.1 and 6.2. Such a region can for instance be found in the gamut mapped destination image at locations where colors are bright, i.e. have a lightness L > 55. The variable intensity image is halftoned with a small diagonally oriented cluster-dot screen (Haines et al. 23a) or with stochastic dots generated with a blue noise dither matrix or by error-diffusion (Haines et al. 23b) thereby obtaining an image made of black and white pixels. By printing the white pixels with the fluorescent ink set and the black pixels with the non-fluorescent ink set, we hide the variable intensity image within the printed image. By observing the printed image incorporating the hidden variable intensity image under a UV illumination, parts printed with the fluorescent ink set emit a colored light depending of the fluorescent ink set used to represent the variable intensity image. For the cmy f ink set, the fluorescent emission yields greenish colors while the fluorescent ink set cm f y yields reddish colors. The intensity of the fluorescent emission is proportional to the surface coverage of printed fluorescent ink. Since these surface coverages have been obtained by halftoning, the intensity of the colored fluorescent emission is proportional to the intensity of the hidden variable intensity image. This reveals the hidden variable intensity image. Figure 6.7 shows an inkjet print of a girl embedding a hidden variable intensity tiger image. The tiger image is halftoned with blue noise dithering (Figure 6.6). Since the left upper part of the destination girl image has bright colors, i.e. a lightness L > 55, we choose this image region to place the hidden tiger. In the destination image we use the halftoned tiger as spatial 67

86 Chapter 6. Hiding patterns with daylight fluorescent inks G cmyk G cmyf Figure 6.6: Variable intensity tiger image halftoned with a blue noise dithering. This halftoned image is incorporated within another security image by printing its white pixels with the cmy f ink set and its black pixels with the cmyk ink set. mask. As is shown in an enlargement of the mask content (Figure 6.6, red rectangle), we print destination image colors with the ink set cmy f where the halftoned tiger has white pixels. The other pixels of the destination image are printed with the ink set cmyk. Since in the destination image region colors are bright, we are able to reproduce all the mapped colors either with classical inks only or with combinations of cyan, magenta and daylight fluorescent yellow inks. While under normal daylight conditions, it is not possible to distinguish the hidden tiger (Figure 6.7a), under a UV illumination the variable intensity tiger image is revealed by a colored greenish fluorescent emission of the parts printed with the fluorescent cmy f ink set (Figure 6.7b). (a) (b) Figure 6.7: Photographs of a printed grayscale girl image incorporating a variable intensity tiger image viewed under (a) normal daylight and (b) under UV illumination. Please observe the images in the electronic version of the thesis. 68

87 6.4. Hiding security patterns under multiple illuminations 6.4 Hiding security patterns under multiple illuminations In this section, we propose a method for hiding patterns under different illuminations and revealing them under UV excitation light or under blue light. We would like to hide the patterns when they are observed under daylight, in an office under a fluorescent tube illumination and in a room illuminated with tungsten lamps. These patterns should be revealed only under a narrow band illumination in the excitation wavelength range of the daylight fluorescent inks, e.g. a UV illumination or a colored blue illumination. We therefore need to optimize the respective amounts of fluorescent and non-fluorescent inks, so as to produce metamers under the A and D65 illuminants. The problem of creating near metamers, called paramers, has been tackled by Urban and Berns (211). They proposed a gamut mapping framework that creates mapped colors which remain substantially similar when observed under different illuminants. Our problem is different, since by reducing the relative amount of fluorescent ink, the resulting color comes closer to the color produced by the non-fluorescent cmyk inks, which is the reference color. In order to create with mixtures of fluorescent halftones and non-fluorescent halftones colors which are close to the reference cmyk colors under the different illuminants, we proceed as follows. In a first step, we create a mapping between desired colors and surface coverages of the fluorescent ink set by minimizing the error between gamut mapped image colors and predicted colors at the same time for the A and D65 illuminants. Since the color differences are large when colors are either viewed under the D65 or the A illuminant, we reduce the amount of printed fluorescent ink within the pattern mask by reducing the ratio of the fluorescent ink set in respect to the non-fluorescent ink set. This is performed by spatially distributing the fluorescent ink set by a dither function, which converts relative amounts to surface coverages. Practical experiments shows that patterns are hidden under various illuminations when no more than 2% of a daylight fluorescent ink set is used within the pattern mask. For hiding patterns under a wide range of illuminants, we first select the A and the D65 illuminants. These illuminants have respectively a very low (A) and a very high (D65) energy in the excitation wavelength range of the daylight fluorescent inks (Chapter 1, Section 1.3). Other natural illuminations, e.g. cloudy daylight as well as artificial light, e.g. F7 and F11 fluorescent tubes, provide within the excitation wavelength range of the daylight fluorescent inks energy that is higher than the energy provided by the A illuminant and lower than the energy provided by the D65 illuminant. Therefore, patterns hidden under these two illuminants will be hidden under various natural and artificial illuminations. For hiding patterns under both the D65 and the A illuminants, while establishing a relationship between destination image gamut mapped colors and corresponding ink surface coverages, we not only minimize the E 94 difference between gamut mapped color and predicted color for the D65 illuminant but also at the same time this difference for the A illuminant. We return as minimization metric the largest minimized E 94 difference obtained for the two considered illuminants. Assuming that the minimized difference obtained for the D65 illuminant is given 69

88 Chapter 6. Hiding patterns with daylight fluorescent inks by E D65 94 and the minimized difference obtained for the A illuminant is given by E A 94, we return as fitting metric the maximal minimized E 94 error, i.e. we return Max { E D65 94, E A 94}. In a similar way as presented in Section 6.2, we store for each ink set this maximal minimized E 94 error under the D65 and A illuminants plus the corresponding ink surface coverages. For generating an image incorporating the pattern hidden under different illuminations we test if the srgb colors mapped to the classical G cmyk gamut colors within the pattern mask can be reproduced by one of the fluorescent ink sets. However, for most gamut mapped srgb colors, E 94 differences are large when colors are viewed either under the A or the D65 illuminant. We therefore choose the ink set by looking at corresponding entry of the cm f y f, cm f y, cmy f lookup tables and select the ink set with the smallest joint D65 and A illuminant E 94 error. In a further step, in order to avoid seeing large color differences induced by the large tolerance on E 94 errors, we reduce the amount of daylight fluorescent inks within the mask. For example, in order to hide the "L" character with combinations of the cyan, magenta and daylight fluorescent yellow inks, we can create a "L" pattern mask at 2% gray level intensity. This pattern mask is halftoned with blue noise dithering (Figure 6.8, red rectangle). We then reduce the amount of printed colors with the cmy f ink set by spatially distributing this fluorecent ink set according to the halftoned mask pattern. By halftoning the pattern mask at the same resolution as the resolution of the input image, we avoid to print big clusters of neighbouring source image pixels with only the fluorescent ink set, i.e. we print isolated image pixels with the fluorescent ink set cmy f surrounded by many image pixels with the non-fluorescent ink set cmyk (see Figure 6.8, distributions of the cmyk and cmy f ink sets). L Gcmyk G cmyf Figure 6.8: Character L of the "VALID" mask message to be hidden under different illuminants at 2% gray level intensity together with an enlargement of a small region of its corresponding blue noise halftone (red rectangle). Figure 6.9 illustrates an offset printed Iceland landscape incorporating the repetitive "VALID" pattern hidden under different white light illuminations. "VALID" patterns are printed with the ink set cmyk or with the daylight fluorescent ink set cmy f with maximal surface coverage of the daylight fluorescent yellow ink set at 2%. In this configuration, within the "VALID" mask, we obtain for the D65 and A illuminants a mean point E 94 prediction error of

89 6.4. Hiding security patterns under multiple illuminations and a quantile 95% prediction error of Since the amount of daylight fluorescent ink is reduced to 2% surface coverage, it is not possible to distinguish the "VALID" mask content under normal daylight (Figure 6.9a), under a tungsten illumination (Figure 6.9b) and under a F7 fluorescent tube illumination (Figure 6.9c). By illuminating the offset printed Iceland landscape with a blue non UV low consumption lamp, the part printed with the daylight fluorescent yellow ink is excited by the blue illuminant and emits light that reveals the "VALID" content. The relative spectral power distribution of the F7 fluorescent tube illumination is shown in Chapter 1, Figure 1.1. (a) (b) (c) (d) Figure 6.9: Photographs of an offset printed Iceland landscape incorporating the repetitive "VALID" pattern viewed under (a) normal daylight, (b) under illumination A, (c) under the fluorescent tube illumination F7 and (d) under a blue low consumption Swiss light classic 55 non UV lamp. Please observe the images in the electronic version of the thesis. Figures 6.1a and b illustrate an inkjet printed Iceland landscape incorporating the repetitive "VALID" pattern viewed under normal daylight. Valid mask content is printed either with the inkjet cmyk ink set or with the cmy f ink set for maximal printed daylight fluorescent cmy f ink set at respectively 1% and at 3%. While it is not possible to distinguish the "VALID" mask content under normal daylight with maximal printed fluorescent cmy f ink set at 1% (Figure 6.1a), the "VALID" mask content starts to be slightly visible at 3%(Figure 6.1b, red rectangle). Figure 6.1c shows the same printed image as shown in Figure 6.1a but viewed under a UV illumination. Since only a small amount of 1% of the daylight fluorescent ink set is printed, it is difficult to reveal the "VALID" pattern with a UV illumination. With an amount of 2% of the printed fluorescent ink set, we obtain an excellent tradeoff between the capability to be hidden under different illuminations (Figure 6.9) and the capability to be revealed when seen under a UV illumination (Figure 6.1d). 71

90 Chapter 6. Hiding patterns with daylight fluorescent inks (a) (b) (c) (d) Figure 6.1: Photographs of an inkjet printed Iceland landscape incorporating the repetitive "VALID" pattern viewed under normal daylight with maximal daylight fluorescent cmy f ink surface coverage at (a) 1%, (b) 3%, (c) photograph of image (a) viewed under UV and (d) photograph of the image with maximal surface coverage of fluorescent cmy f ink set at 2% and viewed under UV illumination. Please observe these images in the electronic version of the thesis. 6.5 Summary We propose a method for hiding security patterns within images by making use of the two daylight fluorescent magenta and yellow inks. The patterns are printed with combinations of these two daylight fluorescent inks and classical inks while the rest of the image is printed with classical inks only. Since the ink surface coverages are calculated with a highly accurate spectral prediction model calibrated under the D65 illuminant, the embedded security patterns are completely hidden under normal daylight. By spatially interpolating between a fluorescent ink set and the classical cmyk non-fluorescent ink set, we are also able to hide a security variable intensity image within a printed image. The verification is performed by putting the security images under a tungsten lamp or under a UV black light and by visually verifying that the security patterns are revealed. With classical inks it is not possible to hide patterns that are revealed both under UV and A illuminations. Therefore, these security images are difficult to reproduce. We also propose a method for hiding the security patterns under different illuminations. By minimizing the E 94 error between gamut mapped colors and predicted colors under at the same time the A and D65 illuminants and by reducing the part printed with a daylight fluorescent ink set to a maximal surface coverage at 2%, we showed that the patterns are hidden under several classical illuminations. The verification is then performed by putting the security images under an illumination having energy only in the excitation wavelength range of the daylight fluorescent inks, e.g. a non UV blue lamp or a UV black lamp. 72

91 7 Conclusion In this thesis, we explored the possibilities offered by adding the daylight fluorescent magenta and yellow inks to classical cmyk prints. We propose a complete framework for printing with daylight fluorescent inks. The solution comprises a spectral prediction model (IS-CYNSN) that has been optimized to predict spectral reflectances of halftones comprising daylight fluorescent inks. With a few calibration patches, we achieve remarkable prediction accuracies for both inkjet and offset halftones printed with classical inks only or with combinations of classical inks and daylight fluorescent inks (Chapter 2). Thanks to the accurate spectral prediction of the IS-CYNSN spectral prediction model, we establish the fluorescent gamut G f comprising colors generated by combining classical inks with daylight fluorescent inks and the classical cmyk gamut combining classical cmyk inks only. By comparing these two gamuts for both an inkjet and offset printer, we show that we considerably expand the domain of printable colors by adding two fluorescent inks to classical cmyk inks. The domain extension of colors is mainly available for green, yellow, red, magenta and orange colors (Chapter 3). These fluorescent inks enable to print high chroma and bright colors that cannot be obtained with classical inks only. They add therefore a new dimension to color prints. In order to fully utilize these high chroma and bright fluorescent colors, we define an adapted gamut mapping from the srgb display gamut to the fluorescent gamut which allows by gamut expansion to print beyond srgb gamut colors. The gamut expansion experience can be user-driven with a prototype software. This software enables user (a) choosing image regions to be enhanced, (b) how far and how fast the chroma should be expanded, (c) creating smooth chroma transitions along the boundary between the gamut expanded and non-gamut expanded image part and (d) displaying a preview of the printable gamut expanded image (Chapter 4). This prototype software is useful for designers working in fields such as photography, advertisement, production of magazine as well as for artists. With this prototype software we designed and printed images such as a Rolex and a lipstick advertising image where colors of the products, respectively the watch, the lips and the lipstick are reinforced in order to attract 73

92 Chapter 7. Conclusion the attention of the observer. We also show that without performing color gamut expansion, the large fluorescent gamut enables better reproducing colors of input srgb images. We compared offset classical cmyk prints with offset fluorescent prints of watches and master painting images. The reproduction was significantly improved thanks to the fluorescent inks. In addition, a graphic designer used our prototype software in order to design and print images with interesting vivid fluorescent colors (Chapter 5). In Chapter 6, we show that with these daylight fluorescent inks it is possible to provide strong optical document security features. Hidden image patterns are printed with daylight fluorescent inks so as to obtain a perfect metameric match with the colors produced by cmyk inks only. Parts of image printed with daylight fluorescent inks define a security pattern or a security variable intensity image that can be revealed under an illuminant having an energy different from the reference illuminant in the excitation wavelengths of the fluorescent inks. The process of hiding and revealing the security patterns can be performed with a "hidding" and a "revealing" illuminant pair. We can for instance hide the security patterns under normal daylight and reveal them under a tungsten illumination or hide the security patterns under a tungsten illumination and reveal them under a fluorescent tube F7 illumination. It is also possible to hide the security patterns at the same time under different natural and artificial white illuminants such as a normal daylight, a tungsten lamp and a fluorescent tube illumination. In this situation, the security patterns are revealed under an illumination having energy only in the excitation wavelengths of the fluorescent inks, such as a blue or a UV illumination. These security features are easily integrable into the workflow of a printing company. Once a printing company has integrated in its printing workflow the additional two fluorescent ink printing stages, it can offer to its customer both higher fidelity printing capabilities and means of hiding security patterns. Future work Additional research is needed to determine user preferences, e.g. which lightness adaptation strategy is preferable for different types of color images and to which extent the printable color domain extension offered by the different inks is really perceived and appreciated by the users. Visual preference experiments would be interesting to investigate the effectiveness of the proposed gamut expansion algorithm compared to the results obtained by other algorithms mostly proposed to exploit the gamut of modern wide gamut displays. Future work should also includes studies about the extent to which chroma expansion of prints improves the communication of its embedded message. Regarding the proposed security feature, the security of the hidden patterns can be further enhanced by establishing a model predicting the fluorescent emission of the daylight fluorescent inks under UV light. By comparing the image captured under a UV illuminant and the predicted fluorescent image, one may obtain a further confirmation of the authenticity of the document. Future work should finally verify if the metameric index can be used for 74

93 expressing the pattern hiding capabilities of different substrates and daylight fluorescent inks under different daylight illuminants. 75

94

95 Bibliography Din deutches institut für normung e. v., prüfung von drucken and druckfarben, teil 2: Anforderungen an die messanordnung von farbdichtemessgeräten und ihre prüfung, ref. nr. din , Practice for evaluating the relative lightfastness and weatherability of printed matter, astm d standard, U. Agar and J. P. Allebach. An iterative cellular ynsn method for color printer characterization. In Proc. 6 th IS&T Color imaging conf., pp J. D. Auslander and R. A. Cordery. Fluorescent hidden indicium US Pat 7,422,158. J. D. Auslander, R. A. Cordery, and C. Zeller. Method and system for validating a security marking US Pat 7,536,553. R. Bala. Optimization of the spectral neugebauer model for printer characterization. In J. Electronic Imaging, Vol. 8, No. 2, pp R. Bala. Digital Color Imaging Handbook. Ed. G. Sharma, Chapter 5, Device Characterization, Section 5.4.5, Lattice-based interpolation, 23a. pp R. Bala. Digital Color Imaging Handbook. Ed. G. Sharma, Chapter 5, Device Characterization, Section 5.2.3, Input device calibration and characterization, 23b. pp R. Bala and E. Dalal. A method for quantifying the color gamut of an output device. In Proceedings of SPIE, Vol. 318, pp R. Bala, R. Eschbach, and Y. Zang. Substrate fluorescence: Bane or boon? In Proc. 15 th IS&T/SID Color Imaging Conference, Albuquerque, NM, 27. pp F. Bernardini, J. Mittleman, H. Rushmeier, C. Silva, and G. Taubin. The ball-pivoting algorithm for surface reconstruction. In IEEE Trans. Visualization and Computer Graphics, Vol. 5, No. 4, pp D. H. Brainard, D. G. Pelli, and T. Robson. Display Characterization. 2 rd ed., 22. pp L. Brehm and H. Erbar. Halftone image produced by printing US Pat. Appl. 1/482,

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99 A Hue planes of the fluorescent gamuts of both an offset and an inkjet printer This appendix shows constant hue planes from to 3 degrees between the non-linearly lightness adapted srgb gamut and the fluorescent gamut G f of the two considered inkjet (Figure A.1) and offset (Figure A.2) printers. 81

100 Appendix A. Hue planes of the fluorescent gamuts of both an offset and an inkjet printer L * hue angle of 1 G f inkjet L * hue angle of 6 1 G f G' srgb G' srgb C * C * L * hue angle of 12 1 G f L * hue angle of 18 1 G' srgb G f 4 G' srgb C * C * L * hue angle of 24 L * hue angle of G' srgb 8 8 G' srgb 6 G f 6 G f C * C * Figure A.1: Comparison of constant hue planes between the fluorescent gamut of an inkjet printer G f (solid lines) and the non-linearly lightness adapted srgb gamut G srgb (dashed lines). 82

101 L * hue angle of 1 offset L * hue angle of 6 1 G f 8 G f G' srgb 4 G' srgb C * C * L * hue angle of 12 1 G' srgb L * hue angle of G'sRGB 6 G f 6 Gf C * C * L * hue angle of 24 1 L * hue angle of 3 1 G' srgb 8 8 G' srgb 6 G f 6 G f C * C * Figure A.2: Comparison of constant hue planes between the fluorescent gamut of an offset printer G f (solid lines) and the non-linearly lightness adapted srgb gamut G srgb (dashed lines). 83