Color and Color Model. Chap. 12 Intro. to Computer Graphics, Spring 2009, Y. G. Shin

Transcription

1 Color and Color Model Chap. 12 Intro. to Computer Graphics, Spring 2009, Y. G. Shin

2 Color Interpretation of color is a psychophysiology problem We could not fully understand the mechanism Physical characteristics of color can be represented by a formal expression presentation by experimental results The concept of color Spectrum of color : discovered in 1666 by Newton Newton s prism decomposition

3 Physics of Light Electromagnetic spectrum: Visible spectrum : red (700nm)-violet (400nm)

4 The Physics of Light Some examples of the spectra of light sources A. Ruby Laser B. Gallium Phosphide Crystal Wavelength (nm.) D. Normal Daylight # Photons # Photons Wavelength (nm.) C. Tungsten Lightbulb # Photons # Photons Stephen E. Palmer, 2002

5 The Physics of Light Some examples of the reflectance spectra of surfaces % Photons Reflected Red Yellow Blue Purple Wavelength (nm)

6 Psychophysical Correspondence Mean Hue # Photons blue green yellow Wavelength Hue: indicates a characteristic related to the dominant wavelength in a mixed light wave, and represents a dominant color as perceived by a human observer Stephen E. Palmer, 2002

7 Psychophysical Correspondence Variance Saturation hi. high # Photons med. low medium low Wavelength Saturation ( 채도, purity, chroma): how much a color is NOT mixed with white light. Stephen E. Palmer, 2002

8 Psychophysical Correspondence Area Brightness B. Area Lightness Stephen E. Palmer, 2002 # Photons bright dark Luminance Wavelength the measured amount of energy perceived by an observer (Lumens) the physical measure of brightness. Stephen E. Palmer, 2002

9 Intensity & Brightness intensity Spectrum nm intensity : the amount of light, or the amount of a particular color actually reflected or transmitted from a physical object. brightness : measured intensity after it is acquired, sampled, and observed (with our eye)

10 Primary colors of a light Red, Green, Blue Why we choose these three Among 6-7 million cone cells of our eye 65% is sensitive to red 33% is sensitive to green 2% is sensitive to blue

11 Colors Magenta Light Primary color: R,G,B Secondary color Red+blue magenta Pigment Green+ Blue cyan Red+green yellow Primary color : Magenta, Cyan, Yellow Secondary colors : R, G, B Yellow These are all hardware oriented Red Green Blue Cyan

12 Perception purpose by a human Instead of relative amount of R,G,B, we use. The components of a human to say colors Brightness: achromatic notion of intensity Hue : Dominant wavelength in a mixture of colors Saturation: purity Hue + Saturation chromaticity of a light e.g., pink red + white

13 Perception purpose Tri-stimulus the amount of R, G, B lights to form a certain color Q λ = r( λ) R + g( λ) G + b( λ) B From the color matching experiment

14 Color matching experiment 1

15 Color matching experiment 1 p 1 p 2 p 3

16 Color matching experiment 1 p 1 p 2 p 3

17 Color matching experiment 1 The primary color amounts needed for a match p 1 p 2 p 3

18 Color matching experiment 2

19 Color matching experiment 2 p 1 p 2 p 3

20 Color matching experiment 2 p 1 p 2 p 3

21 Color matching experiment 2 We say a negative amount of p 2 was needed to make the match, because we added it to the test color s side. The primary color amounts needed for a match: p 1 p 2 p 3 p 1 p 2 p 3 p 1 p 2 p 3

22 CIE XYZ space The dominant international standard for color specification (1931, by CIE) Based on experimental results When we use R,G,B color, we have negative primaries (require adding light to the test color s side in a color matching experiment)

23 CIE XYZ space The following transformation [Dean B. Judd] makes color matching functions positive everywhere x( λ) = r( λ) g( λ) b( λ) y( λ) = r( λ) g( λ) b( λ) z( λ) = r( λ) g( λ) b( λ)

24 CIE XYZ space Normalize x( λ), y( λ), z( λ) x( λ) y( λ) z( λ) = = = x( λ) x( λ) + y( λ) + z( λ) y( λ) x( λ) + y( λ) + z( λ) z( λ) x( λ) + y( λ) + z( λ) All real colors can be represented as positive combinations of x, y since x+y+z=1

25 CIE chromaticity diagram encompasses all the perceivable colors in 2D space (x,y)

26 Color gamut : range of colors a color model can describe CIE color model is useful for comparing color gamuts for different sets of primaries.

27 Complementary Colors Complementary colors : together produce white color. Illuminant C (Average sunlight)

28 Dominant Wavelength The spectral color which can be mixed with white light in order to reproduce the desired color The dominant color of C 2 is C p

29 RGB Color Model Red, Green, Blue lights Additive model Used in display devices Gray scale

30 CMY color model Subtractive model (colors of pigments are subtracted) Used in color output devices C 1 R M 1 G = Y 1 B CMYK color model - K for black ink for reducing the amount of ink

31 CMYK Example

32 HSI color model Hue, Saturation, Intensity Hue & Saturation give a chromatic information Hue: [0,360] Saturation, Intensity : [0,1]

33 An RGB image of the John Moulton Barn at the base of the Teton Range, along with its h, s and l components.(from Wikipedia)

34 RGB-HSI Relation

35 HSV color model Hue, Saturation, Value Value 0-1 represents the relative brightness or gray scale information RGB color cube viewed along principal diagonal is value = 1.0 plane

36 RGB HSV

37 RGB-HSI-HSV Relation RGB HSI HSV Result (1, 0, 0) (0, 1, 0.5) (0, 1, 1) (0.5, 1, 0.5) (120, 1, 0.75) (120, 0.5, 1) (0, 0, 0.5) (240, 1, 0.25) (240, 1, 0.5)

38 Effective Use of Color on a Visual Display

39 Opponent colors go well together red-green, yellow-blue good red-yellow, green-blue poor Magenta Red Blue Yellow Green Cyan

40 Avoid the simultaneous display of highly saturated(pure), spectrally extreme colors. visual refocusing caused by mixing extreme color pairs causes fatigue avoid red-blue, yellow-purple [solution: avoid pairs (add white)] Pure blue should be avoided for text, thin lines, and small shapes. Fovea is blue-blind. Blue is absorbed in eye. Can't focus on blue. Blue is an excellent background color. Raster points less noticeable in blue Blue perceived clearly in peripheral vision For color-deficient observers, avoid single-color distinctions.

41 Color Temperatures Low color temperature implies warmer (more yellow/red) light while high color temperature implies a colder (more blue) light. Cool colors (blues, greens) useful for background Warm colors (reds, yellows) useful for foreground elements GOOD warm foreground + cool background color POOR cool foreground + a warm background color

42 Contrasts Greatest contrast pairs of primary colors Least contrast pairs of secondary colors Magenta GOOD POOR Red Blue Green Yellow Cyan

43 Complimentary contrasting pairs Secondary color + primary color that falls opposite on the wheel Magenta Red Blue Green Yellow Cyan Greatest light/dark contrast Yellow / Blue Greatest cold/warm contrast Red / Cyan Most vivid contrast Magenta / Green Greater contrast energy, clarity & sharpness Too much contrast confusing & overwhelming Lower contrast soothing & subtle Too little contrast boring & bland Copyright 2000 Cynthia D. Hollingsworth

44 COLOR ASSOCIATIONS & MEANINGS Red Green Blue Purple Brown White.Vibrancy, energy.growth.serenity.royalty.earthy.purity National, cultural, religious, holidays implications