Colors in images. Color spaces, perception, mixing, printing, manipulating...

Transcription

1 Colors in images Color spaces, perception, mixing, printing, manipulating... Tomáš Svoboda Czech Technical University, Faculty of Electrical Engineering Center for Machine Perception, Prague, Czech Republic

2 rather an overview lecture Warning 2/30 pictorial, math kept on minimum knowing keywords you may dig deeper Thanks Wikipedia for many images.

3 Color Spectrum Color is a human interpretation of a mixture of light with different wavelength λ (projected into a retina or camera photoreceptors). 3/30 Isaac Newton s experiment (1666).

4 Perception of Light Human eye contains three types color receptor cells, or cones. 4/30 Their sensitivity is a function of wavelength. Three peaks may be approximately identified in BLUE, GREEN, RED. Combination of the responses give us our color perception. tristimulus model of color vision. Marking according to wavelengths S short M medium L long

5 RGB color model A color point is represented by three numbers [R, G, B] 5/30 [R, G, B] have typically range for most common 8-bit images

6 RGB color model A color point is represented by three numbers [R, G, B] 5/30 [R, G, B] have typically range for most common 8-bit images

7 RGB color model A color point is represented by three numbers [R, G, B] 5/30 [R, G, B] have typically range for most common 8-bit images

8 RGB color model A color point is represented by three numbers [R, G, B] 5/30 [R, G, B] have typically range for most common 8-bit images

9 RGB color model A color point is represented by three numbers [R, G, B] 5/30 [R, G, B] have typically range for most common 8-bit images

10 RG only, B zero 6/30

11 RB only, G zero 7/30

12 GB only, R zero 8/30

13 Additive mixing computer screens, TV, projectors 9/30 Primary colors: ones used to define other colors, [R,G,B] Secondary colors: pairwise combination of primaries, [C,M,Y] (Cyan, Magenta, Yellow)

14 Additive mixing computer screens, TV, projectors 9/30 Primary colors: ones used to define other colors, [R,G,B] Secondary colors: pairwise combination of primaries, [C,M,Y] (Cyan, Magenta, Yellow)

15 Additive mixing computer screens, TV, projectors 9/30 Primary colors: ones used to define other colors, [R,G,B] Secondary colors: pairwise combination of primaries, [C,M,Y] (Cyan, Magenta, Yellow)

16 Additive mixing computer screens, TV, projectors 9/30 Primary colors: ones used to define other colors, [R,G,B] Secondary colors: pairwise combination of primaries, [C,M,Y] (Cyan, Magenta, Yellow)

17 Additive mixing computer screens, TV, projectors 9/30 Primary colors: ones used to define other colors, [R,G,B] Secondary colors: pairwise combination of primaries, [C,M,Y] (Cyan, Magenta, Yellow)

18 RG only, B zero 10/30

19 RB only, G zero 11/30

20 GB only, R zero 12/30

21 Subtractive mixing it works through light absorption the colors that are seen are from the part of light that is not absorbed paintings, printing,... Primary colors: ones used to define other colors [C,M,Y] Secondary colors: pairwise combination of primaries [R,G,B] 13/30

22 CMYK model color primaries [C.M,Y] should result black when all mixed together 14/30 in practice, such black is not dense enough K = key (black) is added to the model

23 CMYK model color primaries [C.M,Y] should result black when all mixed together in practice, such black is not dense enough K = key (black) is added to the model 14/30 CMYK printing

24 Capturing RGB values We know how to display, print color... 15/30 How to capture? CCD generates output proportionally to amount of energy

25 Capturing RGB values We know how to display, print color... 15/30 How to capture? CCD generates output proportionally to amount of energy 3CCD camera with separating dichroic beam splitter 3CCD chip camera dichroic prism

26 Capturing RGB values We know how to display, print color... 15/30 How to capture? CCD generates output proportionally to amount of energy 3CCD camera with separating dichroic beam splitter 3CCD chip camera dichroic prism Good: Color quality, Problem: price...

27 use one chip 1CCD camera with Bayer filter 16/30 place a selective filter in front of it 2:1:1, 2 to green, human eye is most sensitive to it combine values to make RGB image Demosaicking cheap but the image quality suffers this is, among other things, what makes difference between digital photo cameras

28 Demosaicking in images 17/30

29 Demosaicking in images 18/30

30 Demosaicking result 19/30 Many demosaicking method exist. 1CCD with a filter is still prevailing solution. Few expensive DV cameras in consumer level. A company Foveon found yet another way...

31 Color from depths 20/30

32 Capturing color revisited Many demosaicking method exist. 21/30 1CCD with a filter is still prevailing solution. Few expensive DV cameras in consumer level. A company called Foveon found yet another way...

33 HSV color space Problem in RGB space: How would you create a color according to your design? 22/30 RGB values do not correspond to human thinking about colors We are saying: pure red, deep purple, sky blue...

34 HSV color space Problem in RGB space: How would you create a color according to your design? 22/30 RGB values do not correspond to human thinking about colors We are saying: pure red, deep purple, sky blue... HSV Hue, Saturation, Value color space Hue is the color type (red, yellow,... ) Saturation refers to color purity or vibrancy Value is the brightness of the color

35 HSV cone 23/30

36 Playing with saturation 24/30 original image what a nice autumn!

37 Playing with saturation 25/30 original image what a sad gray autumn!

38 Additive mixing revisited Can we, assuming properly chosen [R,G,B], mix any color? 26/30

39 Additive mixing revisited Can we, assuming properly chosen [R,G,B], mix any color? 26/30 Well, almost any. What is wrong?

40 Additive mixing revisited Can we, assuming properly chosen [R,G,B], mix any color? 26/30 Well, almost any. What is wrong? Blue and Green makes Cyan. But how to make monochromatic Cyan? blue + green - (little red) = monochromatic cyan but how to make negative values on screens?

41 What do you need to match any color? 27/30 Color spectrum S(λ) wavelength [nm] 1 Table of S(λ) in predefined λ S(λ) = P1 f1(λ)s(λ)dλ + P2 f2(λ)s(λ)dλ + P3 f3(λ)s(λ)dλ which gives us [P 1, P 2, P 3 ] representation. 1 Data tables can downloaded from

42 What RGB do you need to match any color? 28/ color matching functions for the RGB primaries r g b S(λ) = R wavelength [nm] r(λ)s(λ)dλ + G g(λ)s(λ)dλ + B 2 b(λ)s(λ)dλ Problem: How to realize devices with negative matching functions? 2 Data tables can downloaded from

43 A way out new primary colors CIE XY Z 29/ color matching functions for the CIE XYZ primaries x y z S(λ) = X wavelength [nm] x(λ)s(λ)dλ + Y y(λ)s(λ)dλ + Z 3 z(λ)s(λ)dλ 3 Data tables can downloaded from

44 CIE chromaticity diagram 30/30 [x, y] = [ X X + Y + Z, ] Y X + Y + Z

45 Do we see all colors on the screen? 31/30

46 No! Do we see all colors on the screen? 31/30 Typical gamut of a CRT monitor

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136 Color spectrum S(λ) wavelength [nm]

137 3.5 3 color matching functions for the RGB primaries r g b wavelength [nm]

138 2.5 2 color matching functions for the CIE XYZ primaries x y z wavelength [nm]

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