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Color Theory What is color?
‣ Interaction of light and eye-brain ‣ What is color? system ‣ How do we perceive it? ‣ Light: electromagnetic phenomenon ‣ How do we describe and match colors? • Discerned by different wavelength ‣ Color spaces
Color Spectra Color Spectra
Pure colors - single wavelength Sample lights:
How do we perceive them?
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Human Visual System Tristimulus Theory of Color
Rods Important principle: - black & white receptors Any color spectra is perceived by sensors with 3 different - peripheral vision response frequencies! -sensitive
Cones Tristimulus theory of color: - 3 type tuned to different Color is inherently a three-dimensional space frequencies - 3 cones have different Metamers: sensitivities If two colors produce the same tristimulus values, then they -central vision are visually indistinguishable - less sensitive
Spectral Response of Human Visual System Color Spectra
Sample lights:
How to describe them numerically?
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Color Spectra Dominant Wavelength
Important principle: ‣ Stating the numbers
Anyyp color spectra is p erceived as: - Dominant wavelength (hue) - a single dominant wavelength - its hue - Luminance - mixed with a certain amount of white light (saturation) (total power) - of a certain intensity or brightness - Saturation (purity)
Luminance and Saturation RGB color description
‣ Luminance (L) = (D-A)B + AW ‣ Use three primary color (r,g,b) ‣ Saturation = (D-A)B/L * 100% - C(⎣) = r(⎣)R + g(⎣)G + b(⎣)B - White lig ht: D = A , i .e., S at . = 0
negative!! g(⎣) r(⎣) b(⎣)
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RGB Primary Colors RGB Color Space
CMY Color Model CMY <-> RGB
‣ C: Cyan; M: Magenta; Y: Yellow ‣ Subtractive primaries - Cyan, Magenta, and C 1 R Yellow are the compliment of Red, Green Blue M = 1 - G ‣ Specified by what is being removed from white Y 1 B ‣ Example: Cyan color = (1,0,0) means red is removed; CMY: (1,1,0) -> red and green is removed => what color? ‣ Sometimes CMYK - K: Black
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CIE Primary “Colors” CIE Primary Colors ‣ (X,Y,Z) - Not real colors ‣ The combination coefficients are positive ‣ PlPerceptual space
C(⎣) = x(⎣)X + y(⎣)Y + z(⎣)Z
CIE Chromaticity Chart CIE Gamut
- The range of colors that can be produced on a device
Green
CRT Gamut
Red
Project to xy plane Blue
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Color Spaces HLS Color Space
‣ CIE model is a good color reference ‣ Not necessarily the most natural one ‣ Many other color spaces are used - RGB - HLS - CMY - HSV - YIQ - ...
HLS Color Space (2)
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