4/29/2009 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? 1 4/29/2009 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? 2 4/29/2009 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(⎣) 3 4/29/2009 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 4 4/29/2009 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 5 4/29/2009 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) 6.