Color Theory What is color? How do we perceive it?

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Presentation transcript:

Color Theory What is color? How do we perceive it? How do we describe and match colors? Color spaces

What is color? Interaction of light and eye-brain system Light: electromagnetic phenomenon Discerned by different wavelength

Color Spectra Pure colors - single wavelength

Color Spectra Sample lights: How do we perceive them?

Human Visual System Rods - black & white receptors - peripheral vision - sensitive Cones - 3 type tuned to different frequencies - 3 cones have different sensitivities - central vision - less sensitive

Tristimulus Theory of Color Important principle: Any color spectra is perceived by sensors with 3 different response frequencies! Tristimulus theory of color: Color is inherently a three-dimensional space Metamers: If two colors produce the same tristimulus values, then they are visually indistinguishable

Spectral Response of Human Visual System

Color Spectra Sample lights: How to describe them numerically?

Color Spectra Important principle: Any color spectra is perceived as: - a single dominant wavelength - its hue - mixed with a certain amount of white light (saturation) - of a certain intensity or brightness

Dominant Wavelength Stating the numbers Dominant wavelength (hue) Luminance (total power) Saturation (purity)

Luminance and Saturation Luminance (L) = (D-A)B + AW Saturation = (D-A)B/L * 100% White light: D = A, i.e., Sat. = 0

RGB color description Use three primary color (r,g,b) C(λ) = r(λ)R + g(λ)G + b(λ)B negative!! r(λ) g(λ) b(λ)

RGB Primary Colors

RGB Color Space

CMY Color Model C: Cyan; M: Magenta; Y: Yellow Subtractive primaries - Cyan, Magenta, and Yellow are the compliment of Red, Green Blue Specified by what is being removed from white 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

CMY <-> RGB C 1 R M = 1 - G Y 1 B

CIE Primary “Colors” (X,Y,Z) - Not real colors The combination coefficients are positive Perceptual space C(λ) = x(λ)X + y(λ)Y + z(λ)Z

CIE Primary Colors

CIE Chromaticity Chart Project to xy plane

CIE Gamut - The range of colors that can be produced on a device Green CRT Gamut Red Blue

Color Spaces 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

HLS Color Space (2)