FRS 123: Technology in Art and Cultural Heritage

FRS 123: Technology in Art and Cultural Heritage Color Color Two types of receptors: rods and cones Rods and cones Cones in fovea Rods and Cones Rods

More sensitive in low light: scotopic vision More dense near periphery Cones Only function with higher light levels: photopic vision Densely packed at center of eye: fovea Different types of cones color vision Color Perception M L 3 types of cones: L, M, S

Tristimulus theory of color S Tristimulus Color Any distribution of light can be summarized by its effect on 3 types of cones Therefore, human perception of color is a 3-dimensional space Metamerism: different spectra, same response Color blindness: fewer than 3 types of cones Most commonly L cone = M cone Color Models RGB

CMY HSV XYZ etc Color Models Different ways of parameterizing 3D space RGB Official standard: R = 645.16 nm, G = 526.32 nm, B = 444.44 nm Most monitors are some approximation to this

RGB Color Model Colors are additive R 0.0 1.0 0.0 0.0 1.0 1.0 0.0 1.0 0.5 1.0 1.0

0.5 G 0.0 0.0 1.0 0.0 1.0 0.0 1.0 1.0 0.0 0.5 0.5 0.3

0.0 0.0 0.0 1.0 0.0 1.0 1.0 1.0 0.0 0.5 0.0 0.1 B Color

Black Red Green Blue Yellow Magenta Cyan White ? ? ? ? CMY Color Model C 0.0

1.0 0.0 0.0 1.0 1.0 0.0 1.0 0.5 Colors are subtractive 1.0 1.0 M 0.0 0.0 1.0 0.0

1.0 0.0 1.0 1.0 0.0 0.5 0.5 0.0 0.0 0.0 1.0 0.0 1.0 1.0 1.0

0.0 0.5 0.0 Y Color White Cyan Magenta Yellow Blue Green Red Black ?

? ? HSV Color Model H 0 120 240 * * * 60 270 270 S

1.0 1.0 1.0 0.0 0.0 * 1.0 0.5 0.0 V 1.0 1.0 1.0 1.0 0.5

0.0 1.0 1.0 0.7 Color Red Green Blue White Gray Black ? ? ?

XYZ Colorspace RGB cant represent all pure wavelengths with positive values Saturated greens would require negative red XYZ colorspace is a linear transform of RGB so that all pure wavelengths have positive values CIE Chromaticity Diagram CIE Chromaticity Diagram (White)

CIE Chromaticity Diagram Compare Color Gamuts Identify Complementary Colors Determine Dominant Wavelength and Purity RGB Color Gamut for Typical Monitor

Colorspaces for Television Differences in brightness more important than differences in color YCrCb, YUV, YIQ colorspaces = linear transforms of RGB Lightness: Y=0.299R+0.587G+0.114B Other color components typically allocated less bandwidth than Y Perceptually-Uniform Colorspaces Most colorspaces not perceptually uniform MacAdam ellipses: color within each ellipse appears

constant (shown here 10X size) Perceptually-Uniform Colorspaces uv space 4X X 15Y 3Z 9Y v' X 15Y 3Z u' Not perfect, but better than XYZ L*a*b* Color Space

Another choice: L*a*b* 1/ 3 Y L* 116 16 Yn 1/ 3 X 1 / 3 Y a* 500 X n Yn 1/ 3 Y 1 / 3 Z

b* 200 Yn Z n L*a*b* Color Space Often used for color comparison when perceptual differences matter Summary Perception and representation of Intensity, frequency, color Color

Tristimulus theory of color CIE Chromaticity Diagram Different color models