COLOR Angel 1.4 and 2.4 J. Lindblad 2001-11-01
Color = The eye’s and the brain’s impression of electromagnetic radiation in the visual spectra. How is color perceived? detector rods & cones light source red-sensitive green-sensitive blue-sensitive reflecting object
The Fovea There are three types of cones, S, M and L
Rods Sense luminance, or ”brightness”, but not color. Are spread out across the whole retina, and dominate when the pupil is large, i.e. night vision. Less color is seen at night. The respons is not linear, but logarithmic. The appearance of an object’s intensity depends on the surroundings; the sensation is relativ and not absolute.
Three kinds of cones r=700nm red-sensitive green-sensitive blue-sensitive g=546.1nm b=435.8nm CIE standard (Comission Internationale de L’Eclairage, 1931) wavelength In order to standardize the description of color, a large number of people were instructed to say what combination of basic colors a certain color sample consisted of in standard lighting. This resulted in the color matching curves, i.e. transform
Color perception • Different spectra can result in identical sensations, called metamers • Color perception results from the simultaneous stimulation of 3 cone types (trichromat) • Our perception of color is affected by surrounding effects and adaptation
standard lightsource object reflectance CIE 1931 standard observer CIE XYZ values z X=14.27 Y=14.31 Z=71.52 x y x x = 400nm 700nm 400nm 700nm 400nm 700nm Each color is represented by a point (X,Y,Z) in the 3D CIE color space. The point is called the tristimulus value.
Projection of the CIE XYZ-space Perceptual equal distances
RGB/CMY color space RGB - for additive color mixing, e.g. computer screen. CMY - for subtractive color mixing, e.g. printing or painting.
Mixing light and mixing pigment Additive Subtractive R G B C M Y (K)
Mixing light and mixing pigment green yellow yellow cyan red green red blue magenta magenta cyan blue   R G B C M Y R+B+G=white (additive) R+G=Y C+M+Y=black (subtractive) C+M=B etc... = 1- (CMYK common in printing, where K is black pigment)
HLS color space HueLightnesSaturation • Important aspects: • Intensity decoupled from color • Related to how humans perceive color Hue=dominant wavelength, tone Lightness=intensity, brightness Saturation=purity, dilution by white
YIQ color space • Y= Lightness • I= Inphase = ammount red-green • Q= Quadrature = ammount blue-yellow • Optimised for transmission (TV broadcast). • Compatible with BW monitors (use only Y component) • Human eye is more sensitive to variations in lightness than variations in hue and saturation and more bandwith (bits) is used for Y.
NCS color description NCS=Natural Color System A psychological more than a physiological description of color. Common among artists, designers etc. 2060-R50B= 20% white 60% black red with 50% blue w 20 40 c 60 b
hjärnanär lättlurad grönlilagulblåsvart
Non-existing colors (without use of psychadelic drugs)
Gamma correction Most displays have non-linear intensity scales. The most common correction method is called gamma correction (usually implemeted with a lookup table) Sometimes in computer graphics this effect is exaggerated to compensate for the adaptation of the eye.
True-color frame buffer Store R,G,B values directly in the frame-buffer. Each pixel requires at least 3 bytes => 2^24 colors.
Indexed-color frame buffer Store index into a color map in the frame-buffer. Each pixel requires at least 1 bytes => 2^8 simultaneous colors. Enables color-map animations.
Different blending versions (how to combine color values)
Additive blending C=A+B e.g. combining light Subtractive blending C=A-(1-B) e.g. filter effect
Average blending C=(A+B)/2 or C=uA+vB e.g. for anti-aliasing Multiplicative blending C=A*B e.g. combining light and matter