Chapter+FOUR+-+COLOUR

=Chapter FOUR - COLOUR=

Trichromacy theory - Cone = active at normal light level (3 cone types = 3 color receptors = trichromacy) o Cone response spaces (imagine the lines on a cube)  (|) Y axis = C1  (-) X axis = C2  (/) Z axis = C3 - Rods = active at low light level - 3 color receptors = basic 3 dimensionality of human colour vision

- 3 colored phosphors in a tv tube – red, green, blue - 3 primary paint colors – red, yellow, blue - 3 minimum colored inks for printing – cyan, magenta, yellow

Color Blindness - Color blindness = 10% male and 1% female - Reason of deficieny: (both are unable to distinguish red and green) o Lack of long wavelength sensitive cone (protanopia) o Lack of medium wavelength sensitive cone (deuteranopia)

Color Measurement - Colorimetry = matching any color with a mixture of no more than 3 primary lights - Gamut = set of all colors that can be produced by a device (ie tv) or sensed by a receptor system o Color space for a gamut (imagine a cube)  X axis = G  Y axis = R  Z axis B

Commmission International de L’Eclairage (CIE) system of color standards - Standard for measuring colored light (ie monitor calibration) - Uses tristimulus values = a set of abstract primaries labelled XYZ (imagine as 3 angles in a triangle with all 3 primaries intersecting each other) o Y tristimus value = luminance - Primaries are chosen for mathematical properties not based on color match - Chromacity coordinates = measures hue and vividness of a color

Features of chromacity diagram - If 2 colored lights are represented by 2 points in a chromacity diagram, the color of mixture of those two lights will lie on a straight line between those 2 points - Any set of 3 lights specifies a triangle in the chromacity diagram o The corners are given by the chromacity coordinates of the three lights - Spectrum locus = chromacity coordinates of pure monochromatic (single wavelength) lights - Purple boundary = straight line connecting chromacity coordinate of 700 nm of red the longest visible wavelength to 400 nm of blue the shortest visible wavelength - Excitation purity = measured distance between pure spectral wavelength and the white point - Complementary wavelength = opposite color in the spectrum locus o Color + complementary color = white

Color differences and uniform color spaces - 3 applications of color space with equal perceptual distances o Specification of color tolerances (ie matching furniture colors with upholstery) o Specification of color codes (ie distinct cable wires to avoid confusion) o Pesudocolor sequences for maps (ie used by paint industry, medical imaging)  Uniform color spaces = CIE lab and CIE luv - Contrast effects can radically alter the shape of the color shape

Opponent process theory (Ewald Hering) - 6 elementary colors arranged perceptually as opponent pairs along 3 axes o Black-white = luminance channel o Red-green = difference of long and mid wavelength cone signals o Yellow-blue = difference between short wavelength cones and the sum of the other 2

Categorical colors = ‘ideal’ shade of a color

Isoluminant/equiluminous pattern = color channels must differ in luminance (ie spatial sensitivity)

Color appearance = color creates visual attributes of obj (ie red berry) = coding of information

- High saturation = vivid - Low saturation = black/white/gray Applications of color in visualization

Application 1: color specification interfaces and color spaces - Color space – adjusted by hue and saturation - Color naming o Natural color system (NCS) – based on Hering’s opponent color theory o Pantone system – used in printing industry o Munsell system – reference for surface colors - Color palette = solution to color selection problem by using contrast effects with diff bgs Application 2: color for labelling - Nominal info coding = technical term for labelling an obj - Perceptual factors considered in choosing a set of color labels: o Distinctness - perceive difference between 2 colors placed close together o Unique hues (ie use distinct colors = red, green, blue VS blue, indigo, violet) o Contrast with background o Color blindness o Number of colors o Field size o Conventions (ie red = warning) Application 3: color sequence for maps - Pseudocoloring = technique to represent values in maps using a sequence of colors o Perceive shape of features (ie roads, valleys) o Classification on basis of color (ie green grass, blue water, beige wheat) Application 4: color reproduction - Process of gamut mapping - Calibration - Range scaling = equate luminance range of source and destination images - Rotation = perception on diff materials based on illumination (ie monitor vs paper) - Saturation scaling = printing is diff from color on monitor Application 5: color for exploring multidimensional discrete data - Use color to create 3d translations of graphs (ie scatter plots)