Color: Intro • To create image, need 3 things: 1. Object 2. Light 3. Viewer • Same thing is true for color { Color of object depends on "natural" color (under white light) and color of light { Color perceived depends on viewer ∗ 2 viewers observing same object under same conditions may perceive different color • Major topics to be discussed: { Nature of light and color { The eye and color perception { Color models 1 Color: Basics • Color attributes { Hue - basic color { Saturation - purity of a color ∗ How much white is mixed with a pure color { Lightness (brightness) - intensity ∗ Lightness pertains to reflected color ∗ Brightness pertains to emitted color • Artists' terminology { These relate to pure pigments (pure colors) { Tint ∗ Result of adding white to pigment ∗ Reduces saturation { Shade ∗ Result of adding black to pigment ∗ Reduces lightness { Tone ∗ Result of adding white and black to pigment • Both terminologies widely used { Both are subjective • Want objective system for describing color 2 Color: Primaries • Represent using color wheel • Additive primaries { Apply to colors of light { Primaries: RGB { Secondaries: CMY { Max combination of 3 primaries: white { Absence of primaries: black { Called additive because as add more primaries, are contributing more colors to the light ∗ i.e., the more wavelengths are represented • Subtractive { Apply to colors of pigments { Primaries: CMY { Secondaries: RGB { Max combination of 3 primaries: black { Absence of primaries: white** { Called subtractive because as add more primaries, are absorbing more colors from the light • Color space is set of colors that can be generated from a set of primaries 3 Color: Light • Characterized by wavelength (λ) and intensity • Visible light λ in range [380, 700] nm • Basic colors of spectrum: ROYGBIV • Given color represented by a spectral (color distribution) curve • Colorimetry: objective, quantitative science of color • Objective characteristics of color { Dominant wavelength - observed color (λ) ∗ Corresponds to hue { Excitation purity - per cent of dominant λ ∗ Corresponds to saturation { Luminance - intensity; total amount of energy ∗ Corresponds to lightness/brightness 4 Color: Eye Structure • Retina covered with 2 types of photoreceptors 1. Rods { Sensitive to low-intensity light { Overloaded by bright light { Only useful for night vision { Densest around perimeter of retina { ∼ 120 million 5 Color: Eye Structure (2) 2. Cones { Only stimulated by bright light { 3 types ∗ Usually referred to as R, G, B types { ∼ 6 million { Most dense in fovea - center of retina ∗ Only place where cone density > rod density { Foveola - center of fovea ∗ 100% cones ∗ Greatest density of cones ∗ Part of eye most sensitive to color ∗ Has greatest visual acuity • Trichromacy theory (3 channel, Young-Helmholtz theory) { Eye has 3 color channels 6 Color: Eye Response to Light • Cones respond to light differently { B cone most sensitive to 440 nm (indigo) { G cone most sensitive to 545 nm (green) { R cone most sensitive to 580 nm (greenish-yellow) { Better labels are S, M, L for short, medium, and long λ sensitivity • Rods most sensitive to 449 nm • Comparatively, eye least sensitive to blue { I.e., given R, G, B light of equal intensities, B will seem much dimmer 7 Color: Eye Response to Light (2) • Luminance efficiency function { Shows eye's response to light of equal intensities as a function of λ { Peak sensitivity 550 nm { Linear combination of 3 cone functions 8 Color: Eye Response to Light (3) • Functions rλ, gλ, bλ represent amounts of additive primaries needed to create perception of all colors of spectrum { Negative values ) cannot produce color from primaries { Such colors only generated by adding the negative amount to color sample { Hence, additive primaries cannot reproduce all colors of spectrum (wrt eye) • Eye can distinguish ∼ 128 different fully saturated hues { Proximity of each hue not linear wrt λ • Eye's response to brightness not linear { Doubling intensity does not result in perception of doubled brightness 9 Color: Tristimulus Theory • Given spectral distribution P (λ) and sensitivity curve S(λ) { Particular cone generates signal A = s S(λ)P (λ)d(λ) { Cone converts continuous distribution into a discrete value { 3 discrete signals generated by eye: AR;AG;AB • Perceived color can be represented as C = TRα + TBβ + TGγ { α; β; γ are primaries, Ti are intensities of each • Ti called tristimulus values • Tristimulus theory states that can produce any color with the right tristimulus values • Many:1 correspondence between tristimulus values and perceived color { Several sets of TSVs can generate same color perception { Such sets called metamers { Implication: 2 spectral distributions may appear the same color • Metamerism depends on light source, object, and viewer { 2 objects may appear same color under one light, but different under an- other light { One viewer may see 2 spectral distributions as same color, while another viewer sees the same distributions as different colors 10 COLOR: CIE • Commission Internationale de l'Eclairage (International Commission on Illu- mination) • Founded in 1920's • Purpose is the study and standardization of color { Intent is to provide objective description of color • Responsible for a number of standards { Standard illuminant - light source ∗ Defined in terms of black body radiators · Ideal object that produces light solely via thermal energy · Observed color only depends on light source ∗ Standards: 1. Illuminant A - tungsten lamp 2. Illuminant B - sunlight with correlated color temperature of 4874oK 3. Illuminant C - sunlight with correlated color temperature of 6774oK 4. Illuminants D - series of various daylight conditions (a) D50 - sunlight with correlated color temperature of 5000oK (b) D65 - sunlight with correlated color temperature of 6504oK 5. Illuminant E - equal energy illuminant · Theoretical ideal 6. Illuminants F - series of various fluorescent lamps { Standard observer ∗ Represents full tristimulus response of typical human ∗ Standards: 1. 2o observer (1931) - observes color swatches that subtend 2o FOV of retina · (Color concentrated on fovea) 2. 10o observer (1964) - observes color swatches that subtend 10o FOV of retina 11 COLOR: CIE Color Systems • Designed to provide 3 primaries that capture full range of visible light • Replace RGB • Systems: 1. XYZ 2. xyY 3. Lab 4. Luv 12 COLOR: CIE XYZ System • Primaries {X, Y, Z { Loosely correspond to R, G, B { Do NOT require negative values to generate all colors • Blending functions { xλ, yλ, zλ { Represent amount of X, Y, Z to generate all colors { yλ chosen to be same as luminance function { Defined for standard 2o observer { xλ, yλ, zλ linear combinations of rλ, gλ, bλ ∗ Can convert between 2 systems 13 COLOR: CIE XYZ System (2) • XYZ space { Given a spectral distribution P (λ), the amount of X needed to match this color is X = k s P (λ)xλdλ { Similarly for Y, Z { k defined by ∗ For emitters, k = 680 lumens/watt ∗ For reflectors, chosen so bright white has Y = 100 · Hence 100 k = s Pw(λ)yλdλ · where Pw is spectral distribution of source used for white { Visible part of XYZ space ∗ Curved surface represents X + Y + Z = 1 plane { Color C = XX + Y Y + ZZ 14 Color: CIE Chromaticity • Chromaticity represents color info only { Excludes luminance aspect (i.e., represents hue and saturation, but not lightness) • Defined as X x = X + Y + Z Y y = X + Y + Z Z z = X + Y + Z • x + y + z = 1 and falls on X + Y + Z = 1 plane • CIE chromaticity diagram 15 Color: CIE Chromaticity (2) { Projection of X + Y + Z = 1 onto X-Y plane represents all visible chro- maticity values { All colors with same hue and saturation map to same point, regardless of luminance { Pure hues lie on perimeter { Central dot represents illuminant C • Given x, y { z = 1 − x − y { Not enough info to recover X, Y , Z { Requires an additional value: Y { XYZ recovered by x X = Y y Y = Y 1 − x − y Z = Y y • THE FOLLOWING MUST BE DISTINGUISHED: {X, Y, Z represent the 3 CIE primaries { xλ, yλ, zλ represent the functions that represent the amount of each CIE primary needed to produce a given visible dominant wavelength { X, Y, Z represent tristimulus values of the primaries { x, y, z represent chromaticity values of a color 16 Color: CIE Chromaticity Diagram Applications • If 2 colors mixed, resultant color lies on straight line connecting them • Finding dominant λ of color { Measure XYZ tristimulus values { Calculate xy chromaticity values { Plot point on diagram { Draw straight line thru point and white { Point of intersection with perimeter is dominant hue • Finding excitation purity of color { Given: chromaticity values of color C and dominant hue H jCW j ep = jHW j • Finding complements { Draw line thru color and white { Complement is on line on opposite side of white • Non-spectral colors { Do not correspond to a visible hue { Represent their dominant hue as a complement, represented λc • Color gamuts { Gamut is set of all possible colors that can be produced from a given set { Gamut's chromaticities represented by triangle defined by chromaticities of primaries { No visible primaries capable of producing all visible colors • Chromaticity diagram does not represent full palette { Luminance values not represented { Infinitely many planes in XYZ space, each with different luminosity, that project to chromaticity diagram 17 Color: CIE Luv • Consider { C1 = (X1;Y1;Z1) { D1 = C1 + ∆C, where ∆C = (∆X; ∆Y; ∆Z) { C2 = (X2;Y2;Z2) { D2 = C2 + ∆C { In general, the perceived difference between C1 and D1 will