Outline Color perception Introduction Theories of color perception Yong-Helmhotz trichromatic theory Hering opponent process theory Dual process theory Physiological mechanisms for color perception

Visual Perception Modeling Color Perception The sensation of color is caused by the brain. One way to get it is the response of the eye to the presence/absence of light at various wavelengths. 11/14/2018 Visual Perception Modeling

Physical description of light Photon A tiny packet of vibrating electro-magnetic energy Characterized by the wavelength of its vibration The photons we experience in visible light cover just a small portion of the electro-magnetic spectrum 11/14/2018 Visual Perception Modeling

Visual Perception Modeling Light Spectra 11/14/2018 Visual Perception Modeling

Physical description of light Photon A tiny packet of vibrating electro-magnetic energy Characterized by the wavelength of its vibration The photons we experience in visible light cover just a small portion of the electro-magnetic spectrum Physical description of light Number of photons it contains at each wavelength 11/14/2018 Visual Perception Modeling

Visual Perception Modeling Sunlight 11/14/2018 Visual Perception Modeling

Psychological description of color Color space The subjective experience of surface colors can be described in terms of three dimensions Hue Saturation Lightness Color space is the three-dimensional coordinate system in which each color experience is represented by a point 11/14/2018 Visual Perception Modeling

Psychophysical correspondence Mapping between physical descriptions and psychological ones Mean wavelength determines hue Spectral area determines lightness The total number of photons Variance determines saturation 11/14/2018 Visual Perception Modeling

Visual Perception Modeling Basic Phenomena Light mixture Only a small portion of the colors correspond to monochromatic lights Two or more colors must be combined in order to non-spectral colors and de-saturated colors By experience, it is possible to match almost all colors using only three primary sources 11/14/2018 Visual Perception Modeling

Additive and subtractive matching Choose colors A, B, C such that no two can be mixed to match the third - Primaries. Many colors can be represented as a mixture of A, B, C write M=a A + b B + c C This is additive matching. Gives a color description system - two people who agree on A, B, C need only supply (a, b, c) to describe a color. Some colors can’t be matched like this: instead, must write M+a A = b B+c C This is subtractive matching. Interpret this as (-a, b, c) Problem for building monitors: Choose R, G, B such that positive linear combinations match a large set of colors 11/14/2018 Visual Perception Modeling

Visual Perception Modeling Some Phenomena Color blindness People who cannot discriminate among all the colors There are several distinct varieties of color blindness Color afterimages Mach bands Chromatic adaptation 11/14/2018 Visual Perception Modeling

Early theories and confusion Widespread: We see things by rays fired out of the eyes, hitting surfaces; colors come from different kinds of rays. Newton: Color is obtained by differential refraction of white light. Artists: Can’t get enough colors from Newton; actually, color is obtained by mixing lights or pigments. 11/14/2018 Visual Perception Modeling

Young-Helmholtz Trichromatic Theory Propose by Young-Helmholtz There are three types of color receptors in the human eye They were hypothesized to produce the psychologically primary color sensations of red, green, and blue All other colors were explained as combinations of these primaries 11/14/2018 Visual Perception Modeling

Opponent Process Theory Trichromatic theory cannot explain very well some facts and observations Yellow is the additive mixture of red and green primaries but the subjective experience does not appear to be that way Color experiences are always lost in certain pairs 11/14/2018 Visual Perception Modeling

Opponent Process Theory – cont. Opponent process theory by Hering There are four chromatic primaries rather than three They are structured in pairs of polar opposites Red/green Blue/yellow Black/write 11/14/2018 Visual Perception Modeling

Dual Process Theory For many decades there were heated debates between two warring factions – Helmholtz vs. Hering In 1957, Hurvich and Jameson proposed a dual processing theory Trichromatic theory Opponent-Process stage 11/14/2018 Visual Perception Modeling

Physiological Mechanisms Three cone systems There are three types of cones in the normal trichromat retina, each of which contains a different light-absorbing pigment Absorption spectra of these three cone types are determined using different techniques 11/14/2018 Visual Perception Modeling

Visual Perception Modeling Color Receptors “Red” cone “Green” cone “Blue” cone 11/14/2018 Visual Perception Modeling

Visual Perception Modeling Color Opponent Cells Color opponent cells in LGN Responses in the LGN of moneys are roughly conformed to the pattern predicted by Hering’s opponent process theory They are also found in the bipolar and ganglion cells in the retina 11/14/2018 Visual Perception Modeling

Theory for Color Perception Trichromatic theory is perhaps the only theory that explains many perceptual phenomena nicely and in a unified framework People have been trying to find theories in the other areas of perception Texture perception 11/14/2018 Visual Perception Modeling

Visual Perception Modeling Representing Colors Accurate color reproduction is commercially valuable - e.g. Kodak yellow, painting a house. Of the order of 10 color names are widely recognized by English speakers - other languages have fewer/more, but not much more. Color reproduction problems increased by prevalence of digital imaging – e.g. digital libraries of art. Choosing pixel values to reproduce/evoke experiences, e.g. an architectural model. Consistency in user interfaces, monitor-printer consistency, etc. 11/14/2018 Visual Perception Modeling

Visual Perception Modeling Color spaces Linear color spaces describe colors as linear combinations of primaries Choice of primaries choice of color matching functions choice of color space RGB primaries are monochromatic, energies are 645.2nm, 526.3nm, 444.4nm Color matching functions have negative parts -> some colors can be matched only subtractively. CIE XYZ Color matching functions are positive everywhere, but primaries are imaginary 11/14/2018 Visual Perception Modeling

Visual Perception Modeling CIE x, y 11/14/2018 Visual Perception Modeling

Qualitative features of CIE x, y Linearity implies that colors obtainable by mixing lights with colors A, B lie on line segment with endpoints at A and B Monochromatic colors (spectral colors) run along the “Spectral Locus” Dominant wavelength = Spectral color that can be mixed with white to match Purity = (distance from C to spectral locus)/(distance from white to spectral locus) Wavelength and purity can be used to specify color. Complementary colors=colors that can be mixed with C to get white 11/14/2018 Visual Perception Modeling

More linear color spaces Monitor RGB: primaries are monitor phosphor colors, primaries and color matching functions vary from monitor to monitor - careful! YIQ: mainly used in television, Y is (approximately) intensity, I, Q are chromatic properties. Linear color space; hence there is a matrix M that transforms XYZ coords to YIQ coords. I and Q can be transmitted with low bandwidth. 11/14/2018 Visual Perception Modeling

Non-linear color spaces HSV: Hue, Saturation, Value are non-linear functions of XYZ. because hue relations are naturally expressed in a circle Uniform: equal (small!) steps give the same perceived color changes. Munsell: describes surfaces, rather than lights - less relevant for graphics. Surfaces must be viewed under fixed comparison light 11/14/2018 Visual Perception Modeling

Visual Perception Modeling Color books 11/14/2018 Visual Perception Modeling

Device independent color imaging Problem: ensure that colors on a display, printer, etc. give the same experience that a viewer would have seeing relevant light spectra Difficulty: limited gamuts of most output devices Strategy: exploit a model of human experience 11/14/2018 Visual Perception Modeling

Visual Perception Modeling Constancy We observe the color of the light reflected from surfaces But we want surface colour problem is known as colour constancy Lightness constancy how light is the surface, independent of the brightness of the illuminant issues spatial variation in illumination absolute standard Human lightness constancy is very good 11/14/2018 Visual Perception Modeling

Finding Skin Using Image Color It is very useful to find human skin in images Gesture-based user interfaces Content-based retrieval Ignore some pictures A computational model of skin 11/14/2018 Visual Perception Modeling

Surface Color from Image Color The color of light in an image is determined by two factors Spectral reflectance of the surface Spectral radiance of the light falling on that surface Color of light falling surfaces can vary very widely Image color can be a bad representation of the color of surfaces being viewed Color constancy in human perception 11/14/2018 Visual Perception Modeling