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COLOR CONSTANCY. VISN2211 David Lewis Sieu Khuu. COLOR CONSTANCY THEORY. Absolute rates of photopigment absorptions do not explain color appearance. Color appearance depends on local contrast of cone absorptions. Color perception of an object is relative to the colors in the surround.
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COLOR CONSTANCY VISN2211 David Lewis Sieu Khuu
COLOR CONSTANCY THEORY Absolute rates of photopigment absorptions do not explain color appearance. Color appearance depends on local contrast of cone absorptions. Color perception of an object is relative to the colors in the surround.
COLOR CONSTANCY • Color constancy is the ability to perceive the color of an object as remaining constant despite variations in illumination. • For example, Outdoor (Sunlight) Indoor (Yellow Light)
NOT JUST A SLIGHT DIFFERENCE • Color constancy produces dramatic effects on a daily basis, which usually go completely unnoticed. • Compare these two Gakos… Indoor (Yellow Light) Forehead Forehead Outdoor (Sunlight) Beak Beak
COLOR CONSTANCY UNDER A SINGLE LIGHT SOURCE • Color constancy also plays a part in the perception of a uniformly colored object, despite real color variations. • Shadows • Highlights • 3D Shape • Depth perception
ILLUMINATION & REFLECTANCE • The illumination (quality of the light source) combined with the reflectance (quality of the object) combine to form color signals. • Without a source of illumination, there is no light. • Without light, nothing can be reflected, so an object’s reflectance is irrelevant. Man, it’s dark in here…
COLOR SIGNALS • A color signal is the portion of the illuminated light that is reflected off of a surface. • It is what ultimately leads to the perception of color. • Our visual system decodes these signals so that we can perceive the color of the objects in our environment. Relative Energy 400 500 600 700 Wavelength (nm) Color Signal
HOW COLOR SIGNALS ARE MADE Reflectance Illumination x Relative Energy Relative Energy Relative Energy 400 400 400 500 500 500 600 600 600 700 700 700 Color Signal Wavelength (nm) Wavelength (nm) Wavelength (nm) =
SURFACE REFLECTANCE • Surface reflectance is a physical quality of an object. • It can be thought of as the “true color” of that object. • Objects that reflect mostly • Long wavelengths = RED • Mid-Long waves = YELLOW • Short waves = BLUE • All waves = WHITE/GRAY Reflectance of Basic Colors
Blue Yellow Red
DICHROMATIC REFLECTION MODEL • Shafer (1985) • Most material is dielectic • Dielectic material consists of a clear substrate with embedded color particles • Light reflected off of the substrate is called Interface Reflection • aka Gloss • Light reflected off of the particles is called Body Reflection. • aka Color
EXTRACTION OF COLOR FROM SIGNALS • It is easy to compute a color signal from it’s luminance and surface reflectance. • L x SR = CS • L = Luminance • SR = Surface Reflectance • CS = Color Signal • But the visual system must perform this function backwards in order to determine the “true color” of an object. • CS/L = SR
THE PROBLEM • The human visual system can detect the “true color” of an object very efficiently, despite the variation in illumination. • Humans do not perceive the colors that are actually entering they eye, they perceive the colors as they “should” be. • Humans can solve for SR: CS/L = SR • When presented with a CS we can somehow determine the values of L and then SR. • How we do this is largely unknown. • Many psychophysical models have been proposed.
DETERMINING ILLUMINATION (L) To determine how the human visual system can find the value of L previous researchers have tried using simple functions. By fitting functions onto previously collected data we can determine which ones work best. These are the ones that are most likely similar to the human visual system. Well fitting functions are developed into Linear Models of visual perception.
LINEAR MODELS • Have been used since early 1980. • Efficiently approximate a set of spectral functions (i.e. illumination). • One such model is that of regular daylight. • Judd et al. 1964 • 600 measurements • Measurements did not vary much. Daylight Spectra
ESTIMATING SURFACE REFLECTANCE (SR) Unlike illumination, surface reflectance cannot be measured in isolation. SR must be combined with L in order to be measured. In theory, you could measure all possible combinations of SR and L. But the universe is very large and full of stuff so it may take a while. For example…
VARIABILITY WITHIN COLOR SIGNALS FOR A SINGLE REFLECTANCE Body Reflectance Illumination Color Signal Yellowish light x = Redish Color Relative Energy 400 500 600 700 Purplish light 400 400 500 500 600 600 700 700 x = Wavelength (nm) x = Bluish light
SIMILARITY IN COLOR SIGNALS AMONG DIFFERENT REFLECTANCIES Body Reflectance Illumination Color Signal Yellowish light x = Redish Color Relative Energy 400 400 400 500 500 500 600 600 600 700 700 700 Bluish Color x = Redish light ??? Light x = ??? Color
Over time, many models have been produced for the SR of special sets of materials. Photocopying/ Clothing/ Paint/ etc. One of the most widely used surface reflectance measurers is the Macbeth ColorChecker, but it is not linear. A standardized measuring device for color. Widely used in industrial applications. SURFACE REFLECTANCE MODELS
MACBETH COLORCHECKER • Colors were selected due to their similar to naturally occurring colors.
HOW THE COLORCHECKER WORKS It consists of 24 printed color squares, which include spectral simulations of light and dark skin, foliage, etc. It is scientifically designed to help determine the true color balance of any color rendition system. You can compare the digital reproduction of a real scene or a test pattern to the 24 colored squares to determine if your imaging hardware needs to be adjusted.
USES FOR THE COLORCHECKER The colorchecker can be used in all forms of photography and film. For example, in some older films you may see a color checker (or someone holding one up to the camera) so that you could adjust your projector to accurately reproduce the colors.
Wandell sought to build a linear model of the ColorChecker. Fitted the functions with a linear model with three dimensions (basis functions). LINEAR MODEL APPROXIMATION OF THE MACBETH COLORCHECKER
BASIS FUNCTIONS OF THE LINEAR MODEL FOR THE MACBETH COLORCHECKER
COLOR CONSTANCY – GREEN SHELL Exact Same Color
COLOR CONSTANCY – BLUE EYE Exact Same Color
COLOR CONTRAST - SQUARES Exact Same Color
COLOR ASSIMILATION - PATCHES Exact Same Color Exact Same Color
COLOR ASSIMILATION - SPIRAL Exact Same Color
