Color constancy is an example of subjective constancy and a feature of the human color perception system which ensures that the perceived color of objects remains relatively constant under varying illumination conditions. A green apple for instance looks green to us at midday, when the main illumination is white sunlight, and also at sunset, when the main illumination is red. This helps us identify objects. Color vision is how we perceive the objective color, which people, animals and machines are able to distinguish objects based on the different wavelengths of light reflected, transmitted, or emitted by the object. In humans, light is detected by the eye using two types of photoreceptors, cones and rods , which send signals to the visual cortex , which in turn processes those colors into a subjective perception.
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Color constancy is an example of subjective constancy and a feature of the human color perception system which ensures that the perceived color of objects remains relatively constant under varying illumination conditions. A green apple for instance looks green to us at midday, when the main illumination is white sunlight, and also at sunset, when the main illumination is red.
This helps us identify objects. Color vision is how we perceive the objective color, which people, animals and machines are able to distinguish objects based on the different wavelengths of light reflected, transmitted, or emitted by the object.
In humans, light is detected by the eye using two types of photoreceptors, cones and rods , which send signals to the visual cortex , which in turn processes those colors into a subjective perception. Color constancy is a process that allows the brain to recognize a familiar object as being a consistent color regardless of the amount or wavelengths of light reflecting from it at a given moment.
The phenomenon of color constancy occurs when the source of illumination is not directly known. This is due to an ignorance of all possible sources of illumination. Although an object may reflect multiple sources of light into the eye, color constancy causes objective identities to remain constant. Foster states, "in the natural environment, the source itself may not be well defined in that the illumination at a particular point in a scene is usually a complex mixture of direct and indirect [light] distributed over a range of incident angles, in turn modified by local occlusion and mutual reflection, all of which may vary with time and position.
Color constancy allows for humans to interact with the world in a consistent or veridical manner  and it allows for one to more effectively make judgements on the time of day. The physiological basis for color constancy is thought to involve specialized neurons in the primary visual cortex that compute local ratios of cone activity, which is the same calculation that Land's retinex algorithm uses to achieve color constancy.
These specialized cells are called double-opponent cells because they compute both color opponency and spatial opponency. Double-opponent cells were first described by Nigel Daw in the goldfish retina. Color constancy works only if the incident illumination contains a range of wavelengths. The different cone cells of the eye register different but overlapping ranges of wavelengths of the light reflected by every object in the scene.
From this information, the visual system attempts to determine the approximate composition of the illuminating light. This illumination is then discounted  in order to obtain the object's "true color" or reflectance : the wavelengths of light the object reflects. This reflectance then largely determines the perceived color.
There are two possible mechanisms for color constancy. The first mechanism is unconscious inference. Cones, specialized cells within the retina, will adjust relative to light levels within the local environment.
Research in monkeys suggest that changes in chromatic sensitivity is correlated to activity in parvocellular lateral geniculate neurons. Metamerism, the perceiving of colors within two separate scenes, can help to inform research regarding color constancy.
For example, when subjects are presented stimuli in a dichoptic fashion, an array of colors and a void color, such as grey, and are told to focus on a specific color of the array, the void color appears different than when perceived in a binocular fashion. The "Land effect" refers to the capacity to see full color if muted images solely by looking at a photo with red and gray wavelengths. The effect was discovered by Edwin H. Land , who was attempting to reconstruct James Clerk Maxwell 's early experiments in full-colored images.
Land realized that, even when there were no green or blue wavelengths present in an image, the visual system would still perceive them as green or blue by discounting the red illumination.
Land described this effect in a article in Scientific American. The word "retinex" is a portmanteau formed from " retina " and " cortex ", suggesting that both the eye and the brain are involved in the processing. Land, with John McCann, also developed a computer program designed to imitate the retinex processes taking place in human physiology.
The effect can be experimentally demonstrated as follows. A display called a "Mondrian" after Piet Mondrian whose paintings are similar consisting of numerous colored patches is shown to a person.
The display is illuminated by three white lights, one projected through a red filter, one projected through a green filter, and one projected through a blue filter. The person is asked to adjust the intensity of the lights so that a particular patch in the display appears white. The experimenter then measures the intensities of red, green, and blue light reflected from this white-appearing patch.
Then the experimenter asks the person to identify the color of a neighboring patch, which, for example, appears green. Then the experimenter adjusts the lights so that the intensities of red, blue, and green light reflected from the green patch are the same as were originally measured from the white patch.
The person shows color constancy in that the green patch continues to appear green, the white patch continues to appear white, and all the remaining patches continue to have their original colors. Color constancy is a desirable feature of computer vision , and many algorithms have been developed for this purpose.
These include several retinex algorithms. One such algorithm operates as follows: the maximal red value r max of all pixels is determined, and also the maximal green value g max and the maximal blue value b max. Assuming that the scene contains objects which reflect all red light, and other objects which reflect all green light and still others which reflect all blue light, one can then deduce that the illuminating light source is described by r max , g max , b max.
The original retinex algorithm proposed by Land and McCann uses a localized version of this principle. Although retinex models are still widely used in computer vision, actual human color perception has been shown to be more complex. Edwin H. Land 's Essays. Springfield, Va. From Wikipedia, the free encyclopedia. Main article: Color vision. Pearson Education, Limited. Archived from the original PDF on Retrieved Vision Research.
Annual Review of Psychology. A vision of the brain. Oxford: Blackwell Science Ltd. Behavioral and Brain Sciences. Conway Journal of Neuroscience. March In Bernice E. Rogowitz; Thrasyvoulos N. Pappas; Scott J. Daly eds. Proceedings of SPIE. Human Vision and Electronic Imaging X.
Journal of the Optical Society of America. Berlin: Springer Trans. Hurvich, L. Population analysis". Experimental Brain Research. Adaptation and surround effects". Coloration Technology. Scientific American. International Journal of Computer Vision. Perceptual information processing system. Springer International Publishing, JOSA A. The contribution of local and global cone-contrasts to colour appearance: a Retinex-like model. Color topics. Color model additive subtractive Color mixing Primary color Secondary color Tertiary color intermediate Quaternary color Quinary color Aggressive color warm Receding color cool Pastel colors Color gradient.
Color tool Monochromatic colors Complementary colors Analogous colors Achromatic colors Neutral Polychromatic colors Impossible colors Light-on-dark Tinctures in heraldry. Chromaticity diagram Color solid Color wheel Color triangle Color analysis art Color realism art style.
Linguistic relativity and the color naming debate Blue—green distinction in language Color history Color in Chinese culture Traditional colors of Japan Human skin color.
Hue Dichromatism Colorfulness chroma and saturation Tints and shades Lightness tone and value Grayscale. Category Index of color-related articles. Categories : Optical illusions Color Vision. Namespaces Article Talk.
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Lightness and Retinex Theory
Retinex is the theory of human color vision proposed by Edwin Land to account for color sensations in real scenes. Color constancy experiments showed that color does not correlate with receptor responses. In real scenes, the content of the entire image controls appearances. A triplet of L, M, S cone responses can appear any color. Further, he showed that color sensations are predicted by three lightnesses observed in long-, middle-, and short-wave illumination.
Lightness and retinex theory.