Color constancy

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Color constancy: The colors of a hot air balloon are recognized as being the same in sun and shade. Hot air balloon - color constancy.jpg
Color constancy: The colors of a hot air balloon are recognized as being the same in sun and shade.
Example of the Land effect. Color constancy makes the above image appear to have red, green and blue hues, especially if it is the only light source in a dark room, even though it is composed of only light and dark shades of red and white. (Click to view the full-size image for the most pronounced effect.) Mountain-spring-redwhite.png
Example of the Land effect. Color constancy makes the above image appear to have red, green and blue hues, especially if it is the only light source in a dark room, even though it is composed of only light and dark shades of red and white. (Click to view the full-size image for the most pronounced effect.)
Constancy makes square A appear darker than square B, when in fact they are both exactly the same shade of gray. See Checker shadow illusion. Checker shadow illusion.svg
Constancy makes square A appear darker than square B, when in fact they are both exactly the same shade of gray. See Checker shadow illusion.
Achieving luminance constancy by retinex filtering for image analysis JohnMartin TheBard RetinexFilter.jpg
Achieving luminance constancy by retinex filtering for image analysis
In these two pictures, the second card from the left seems to be a stronger shade of pink in the upper one than in the lower one. In fact they are the same color (since they have the same RGB values under white light), but perception is affected by the color cast of the surrounding photo. ColourIllusion2.jpg
In these two pictures, the second card from the left seems to be a stronger shade of pink in the upper one than in the lower one. In fact they are the same color (since they have the same RGB values under white light), but perception is affected by the color cast of the surrounding photo.

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.

Contents

History

Ibn al-Haytham gave an early explanation of color constancy by observing that the light reflected from an object is modified by the object's color. He explained that the quality of the light and the color of the object are mixed, and the visual system separates light and color. He writes:

Again the light does not travel from the colored object to the eye unaccompanied by the color, nor does the form of the color pass from the colored object to the eye unaccompanied by the light. Neither the form of the light nor that of the color existing in the colored object can pass except as mingled together and the last sentient can only perceive them as mingled together. Nevertheless, the sentient perceives that the visible object is luminous and that the light seen in the object is other than the color and that these are two properties. [1]

Monge (1789), Young (1807), von Helmholtz (1867), Hering (1920), and von Kries (1902, 1905), as well as subsequent researchers Helson and Jeffers (1940), Judd (1940), and Land and McCann (1971), have all made significant contributions to the investigation of colour constancy. The idea that the occurrence of colour constancy was the consequence of unconscious inference (Judd, 1940; von Helmholtz, 1867) and the idea that it was the result of sensory adaptation (Helson, 1943; Hering, 1920) coexisted for a significant portion of this time. To clarify the nature of observers' color-constancy judgements, Arend and Reeves (1986) conducted the first systematic behavioural experiments. Subsequently, new colour constancy models, physiological information on cortical mechanisms, and photographic colorimetric measurements of natural scenes all appeared. [2]

Color vision

Color vision is how we perceive the objective color, which people, animals and machines are able to distinguish 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 signals 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. [3] [4]

Object illuminance

The phenomenon of color constancy occurs when the source of illumination is not directly known. [5] It is for this reason that color constancy takes a greater effect on days with sun and clear sky as opposed to days that are overcast. [5] Even when the sun is visible, color constancy may affect color perception. 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. [6]

D. H. Foster (2011) 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." [5] The wide spectrum of possible illuminances in the natural environment and the limited ability of the human eye to perceive color means that color constancy plays a functional role in daily perception. Color constancy allows for humans to interact with the world in a consistent or veridical manner [7] and it allows for one to more effectively make judgements on the time of day. [6] [8]

Physiological basis

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. [9] [10] There was considerable debate about the existence of these cells in the primate visual system; their existence was eventually proven using reverse-correlation receptive field mapping and special stimuli that selectively activate single cone classes at a time, so-called "cone-isolating" stimuli. [11] [12] Human brain imaging evidence strongly suggests that a critical cortical locus for generating color constancy is located in cortical area V4, [13] damage in which leads to the syndrome of cerebral achromatopsia.

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 [14] 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.

Neural mechanism

There are two possible mechanisms for color constancy. The first mechanism is unconscious inference. [15] The second view holds this phenomenon to be caused by sensory adaptation. [16] [17] Research suggests color constancy to be related changes in retinal cells as well as cortical areas related to vision. [18] [19] [20] This phenomenon is most likely attributed to changes in various levels of the visual system. [5]

Cone adaptation

Cones, specialized cells within the retina, will adjust relative to light levels within the local environment. [20] This occurs at the level of individual neurons. [21] However, this adaptation is incomplete. [5] Chromatic adaptation is also regulated by processes within the brain. Research in monkeys suggest that changes in chromatic sensitivity is correlated to activity in parvocellular lateral geniculate neurons. [22] [23] Color constancy may be both attributed to localized changes in individual retinal cells or to higher level neural processes within the brain. [21]

Metamerism

Metamerism, the perceiving of colors within two separate scenes, can help to inform research regarding color constancy. [24] [25] Research suggests that when competing chromatic stimuli are presented, spatial comparisons must be completed early in the visual system. 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. [26] This means that color judgements, as they relate to spatial comparisons, must be completed at or prior to the V1 monocular neurons. [26] [27] [28] If spatial comparisons occur later in the visual system such as in cortical area V4, the brain would be able to perceive both the color and void color as though they were seen in a binocular fashion.

Retinex theory

The "Land effect" is the capacity to see full color images solely by looking at superimposed images of black and white transparancies of the same scene, one taken through a red filter and the other taken through a green filter, and illuminated by red and white light, respectively (or even by two different yellow 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 saw that, even when only yellow light illuminated the superimposed images, the visual system would still perceive a full (if muted) range of color. Land described this effect in a 1959 article in Scientific American. [29] , [4] In 1977, Land wrote another Scientific American article that described a generalized Land effect, leading to formulation of his "Retinex Theory" to explain what he believed was main basis of human color vision. [30] The word "retinex" is a blend of "retina" and "cortex", suggesting that both the eye and the brain are involved in the processing.

The generalized Land 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.

Land, with John McCann, also developed a computer program designed to imitate the retinex processes thought to be taking place in human physiology. [31] Color constancy is a desirable feature of computer vision, and many algorithms have been developed for this purpose. These include several retinex algorithms. [32] [33] [34] [35] These algorithms receive as input the red/green/blue values of each pixel of the image and attempt to estimate the reflectances of each point. One such algorithm operates as follows: the maximal red value rmax of all pixels is determined, and also the maximal green value gmax and the maximal blue value bmax. 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 (rmax, gmax, bmax). For each pixel with values (r, g, b) its reflectance is estimated as (r/rmax, g/gmax, b/bmax). The original retinex algorithm proposed by Land and McCann uses a localized version of this principle. [36] [37]

Although retinex models are still widely used in computer vision, actual human color perception has been shown to be more complex. [38]

See also

Related Research Articles

<span class="mw-page-title-main">Color</span> Visual perception of the light spectrum

Color or colour is the visual perception based on the electromagnetic spectrum. Though color is not an inherent property of matter, color perception is related to an object's light absorption, reflection, emission spectra, and interference. For most humans, colors are perceived in the visible light spectrum with three types of cone cells (trichromacy). Other animals may have a different number of cone cell types or have eyes sensitive to different wavelengths, such as bees that can distinguish ultraviolet, and thus have a different color sensitivity range. Animal perception of color originates from different light wavelength or spectral sensitivity in cone cell types, which is then processed by the brain.

<span class="mw-page-title-main">Optical illusion</span> Visually perceived images that differ from objective reality

In visual perception, an optical illusion is an illusion caused by the visual system and characterized by a visual percept that arguably appears to differ from reality. Illusions come in a wide variety; their categorization is difficult because the underlying cause is often not clear but a classification proposed by Richard Gregory is useful as an orientation. According to that, there are three main classes: physical, physiological, and cognitive illusions, and in each class there are four kinds: Ambiguities, distortions, paradoxes, and fictions. A classical example for a physical distortion would be the apparent bending of a stick half immerged in water; an example for a physiological paradox is the motion aftereffect. An example for a physiological fiction is an afterimage. Three typical cognitive distortions are the Ponzo, Poggendorff, and Müller-Lyer illusion. Physical illusions are caused by the physical environment, e.g. by the optical properties of water. Physiological illusions arise in the eye or the visual pathway, e.g. from the effects of excessive stimulation of a specific receptor type. Cognitive visual illusions are the result of unconscious inferences and are perhaps those most widely known.

<span class="mw-page-title-main">Eye</span> Organ that detects light and converts it into electro-chemical impulses in neurons

An eye is a sensory organ that allows an organism to perceive visual information. It detects light and converts it into electro-chemical impulses in neurons (neurones). It is part of an organism's visual system.

<span class="mw-page-title-main">Color vision</span> Ability to perceive differences in light frequency

Color vision, a feature of visual perception, is an ability to perceive differences between light composed of different frequencies independently of light intensity.

<span class="mw-page-title-main">Visual system</span> Body parts responsible for vision

The visual system is the physiological basis of visual perception. The system detects, transduces and interprets information concerning light within the visible range to construct an image and build a mental model of the surrounding environment. The visual system is associated with the eye and functionally divided into the optical system and the neural system.

Chromatic adaptation is the human visual system’s ability to adjust to changes in illumination in order to preserve the appearance of object colors. It is responsible for the stable appearance of object colors despite the wide variation of light which might be reflected from an object and observed by our eyes. A chromatic adaptation transform (CAT) function emulates this important aspect of color perception in color appearance models.

The flicker fusion threshold, also known as critical flicker frequency or flicker fusion rate, is the frequency at which a flickering light appears steady to the average human observer. It is a concept studied in vision science, more specifically in the psychophysics of visual perception. A traditional term for "flicker fusion" is "persistence of vision", but this has also been used to describe positive afterimages or motion blur. Although flicker can be detected for many waveforms representing time-variant fluctuations of intensity, it is conventionally, and most easily, studied in terms of sinusoidal modulation of intensity.

<span class="mw-page-title-main">Tetrachromacy</span> Type of color vision with four types of cone cells

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<span class="mw-page-title-main">Afterimage</span> Image that continues to appear in the eyes after a period of exposure to the original image

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<span class="mw-page-title-main">Trichromacy</span> Possessing of three independent channels for conveying color information

Trichromacy or trichromatism is the possession of three independent channels for conveying color information, derived from the three different types of cone cells in the eye. Organisms with trichromacy are called trichromats.

<span class="mw-page-title-main">Cone cell</span> Photoreceptor cells responsible for color vision made to function in bright light

Cone cells or cones are photoreceptor cells in the retinas of vertebrates' eyes. They respond differently to light of different wavelengths, and the combination of their responses is responsible for color vision. Cones function best in relatively bright light, called the photopic region, as opposed to rod cells, which work better in dim light, or the scotopic region. Cone cells are densely packed in the fovea centralis, a 0.3 mm diameter rod-free area with very thin, densely packed cones which quickly reduce in number towards the periphery of the retina. Conversely, they are absent from the optic disc, contributing to the blind spot. There are about six to seven million cones in a human eye, with the highest concentration being towards the macula.

<span class="mw-page-title-main">Visual acuity</span> Clarity of vision

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The opponent process is a color theory that states that the human visual system interprets information about color by processing signals from photoreceptor cells in an antagonistic manner. The opponent-process theory suggests that there are three opponent channels, each comprising an opposing color pair: red versus green, blue versus yellow, and black versus white (luminance). The theory was first proposed in 1892 by the German physiologist Ewald Hering.

<span class="mw-page-title-main">Colour centre</span> Brain region responsible for colour processing

The colour centre is a region in the brain primarily responsible for visual perception and cortical processing of colour signals received by the eye, which ultimately results in colour vision. The colour centre in humans is thought to be located in the ventral occipital lobe as part of the visual system, in addition to other areas responsible for recognizing and processing specific visual stimuli, such as faces, words, and objects. Many functional magnetic resonance imaging (fMRI) studies in both humans and macaque monkeys have shown colour stimuli to activate multiple areas in the brain, including the fusiform gyrus and the lingual gyrus. These areas, as well as others identified as having a role in colour vision processing, are collectively labelled visual area 4 (V4). The exact mechanisms, location, and function of V4 are still being investigated.

Visual perception is the ability to interpret the surrounding environment through photopic vision, color vision, scotopic vision, and mesopic vision, using light in the visible spectrum reflected by objects in the environment. This is different from visual acuity, which refers to how clearly a person sees. A person can have problems with visual perceptual processing even if they have 20/20 vision.

Visual object recognition refers to the ability to identify the objects in view based on visual input. One important signature of visual object recognition is "object invariance", or the ability to identify objects across changes in the detailed context in which objects are viewed, including changes in illumination, object pose, and background context.

<span class="mw-page-title-main">Impossible color</span> Color that cannot be perceived under ordinary viewing conditions

Impossible colors are colors that do not appear in ordinary visual functioning. Different color theories suggest different hypothetical colors that humans are incapable of perceiving for one reason or another, and fictional colors are routinely created in popular culture. While some such colors have no basis in reality, phenomena such as cone cell fatigue enable colors to be perceived in certain circumstances that would not be otherwise.

<span class="mw-page-title-main">Parasol cell</span>

A parasol cell, sometimes called an M cell or M ganglion cell, is one type of retinal ganglion cell (RGC) located in the ganglion cell layer of the retina. These cells project to magnocellular cells in the lateral geniculate nucleus (LGN) as part of the magnocellular pathway in the visual system. They have large cell bodies as well as extensive branching dendrite networks and as such have large receptive fields. Relative to other RGCs, they have fast conduction velocities. While they do show clear center-surround antagonism, they receive no information about color. Parasol ganglion cells contribute information about the motion and depth of objects to the visual system.

Bevil Conway, is a Zimbabwean neuroscientist, visual artist, and an expert in color. Conway specialises in visual perception in his scientific work, and he often explores the limitations of the visual system in his artwork. At Wellesley College, Conway was Knafel Assistant Professor of Natural Science from 2007 to 2011, and associate professor of Neuroscience until 2016. He was a founding member of the Neuroscience Department at Wellesley. Prior to joining the Wellesley faculty, Conway helped establish the Kathmandu University Medical School in Nepal, where he taught as assistant professor in 2002–03. He currently runs the Sensation, Cognition and Action Unit in the Laboratory of Sensorimotor Research at the National Eye Institute and the National Institute of Mental Health.

<span class="mw-page-title-main">Memory color effect</span>

The memory color effect is the phenomenon that the canonical hue of a type of object acquired through experience can directly modulate the appearance of the actual colors of objects.

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Retinex

Here "Reprinted in McCann" refers to McCann, M., ed. 1993. Edwin H. Land's Essays. Springfield, Va.: Society for Imaging Science and Technology.