Editing Color constancy

{{short description|How humans perceive color}}
{{Use American English|date=March 2021}}
{{Use mdy dates|date=March 2021}}

[[Image:Hot air balloon - color constancy.jpg|upright=1.2|thumb|Color constancy: The colors of a [[hot air balloon]] are recognized as being the same in sun and shade.]]
[[Image:Mountain-spring-redwhite.png|upright=1.2|thumb|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.)]]
<!-- [[Image:Stühle Froschperspektive.jpg|upright=1.2|thumb|Automatically making assumptions about [[reflectance]] and the type and source of light]] --> <!-- I don't see the point of this image, as there is no comparison. -->
[[File:Checker shadow illusion.svg|upright=1.2|thumb|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]].]]
[[Image:JohnMartin TheBard RetinexFilter.jpg|thumb|upright=1.2|Achieving luminance constancy by retinex filtering for image analysis]]
[[Image:ColourIllusion2.jpg|thumb|upright=1.2|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.

==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:<blockquote>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.<ref>{{Cite book |last1=Boudrioua |first1=Azzedine |url=https://books.google.com/books?id=WD0PEAAAQBAJ&dq=Al-Haytham+described+color+constancy+by+observing+that+light+reflected+by+an+object+is+modified+by+the+color+of+the+object&pg=PA78 |title=Light-Based Science: Technology and Sustainable Development, The Legacy of Ibn al-Haytham |last2=Rashed |first2=Roshdi |last3=Lakshminarayanan |first3=Vasudevan |year=2017 |publisher=CRC Press |isbn=978-1-4987-7940-1 |language=en}}</ref></blockquote> 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.<ref>{{Cite journal |last=Foster |first=David H. |date=2011-04-13 |title=Color constancy |url=https://www.sciencedirect.com/science/article/pii/S0042698910004402 |journal=Vision Research |series=Vision Research 50th Anniversary Issue: Part 1 |language=en |volume=51 |issue=7 |pages=674–700 |doi=10.1016/j.visres.2010.09.006 |pmid=20849875 |s2cid=1399339 |issn=0042-6989}}</ref>

==Color vision==
{{main|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, [[Cone cells|cones]] and [[Rod cells|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.<ref name=krantz>{{cite book |last=Krantz |first=John |title=Experiencing Sensation and Perception |year=2009 |publisher=Pearson Education, Limited |isbn=978-0-13-097793-9 |pages=9.9–9.10 |url=http://www.saylor.org/content/krantz_sensation/Experiencing_Sensation_and_Perception.pdf |access-date=2012-01-23 |archive-url=https://web.archive.org/web/20171117002814/http://www.saylor.org/content/krantz_sensation/Experiencing_Sensation_and_Perception.pdf |archive-date=2017-11-17 |url-status=dead }}</ref><ref name=wendy>{{Cite web|url=http://www.wendycarlos.com/colorvis/color.html|title = Wendy Carlos ColorVision1}}</ref>

== Object illuminance ==
The phenomenon of color constancy occurs when the source of illumination is not directly known.<ref name=":1">{{cite journal | last1 = Foster | first1 = David H. | year = 2011| title = Color Constancy | journal = Vision Research | volume = 51 | issue = 7| pages = 674–700 | doi = 10.1016/j.visres.2010.09.006 | pmid = 20849875 | s2cid = 1399339 | doi-access = free }}</ref> 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.<ref name=":1"/> Even when the sun is visible, color constancy may affect [[Color vision|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.<ref name="Jameson 1989">{{cite journal | last1 = Jameson | first1 = D. | last2 = Hurvich | first2 = L. M. | year = 1989 | title = Essay concerning color constancy | journal = [[Annual Review of Psychology]] | volume = 40 | pages = 1–22 | doi=10.1146/annurev.psych.40.1.1| pmid = 2648972 }}</ref>

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."<ref name=":1"/> 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<ref>{{cite book |last=Zeki |first=Semir |year=1993 |title=A vision of the brain |location=Oxford |publisher=Blackwell Science Ltd.|isbn=0632030542}}{{page?|date=August 2024}}</ref> and it allows for one to more effectively make judgements on the time of day.<ref name="Jameson 1989" /><ref>{{cite journal | last1 = Reeves | first1 = A | year = 1992 | title = Areas of ignorance and confusion in color science | journal = Behavioral and Brain Sciences | volume = 15 | pages = 49–50 | doi=10.1017/s0140525x00067510| s2cid = 146841846 }}</ref>

==Physiological basis==
The physiological basis for color constancy is thought to involve specialized [[neuron]]s 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.<ref>{{Cite journal |doi=10.1126/science.158.3803.942 |title=Goldfish Retina: Organization for Simultaneous Colour Contrast |first=Nigel W. |last=Daw   |journal=Science |date=17 November 1967 |volume=158 |issue=3803 |pages=942–944 |pmid=6054169|bibcode=1967Sci...158..942D |s2cid=1108881 }}</ref><ref>{{Cite book |title=Neural Mechanisms of Color Vision: Double-Opponent Cells in the Visual Cortex |author=Bevil R. Conway |url=https://books.google.com/books?id=pFodUlHfQmcC&pg=PR7 |publisher=Springer |year=2002 |isbn=978-1-4020-7092-1}}</ref> 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.<ref>{{cite journal | last1 = Conway | first1 = BR | last2 = Livingstone | first2 = MS | year = 2006 | title = Spatial and Temporal Properties of Cone Signals in Alert Macaque Primary Visual Cortex (V1) | journal = Journal of Neuroscience | volume = 26 | issue = 42| pages = 10826–10846 | doi=10.1523/jneurosci.2091-06.2006| pmid = 17050721 | pmc = 2963176 }} [cover illustration].</ref><ref>{{cite journal | last1 = Conway | first1 = BR | year = 2001 | title = Spatial structure of cone inputs to color cells in alert macaque primary visual cortex (V-1) | journal = Journal of Neuroscience | volume = 21 | issue = 8| pages = 2768–2783 | doi = 10.1523/JNEUROSCI.21-08-02768.2001 | pmid = 11306629 | pmc = 6762533 }} [cover illustration].</ref> Human brain imaging evidence strongly suggests that a critical cortical locus for generating color constancy is located in cortical area V4,<ref>{{Cite journal|last1=Bartels|first1=A.|last2=Zeki|first2=S.|date=2000|title=The architecture of the colour centre in the human visual brain: new results and a review *|url=https://onlinelibrary.wiley.com/doi/abs/10.1046/j.1460-9568.2000.00905.x|journal=European Journal of Neuroscience|language=en|volume=12|issue=1|pages=172–193|doi=10.1046/j.1460-9568.2000.00905.x|pmid=10651872|s2cid=6787155|issn=1460-9568|url-access=subscription}}</ref> 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 cell]]s of the [[Human eye|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''<ref>"Discounting the illuminant" is a term introduced by [[Helmholtz]]: {{cite conference |title=Do humans discount the illuminant? |book-title=Proceedings of [[SPIE]] |volume=5666 |conference=Human Vision and Electronic Imaging X |first=John J. |last=McCann |editor1=Bernice E. Rogowitz |editor2=Thrasyvoulos N. Pappas |editor3=Scott J. Daly |date=March 2005 |pages=9–16 |doi=10.1117/12.594383}}</ref> 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.<ref>{{cite journal | last1 = Judd | first1 = D. B. | year = 1940 | title = Hue saturation and lightness of surface colors with chromatic illumination | journal = Journal of the Optical Society of America | volume = 30 | issue = 1 | pages = 2–32 | doi = 10.1364/JOSA.30.000002 | bibcode = 1940JOSA...30....2J }}</ref> The second view holds this phenomenon to be caused by sensory adaptation.<ref>{{cite journal | last1 = Helson | first1 = H | year = 1943 | title = Some factors and implications of color constancy | journal = Journal of the Optical Society of America | volume = 33 | issue = 10| pages = 555–567 | doi=10.1364/josa.33.000555| bibcode = 1943JOSA...33..555H }}</ref><ref>{{cite book |last=Hering |first=E. |orig-date=1920 |title=Grundzüge der Lehre vom Lichtsinn |location=Berlin |publisher=Springer |translator-last1=Hurvich |translator-first1=L. M.  |translator-last2=Jameson |translator-first2=D. |date=1964 |trans-title=Outlines of a theory of the light sense}}</ref>   Research suggests color constancy to be related changes in [[retina]]l cells as well as cortical areas related to vision.<ref>{{cite journal | last1 = Zeki | first1 = S | year = 1980 | title = The representation of colours in the cerebral cortex | journal = Nature | volume = 284 | issue = 5755| pages = 412–418 | doi=10.1038/284412a0| pmid = 6767195 | bibcode = 1980Natur.284..412Z | s2cid = 4310049 }}</ref><ref>{{cite journal | last1 = Zeki | first1 = S | year = 1983 | title = Colour coding in the cerebral cortex: The reaction of cells in monkey visual cortex to wavelengths and colours | journal = Neuroscience | volume = 9 | issue = 4| pages = 741–765 | doi=10.1016/0306-4522(83)90265-8| pmid = 6621877 | s2cid = 21352625 }}</ref><ref name=":02">{{Cite journal|last=Hood|first=D.C.|s2cid=12490019|date=1998|title=Lower-Level Visual Processing and Models of Light Adaptation|journal=[[Annual Review of Psychology]]|volume=49|pages=503–535|doi=10.1146/annurev.psych.49.1.503|pmid=9496631}}</ref> This phenomenon is most likely attributed to changes in various levels of the visual system.<ref name=":1"/>

==== Cone adaptation ====
Cones, specialized cells within the retina, will adjust relative to light levels within the local environment.<ref name=":02" /> This occurs at the level of individual neurons.<ref name="ReferenceA">{{cite journal | last1 = Lee | first1 = B. B. | last2 = Dacey | first2 = D. M. | last3 = Smith | first3 = V. C. | last4 = Pokorny | first4 = J. | year = 1999 | title = Horizontal cells reveal cone type-specific adaptation in primate retina | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 96 | issue = 25| pages = 14611–14616 | doi=10.1073/pnas.96.25.14611| pmid = 10588753 | pmc = 24484 | bibcode = 1999PNAS...9614611L | doi-access = free }}</ref> However, this adaptation is incomplete.<ref name=":1" /> [[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 cell|parvocellular]] [[Lateral geniculate nucleus|lateral geniculate]] neurons.<ref>{{cite journal | last1 = Creutzfeldt | first1 = O. D. | last2 = Crook | first2 = J. M. | last3 = Kastner | first3 = S. | last4 = Li | first4 = C.-Y. | last5 = Pei | first5 = X. | year = 1991 | title = The neurophysiological correlates of colour and brightness contrast in lateral geniculate neurons: 1. Population analysis | journal = Experimental Brain Research | volume = 87 | issue = 1| pages = 3–21 | doi=10.1007/bf00228503| pmid = 1756832 | s2cid = 1363735 }}</ref><ref>{{cite journal | last1 = Creutzfeldt | first1 = O. D. | last2 = Kastner | first2 = S. | last3 = Pei | first3 = X. | last4 = Valberg | first4 = A. | year = 1991 | title = The neurophysiological correlates of colour and brightness contrast in lateral geniculate neurons: II. Adaptation and surround effects | journal = Experimental Brain Research | volume = 87 | issue = 1 | pages = 22–45 | doi=10.1007/bf00228504| pmid = 1756829 | s2cid = 75794 }}</ref> Color constancy may be both attributed to localized changes in individual retinal cells or to higher level neural processes within the brain.<ref name="ReferenceA"/>

== Metamerism ==
Metamerism, the perceiving of colors within two separate scenes, can help to inform research regarding color constancy.<ref>{{cite journal |last=Kalderon |first=Mark Eli |title=Metamerism, Constancy, and Knowing Which |journal=Mind |volume=117 |issue=468 |year=2008 |pages=935–971 |jstor=20532701 |doi=10.1093/mind/fzn043|url=http://sas-space.sas.ac.uk/616/1/M_Kalderon_Metamerism.pdf }}</ref><ref>{{Cite journal|last=Gupte|first=Vilas|date=2009-12-01|title=Color Constancy, by Marc Ebner (Wiley; 2007) pp 394 {{Text|ISBN}} 978-0-470-05829-9 (HB)|journal=Coloration Technology|language=en|volume=125|issue=6|pages=366–367|doi=10.1111/j.1478-4408.2009.00219.x|issn=1478-4408}}</ref> 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 presentation|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.<ref name="ReferenceB">{{cite journal | last1 = Moutoussis | first1 = K. | last2 = Zeki | first2 = S. | year = 2000 | title = A psychophysical dissection of the brain sites involved in color-generating comparisons | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 97 | issue = 14| pages = 8069–8074 | doi=10.1073/pnas.110570897| pmid = 10859348 | pmc = 16671 | bibcode = 2000PNAS...97.8069M | doi-access = free }}</ref> This means that color judgements, as they relate to spatial comparisons, must be completed at or prior to the [[Visual cortex|V1]] monocular neurons.<ref name="ReferenceB"/><ref>{{cite journal | last1 = Hurlbert | first1 = A. C. | last2 = Bramwell | first2 = D. I. | last3 = Heywood | first3 = C. | last4 = Cowey | first4 = A. | year = 1998 | title = Discrimination of cone contrast changes as evidence for colour constancy in cerebral achromatopsia | journal = Experimental Brain Research | volume = 123 | issue = 1–2| pages = 136–144 | doi=10.1007/s002210050554| pmid = 9835402 | s2cid = 1645601 }}</ref><ref>{{cite journal | last1 = Kentridge | first1 = R. W. | last2 = Heywood | first2 = C. A. | last3 = Cowey | first3 = A. | year = 2004 | title = Chromatic edges, surfaces and constancies in cerebral achromatopsia | journal = Neuropsychologia | volume = 42 | issue = 6| pages = 821–830 | doi=10.1016/j.neuropsychologia.2003.11.002| pmid = 15037060 | s2cid = 16183218 }}</ref> 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 transparencies 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]].''<ref>{{Cite journal|url=http://www.psy.vanderbilt.edu/courses/psy236/ColorVision/Land1959.pdf|title=Experiments in Color Vision|last=Land|first=Edwin|date=May 1959|journal=Scientific American|volume=200|issue=5|pages=84–94 passim|doi=10.1038/scientificamerican0559-84|pmid=13646648|bibcode=1959SciAm.200e..84L}}</ref><ref name=wendy /> 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.<ref>{{Cite journal |last=Land |first=Edwin |date=December 1977 |title=The Retinex Theory of Color Vision |url=https://www.jstor.org/stable/24953876 |journal=Scientific American |volume=237 |issue=6 |pages=108–128 |bibcode=1977SciAm.237f.108L |doi=10.1038/scientificamerican1277-108 |jstor=24953876 |pmid=929159 |archive-url=https://web.archive.org/web/20240311155053/https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=d34b21123f86b8515e1150472efbb42147654a18 |archive-date=2024-03-11|url-access=subscription }}</ref> The word "retinex" is a [[blend word|blend]] of "[[retina]]" and "[[Cerebral cortex|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.<ref>J. McCann, S.P. McKee & T. Taylor, "[https://mccannimaging.com/Retinex/Land_McCann_Retinex_files/76MMT%20VisRes.pdf Quantitative Studies in Retinex Theory, A Comparison Between Theoretical Predictions and Observer Responses to Color Mondrian Experiments"], Vision Res. '''16''': 445–458, (1976)</ref> Color constancy is a desirable feature of [[computer vision]], and many algorithms have been developed for this purpose. These include several retinex algorithms.<ref>{{cite journal | doi = 10.1117/12.805474 | title=Fast implementation of color constancy algorithms | year=2009 | journal=Color Imaging XIV: Displaying, Processing, Hardcopy, and Applications | volume=7241 | pages=724106 | last1 = Morel | first1 = Jean-Michel | last2 = Petro | first2 = Ana B. | last3 = Sbert | first3 = Catalina| bibcode=2009SPIE.7241E..06M | editor4-first=Alessandro | editor4-last=Rizzi | editor3-first=Shoji | editor3-last=Tominaga | editor2-first=Gabriel G | editor2-last=Marcu | editor1-first=Reiner | editor1-last=Eschbach | citeseerx=10.1.1.550.4746 | s2cid=19950750 }}</ref><ref>{{cite journal |first1=R. |last1=Kimmel |first2=M. |last2=Elad |first3=D. |last3=Shaked |first4=R. |last4=Keshet |first5=I. |last5=Sobel |url=https://www.cs.technion.ac.il/~ron/PAPERS/retinex_ijcv2003.pdf |title=A Variational Framework for Retinex |journal=International Journal of Computer Vision |volume=52 |issue=1 |pages=7–23 |year=2003 |doi=10.1023/A:1022314423998|s2cid=14479403 }}</ref><ref>{{cite patent |invent1=Barghout, Lauren  |invent2=Lawrence Lee |title=Perceptual information processing system |country=US |status=Patent |number=20040059754A1}}</ref><ref>{{cite book | last=Barghout | first=Lauren | series=Communications in Computer and Information Science | volume=443 | title=Information Processing and Management of Uncertainty in Knowledge-Based Systems | chapter=Visual Taxometric Approach to Image Segmentation Using Fuzzy-Spatial Taxon Cut Yields Contextually Relevant Regions | publisher=Springer International Publishing | publication-place=Cham | year=2014 | isbn=978-3-319-08854-9 | issn=1865-0929 | doi=10.1007/978-3-319-08855-6_17 | pages=163–173}}</ref> 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 ''r''<sub>max</sub> of all pixels is determined, and also the maximal green value ''g''<sub>''max''</sub> and the maximal blue value {{not a typo|''b''<sub>max</sub>}}. 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''<sub>max</sub>, ''g''<sub>max</sub>, ''b''<sub>max</sub>). For each pixel with values (''r'', ''g'', ''b'') its reflectance is estimated as (''r''/''r''<sub>max</sub>, ''g''/''g''<sub>max</sub>, ''b''/''b''<sub>max</sub>). The original retinex algorithm proposed by Land and McCann uses a localized version of this principle.<ref>{{cite journal | last1 = Provenzi | first1 = Edoardo | last2 = De Carli | first2 = Luca | last3 = Rizzi | first3 = Alessandro | last4 = Marini | first4 = Daniele | year = 2005 | title = Mathematical definition and analysis of the Retinex algorithm | journal = JOSA A | volume = 22 | issue = 12| pages = 2613–2621 | doi=10.1364/josaa.22.002613| pmid = 16396021 | bibcode = 2005JOSAA..22.2613P }}</ref><ref>{{cite journal | last1 = Bertalmío | first1 = Marcelo | last2 = Caselles | first2 = Vicent | last3 = Provenzi | first3 = Edoardo | year = 2009 | title = Issues About Retinex Theory and Contrast Enhancement | journal =  International Journal of Computer Vision| volume = 83 | pages = 101–119 | doi=10.1007/s11263-009-0221-5| s2cid = 4613179 }}</ref>

Although retinex models are still widely used in computer vision, actual human color perception has been shown to be more complex.<ref>{{cite conference | last1=Hurlbert | first1=Anya C. | last2=Wolf | first2=Christopher J. L. | editor-first1=Bernice E. | editor-first2=Thrasyvoulos N. | editor-last1=Rogowitz | editor-last2=Pappas | title=Contribution of local and global cone-contrasts to color appearance: a Retinex-like model | series=Human Vision and Electronic Imaging VII | publisher=SPIE | date=2002-06-03 | volume=4662 | pages=286–297 | issn=0277-786X | doi=10.1117/12.469525 }}</ref>

==See also==
*[[Chromatic adaptation]]
*[[Memory color effect]]
*[[Shadow and highlight enhancement]]
*[[Trichromacy]]
*[[Tetrachromacy]]
*''[[Theory of Colours]]''<ref>{{Cite journal|last1=Ribe|first1=N.|last2=Steinle|first2=F.|year=2002|title=Exploratory Experimentation: Goethe, Land, and Color Theory|journal=Physics Today|volume=55|issue=7|pages=43|bibcode=2002PhT....55g..43R|doi=10.1063/1.1506750|doi-access=free}}</ref>

==References==
{{Reflist}}

===Retinex===
Here "Reprinted in McCann" refers to McCann, M., ed. 1993. ''[[Edwin H. Land]]'s Essays.'' Springfield, Va.: Society for Imaging Science and Technology.
*(1964) "The retinex" ''Am. Sci.'' 52(2): 247–264. Reprinted in McCann, vol. III, pp.&nbsp;53–60. Based on acceptance address for [[William Procter Prize for Scientific Achievement]], Cleveland, Ohio, December 30, 1963.
*with L.C. Farney and M.M. Morse. (1971) "Solubilization by incipient development" ''Photogr. Sci. Eng.'' 15(1):4–20. Reprinted in McCann, vol. I, pp.&nbsp;157–173. Based on lecture in Boston, June 13, 1968.
*with J.J. McCann. (1971) "Lightness and retinex theory" ''J. Opt. Soc. Am.'' 61(1):1–11. Reprinted in McCann, vol. III, pp.&nbsp;73–84. Based on the Ives Medal lecture, October 13, 1967.
*(1974) "The retinex theory of colour vision" ''Proc. R. Inst. Gt. Brit.'' 47:23–58. Reprinted in McCann, vol. III, pp.&nbsp;95–112. Based on Friday evening discourse, November 2, 1973.
*(1977) "The retinex theory of color vision" ''Sci. Am.'' 237:108–128. Reprinted in McCann, vol. III, pp.&nbsp;125–242.
*with H.G. Rogers and V.K. Walworth. (1977) "One-step photography" In ''Neblette's Handbook of Photography and Reprography, Materials, Processes and Systems,'' 7th ed., J. M. Sturge, ed., pp.&nbsp;259–330. New York: Reinhold. Reprinted in McCann, vol. I, pp.&nbsp;205–263.
*(1978) "Our 'polar partnership' with the world around us: Discoveries about our mechanisms of perception are dissolving the imagined partition between mind and matter" ''Harv. Mag.'' 80:23–25. Reprinted in McCann, vol. III, pp.&nbsp;151–154.
*with D.H. Hubel, M.S. Livingstone, S.H. Perry, and M.M. Burns. (1983) "Colour-generating interactions across the corpus callosum" ''Nature'' 303(5918):616–618. Reprinted in McCann, vol. III, pp.&nbsp;155–158.
*(1983) "Recent advances in retinex theory and some implications for cortical computations: Color vision and the natural images" ''Proc. Natl. Acad. Sci. U.S.A.'' 80:5136–5169. Reprinted in McCann, vol. III, pp.&nbsp;159–166.
*(1986) "An alternative technique for the computation of the designator in the retinex theory of color vision" ''Proc. Natl. Acad. Sci. U.S.A.'' 83:3078–3080.

==External links==
{{Commons category}}
*[https://web.archive.org/web/20100214093958/http://web.me.com/mccanns/Color/Color_Constancy.html Color constancy – McCann]
*[http://www.colorconstancy.com Color constancy – Illuminant Estimation]
*[https://web.archive.org/web/20010212022353/http://dragon.larc.nasa.gov/retinex/retinex.html Retinex Image Processing]
*[http://www.ipol.im/pub/art/2011/lmps_rpe/ Retinex implemented via a partial differential equation and Fourier transform, with code and on-line demonstration]
*[[List of Horizon episodes|BBC Horizon 21x08 Colourful notions 1985]]

{{Color vision}}

{{DEFAULTSORT:Color Constancy}}
[[Category:Optical illusions]]
[[Category:Color vision]]

[[de:Farbwahrnehmung#Farbkonstanz]]