Editing Color constancy (section)

{{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.