What is "color constancy"

n. the tendency for a color to look the same under widely different viewing conditions [syn: colour constancy]

upright=1.2|thumb|Color constancy: The colors of a hot air balloon are recognized as being the same in sun and shade upright=1.2|thumb|Automatically making assumptions about reflectance and the type and source of light upright=1.2|thumb|Constancy makes square A appear darker than square B, when in fact they are both exactly the same shade of grey. See Same color illusion. thumb|upright=1.2|Achieving luminance constancy by retinex filtering for image analysis thumb|upright=1.2|The second card from the left seems to be a stronger shade of pink in the top picture. In fact they are the same color, but the brain changes its assumption about color due to 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.