Color constancy

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.

[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.

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.

[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.

The different cone cells of the eye register different but overlapping ranges of wavelengths of the light reflected by every object in the scene.

[5] 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.

[22][23] Color constancy may be both attributed to localized changes in individual retinal cells or to higher level neural processes within the brain.

[24][25] Research suggests that when competing chromatic stimuli are presented, spatial comparisons must be completed early in the visual system.

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.

[30] The word "retinex" is a blend of "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 thought to be taking place in human physiology.

[36][37] Although retinex models are still widely used in computer vision, actual human color perception has been shown to be more complex.

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.)
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
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.