Editing Optical illusion (section)

=== Color and brightness constancies ===
[[File:Gradient-optical-illusion.svg|thumb|Simultaneous contrast illusion. The background is a [[color gradient]] and progresses from dark gray to light gray. The horizontal bar appears to progress from light grey to dark grey, but is in fact just one color.]]
Perceptual constancies are sources of illusions. [[Color constancy]] and brightness constancy are responsible for the fact that a familiar object will appear the same color regardless of the amount of light or color of light reflecting from it. An illusion of color difference or luminosity difference can be created when the luminosity or color of the area surrounding an unfamiliar object is changed. The luminosity of the object will appear brighter against a black field (that reflects less light) than against a white field, even though the object itself did not change in luminosity. Similarly, the eye will compensate for color contrast depending on the color cast of the surrounding area.

In addition to the gestalt principles of perception, water-color illusions contribute to the formation of optical illusions. Water-color illusions consist of object-hole effects and coloration. Object-hole effects occur when boundaries are prominent where there is a figure and background with a hole that is 3D volumetric in appearance. Coloration consists of an assimilation of color radiating from a thin-colored edge lining a darker chromatic contour. The water-color illusion describes how the human mind perceives the wholeness of an object such as top-down processing. Thus, contextual factors play into perceiving the brightness of an object.<ref>{{cite journal | last1 = Tanca | first1 = M. | last2 = Grossberg | first2 = S. | last3 = Pinna | first3 = B. | year = 2010 | title = Probing Perceptual Antinomies with the Watercolor Illusion and Explaining How the Brain Resolves Them | url = http://cns-web.bu.edu/%7Esteve/TanGroPin2010.pdf | journal = Seeing & Perceiving | volume = 23 | issue = 4 | pages = 295–333 | doi = 10.1163/187847510x532685 | pmid = 21466146 | url-status = live | archive-url = https://web.archive.org/web/20170921215931/http://cns-web.bu.edu/%7Esteve/TanGroPin2010.pdf | archive-date = 2017-09-21 | citeseerx = 10.1.1.174.7709 }}</ref>