The dress in Figure 6.20 showed an extreme case that results in color confusion across people due to the strange lighting conditions. Ordinarily, human color perception is surprisingly robust to the source of color. A red shirt appears to be red whether illuminated under indoor lights at night or in direct sunlight. These correspond to vastly different cases in terms of the spectral power distribution that reaches the retina. Our ability to perceive an object as having the same color over a wide variety of lighting conditions is called color constancy. Several perceptual mechanisms allow this to happen. One of them is chromatic adaptation, which results in a shift in perceived colors due to prolonged exposure to specific colors. Another factor in the perceived color is the expectation from the colors of surrounding objects. Furthermore, memory about how objects are usually colored in the environment biases our interpretation.

Figure 6.23: (a) The perceived hot air balloon colors are perceived the same regardless of the portions that are in direct sunlight or in a shadow. (Figure by Wikipedia user Shanta.) (b) The checker shadow illusion from Section 2.3 is explained by the lightness constancy principle as the shadows prompt compensation of the perceived lightness. (Figure by Adrian Pingstone; original by Edward H. Adelson.)
\psfig{file=figs/hotairballoon....,width=2.7truein} \\
(a) & (b)

The constancy principle also appears without regard to particular colors. Our perceptual system also maintains lightness constancy so that the overall brightness levels appear to be unchanged, even after lighting conditions are dramatically altered; see Figure 6.23(a). Under the ratio principle theory, only the ratio of reflectances between objects in a scene are perceptually maintained, whereas the overall amount of reflected intensity is not perceived. Further complicating matters, our perception of object lightness and color are maintained as the scene contains uneven illumination. A clear example is provided from shadows cast by one object onto another. Our perceptual system accounts for the shadow and adjusts our perception of the object shade or color. The checker shadow illusion shown in Figure 6.23 is caused by this compensation due to shadows.

Steven M LaValle 2020-01-06