Lightness Perception and Lightness Illusions

A gray surface in sunlight may have much higher luminance than it has in the
shade, but it still looks gray. To achieve the task of "lightness constancy,"
the visual system must discount the illumination and other viewing conditions
and estimate the reflectance. Many different physical situations, such as shadows,
filters, or haze, can be combined to form a single, simple mapping from luminance
to reflectance. The net effect of the viewing conditions, including additive
and multiplicative effects, may be termed an "atmosphere." An "atmospheric transfer
function" maps reflectance into luminance. To correctly estimate lightness,
a visual system must determine a "lightness transfer function" that performs
the inverse. Human lightness computation is imperfect, but performs well in
most natural situations. Lightness illusions can reveal the inner workings of
the estimation process, which may involve low-level, mid-level, and high-level
mechanisms. Mid-level mechanisms, involving contours, junctions, and grouping,
appear to be critical in explaining many lightness phenomena.