Illusion defined
Illusions of lightness and brightness can help reveal the nature of lightness computation in the human visual system.
It appears that low-level, mid-level, and high-level factors can all be involved. In this chapter we have emphasized the
phenomena related to mid-level processing. Our evidence, along with the evidence of other researchers, supports the notion that statistical and configural information are combined to estimate the lightness mapping at a given image location. In outline, picture looks like this:
• At every point in an image, there exists an apparent atmospheric transfer function (ATF) mapping reflectance
into luminance. To estimate reflectance given luminance, the visual system must invert the mapping, implicitly or explicitly.
The inverting function at each point may be called the lightness transfer function (LTF).
• The lightness of a given patch is computed by comparing its luminance to a weighted distribution of neighboring
luminances. The exact computation remains unknown.
• Classical mechanisms of perceptual grouping can influence the weights assigned to patches during the lightness computation. The mechanisms may include proximity, good continuation, similarity, and so on. However, the grouping used by the lightness system apparently differs from ordinary perceptual grouping.
• The luminance statistics are gathered within an adaptive window. When the samples are plentiful the window remains
small, but when the samples are sparse the window expands. The window is soft-edged.
• The adaptive window can change shape and size in order to avoid mixing information from different atmospheres.
• Certain junction types offer evidence that a given contour is the result of a change in atmosphere. The contour then acts as an atmospheric boundary, preventing the information on one side from mixing with that on the other. A series of junctions aligned consistently along a contour produce a strong atmospheric boundary. Some evidence suggests that straight contours make better atmospheric boundaries than curved ones.
