Lightness Perception and Lightness Illusions

“Every light is a shade, compared to the higher lights, till you come to the sun; and every shade is a light, compared to the deeper shades, till you come to the night.”

—John Ruskin, 1879.

Image18.gif
Figure 18: White’s illusion. The gray strips are all the same shade of gray.

“White’s illusion is shown in figure 24.18. The gray rectangles are the same. This is surprising: By local contrast, the left ones should look darker than the right ones. The left rectangles have a long border with white and a short border with black. The illusion is reversed from the usual direction. This effect has been interpreted in terms of the T-junctions (Todorovic, 1997, Gilchrist et al., in press). Patches straddling the stem of a T are grouped together for the lightness computation, and the cross-bar of the T serves as an atmospheric boundary. (cf. Anderson, 1997, for an alternate approach).

Zaidi,Spehar, and Shy (1997) have shown that the action of T-junctions can be so strong that it overpowers traditional grouping cues such as coplanarity. Therefore the grouping rules for the lightness computation evidently differ from those underlying subjective belongingness.”

T-junctions and White’s illusion

Lightness Perception and Lightness Illusions

by Edward H. Adelson
Department of Brain and Cognitive Sciences
Massachusetts Institute of Technology

Be forewarned, this is not light reading.

Print it in Moleskine MSK format
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