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.

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.