Notes on the Illusions - Java Puzzlers: Traps, Pitfalls, and Corner Cases - page 329

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Subjective Contours
A subjective contour , also known as an illusory contour , is a perceived edge that does not exist. The
classic example is found in the Kanizsa triangle (pages 146 and 242), devised by Gestalt
psychologist Gaetano Kanizsa in 1955. A white triangle appears to float above a black triangular
outline, but the lines that form the white triangle don't exist. Your mind constructs them from the
contours implied by the "Pac-Man" figures. The variant on page 147 is based on a figure devised by
Branka Spehar [Spehar00] , and the variant on pages 15 and 168 is based on a figure devised by
Takeo Watanabe and Patrick Cavanagh [Watanabe92] . The three-dimensional variants on pages
100 and 101 are based loosely on the Subjective Necker cube of Bradley and Petry, discussed later
in this appendix. The Kanizsa Dot Window on page 69 is based on a figure drawn by Kanizsa in
1979.
The Shadow Letters on page 27 are also a subjective contour illusion. Your mind perceives the
letters A, B, and C, when all that is present is the shadows that these letters would cast.
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