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The Oxford Compendium of Visual Illusions$
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Arthur G. Shapiro and Dejan Todorovic

Print publication date: 2017

Print ISBN-13: 9780199794607

Published to Oxford Scholarship Online: June 2017

DOI: 10.1093/acprof:oso/9780199794607.001.0001

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The Invisible Saddle, or the Cap-or-Cup Illusion

The Invisible Saddle, or the Cap-or-Cup Illusion

Chapter:
(p.227) Chapter 22 The Invisible Saddle, or the Cap-or-Cup Illusion
Source:
The Oxford Compendium of Visual Illusions
Author(s):

Jan Koenderink

Andrea van Doorn

Johan Wagemans

Publisher:
Oxford University Press
DOI:10.1093/acprof:oso/9780199794607.003.0022

The linear luminance gradient in a circular disk has become a standard example of the “shape from shading cue” in vision science. It is generally supposed to give rise to one of three possible 3D responses, namely “flat” (cue does not work), “cap” (or convexity), or “cup (concavity). From the perspective of ecological optics, there is an infinite set of possibilities, one of which, “saddle,” has—to the best of our knowledge—never been suggested by any human observer. Bayesian convictions do not come to the rescue, because saddles are actually more frequent than caps or cups. The “illusion” is a strong one, because even if we know (e.g., have programmed it) to look at a saddle, we will see a cap (mostly) or cup (sometimes). Thus not only is the shading cue infinitely ambiguous; it is associated with an extreme bias, which might properly be considered an aphasia (soul blindness).

Keywords:   shading cue, shape from shading, saddle, soul blindness, shape, luminance gradient

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