Jump to ContentJump to Main Navigation
The Oxford Compendium of Visual Illusions$
Users without a subscription are not able to see the full content.

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

Show Summary Details
Page of

PRINTED FROM OXFORD SCHOLARSHIP ONLINE (oxford.universitypressscholarship.com). (c) Copyright Oxford University Press, 2021. All Rights Reserved. An individual user may print out a PDF of a single chapter of a monograph in OSO for personal use. date: 28 September 2021

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

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

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

Jan Koenderink

Andrea van Doorn

Johan Wagemans

Oxford University Press

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

Oxford Scholarship Online requires a subscription or purchase to access the full text of books within the service. Public users can however freely search the site and view the abstracts and keywords for each book and chapter.

Please, subscribe or login to access full text content.

If you think you should have access to this title, please contact your librarian.

To troubleshoot, please check our FAQs , and if you can't find the answer there, please contact us .