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. 2015 Jul 1;9:387. doi: 10.3389/fnhum.2015.00387

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Copyright © 2015 Smith, Ropar and Allen.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution and reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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When a cup is viewed obliquely, the retinal projection of the lip of the cup forms an ellipse. Despite the apparent transformation, the viewer knows that the “real shape” of the lip of the cup is circular—this is the phenomenon of shape constancy. When asked to reproduce the retinal projection of the ellipse, shape constancy means that the observer is unable to accurately determine the true retinal projection. Instead, the observer will reproduce what is labeled the “perceived shape”, represented in the above image as being between the retinal projection and the real shape.