Abstract
Competent realistic drawings preserve viewpoint-invariant shape characteristics of simple parts, such that a contour in the object that is straight or curved, for example, is depicted that way in the drawing. A more subtle invariant—a V-shaped singularity of the occluding boundary, containing a T-junction and a contour termination—is produced at the junction between articulated smooth surfaces, as with the leg joining the body of a horse. 45% of the drawings made in 2007 by individuals with only minimal art education correctly depicted such junctions, a proportion that is not reliably different from the incidence (42%) of correct depictions in a large sample of cave art made 17 000 years ago. Whether a person did or did not include the invariant in their drawing, all agreed that it made for a better depiction.
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