Systematic distortions of shape from stereopsis.

The effectiveness of disparity information in defining 3-D shape was investigated by means of judgements of the shape of cylindrical continuous curved surfaces presented as random dot stereograms. At a close viewing distance, truly circular cylinders appeared elongated; at an intermediate distance, perception was close to veridical; and, at a far distance, cylinders appeared flattened. Indirect measures of scaling distance were calculated from these data. The results strongly suggest that the observed shape distortions are a consequence of scaling horizontal disparities with an incorrect measure of egocentric distance.