THE VISUAL-PERCEPTION OF 3-DIMENSIONAL LENGTH

A set of 4 experiments evaluated observers' sensitivity to three-dimen sional (3-D) length, using both discrimination and adjustment paradigm s with computer-generated optical pat terns and real objects viewed di rectly in a natural environment. Although observers were highly sensit ive to small differences in two-dimensional length for line segments p resented in the frontoparallel plane, their discrimination thresholds increased by an order of magnitude when the line segments were present ed at random orientations in 3-D space. There were also large failures of constancy, such that the perception of 3-D length varied systemati cally with viewing distance, even under full-cue conditions.