The role of stereopsis, motion parallax, perspective and angle polarity in perceiving 3-D shape.

We studied how stimulus attributes (angle polarity and perspective) and data-driven signals (motion parallax and binocular disparity) affect recovery of 3-D shape. We used physical stimuli, which consisted of two congruent trapezoids forming a dihedral angle. To study the effects of the stimulus attributes, we used 2 × 2 combinations of convex/concave angles and proper/reverse perspective cues. To study the effects of binocular disparity and motion parallax, we used 2 × 2 combinations of monocular/binocular viewing with moving/stationary observers. The task was to report the depth of the right vertical edge relative to a fixation point positioned at a different depth. In Experiment 1 observers also had the option of reporting that the right vertical edge and fixation point were at the same depth. However, in Experiment 2, observers were only given two response options: is the right vertical edge in front of/behind the fixation point? We found that across all stimulus configurations, perspective is a stronger cue than angle polarity in recovering 3-D shape; we also confirm the bias to perceive convex compared to concave angles. In terms of data-driven signals, binocular disparity recovered 3-D shape better than motion parallax. Interestingly, motion parallax improved performance for monocular viewing but not for binocular viewing.