Abstract
Early research employing computer-generated dihedrals suggested that depth from motion parallax (MP) is unstable and depth-sign ambiguous. The pursuit theory of motion parallax suggests that perceptual instability and depth-sign reversals are the result of stimulus parameters exceeding the physically-possible conditions for a rigid stimulus as described by the Motion/Pursuit Law. When the Motion/Pursuit Ratio approaches 1, depth from MP approaches infinity. The visual system may recognize when stimulus retinal image velocity exceeds pursuit velocity (M/PR > 1,) representing an impossible rigid object. Such conditions are easily generated with computer-generated MP stimuli, producing depth-sign reversals and perhaps concomitant changes to perceived depth magnitude. To test this, we are measured perceived depth magnitude, in addition to depth-sign, with dihedral stimuli.
Psychophysical stimuli included both physical and virtual dihedrals having two planes intersecting with a vertex on the horizontal meridian, facing or opposing the observer. With the 40 cm viewing distance, both stimulus types subtended 10.6 degrees. The virtual random-dot MP stimuli used M/PR gradients to depict the two slanted planes with varying relative depth magnitudes (60-110 mm) and therefore varying slants (19-30 degrees). Physical stimuli were 3D printed with varying slants (15-75 degrees). Observers indicated both slant of the front-facing plane using orientation of their palm, and perceived depth-sign of the vertex.
With full-cue viewing of physical stimuli, observers underestimated slant by half (individuals varied between 0.3-0.7). Because an underestimate of slant signals an increase in perceived depth, the reported slants of MP stimuli were corrected with functions derived from the physical stimuli. Subsequently with MP stimuli, observers overestimated slopes indicating, similar to previous work, remarkable underestimation of perceived depth magnitude. In addition, increasing MP stimulus parameters to represent depth magnitudes that are impossible within the framework of the Motion/Pursuit Law produces both depth-sign reversals and reductions in perceived depth magnitude.