Recovering three-dimensional shape from perspective translations and orthographic rotations.

Perceived orientation in depth and 3D shape was investigated for perspective projections of translations and orthographic projections of rotations of 3D dihedral angles. The principal findings were that (a) perceived orientation in depth depends on the sign of the velocity gradient, even in the case of orthographic projections; (b) the relationship between perceived orientation and the sign of the velocity gradient is greater for shallower gradients in orthographic projections of rotations, consistent with previous findings for perspective translations; (c) the magnitudes of simulated dihedral angles were underestimated (relative depth overestimated) for orthographic projections of rotations but were overestimated for perspective projections of translations; and (d) the judged magnitude of the dihedral angle depends on the velocity ratio and on image compression; it cannot be predicted from the velocity ratio or the velocity gradient alone.