Observers viewed monocular animations of rotating dihedral angles and were
required to indicate their perceived structures by adjusting the magnitude
and orientation of a stereoscopic dihedral angle. The motion displays were
created by directly manipulating various aspects of the image velocity fiel
d, including the mean translation, the horizontal and vertical velocity gra
dients, and the manner in which these gradients changed over time. The adju
sted orientation of each planar facet was decomposed into components of sla
nt and tilt. Although the tilt component was estimated with a high degree o
f accuracy, the judgments of slant exhibited large systematic errors. The m
agnitude of perceived slant was determined primarily by the magnitude of th
e velocity gradient scaled by its direction. The results also indicate that
higher order temporal derivatives of the moving elements had little effect
on observers' judgments.