The Effects of 2D and 3D Smoothness on Motion Segmentation

M.J. Bravo and H. Farid

Investigative Opthalmology and Visual Science (ARVO), Fort Lauderdale, FL, 1998

Purpose: To measure the sensitivity of observers to small, local perturbations in the flow field produced by the rotation of a rigid plane and to determine whether performance is based solely on detecting deviations in the smoothness of the 2D flow field.

Methods: Test stimuli simulated a textured plane rotating about a vertical or horizontal axis viewed under perspective projection. The plane's texture consisted of eight patches of dots arranged in a circle around the fixation point. As the plane rotated, the patches moved and their shapes changed, however, the shape of one patch, the target, did not change appropriately. That is, at the center of the target patch, the velocity was consistent with the plane, but the spatial derivatives of the velocity were not. The observer's task was to locate this target patch. Control stimuli were generated by transforming the test flow field in two ways: either each vector of the flow field was rotated by 90 degrees or the sign of either the Vx or Vy component of the flow field was inverted. Both transformations preserve the 2D smoothness of the flow, but destroy the 3D percept of a rigid plane.

Results: Subjects were able to locate the target patch in all three stimuli, but they required a larger deviation with the control stimuli compared with the test stimuli.

Conclusions: Observers are sensitive to local perturbations in a smooth 2D flow field, but they appear to be more sensitive to such perturbations when the flow field corresponds to a rigid 3D plane.