Optic flow and the metric of the visual ground plane.

A theory is developed in which the optic flow of an observer translating over the ground plane determines the metric of egocentric visual space. Optic flow is used to operationalize the equality of spatial intervals not unlike physicists use time to compare spatial intervals. The theory predicts empirical matching ratios for collinear, sagittal intervals to within 2% of the mean (eight subjects, standard error also 2%). The theory predicts that frontoparallel intervals on the ground plane will match sagittal intervals if their relative image motions match, which was found empirically. It is suggested that the optic flow metric serves to calibrate static depth cues such as angular elevation and binocular parallax.