The relationships between differential optic flow, motion and surface shape have been derived by Koenderink and van Doorn and others, but relatively little use has been made of them. The present paper has three purposes: First, a review of first-order flow and its relationship to planar structure and observer motion is given, with a minimum of mathematical formalism, in Sections 2-4. Second, a summary of the information that can be obtained from the first-order flow for a single planar surface patch is given in Section 5. Third, for those interested in the derivation of the equations for first-order flow, the details are given in Section 6.
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