Approximationorder of the lap optical flow algorithm

Estimating the displacements between two images is often addressed using a small displacement assumption, which leads to what is known as the optical flow equation. We study the quality of the underlying approximation for the recently developed Local All-Pass (LAP) optical flow algorithm, which is based on another approach — displacements result from filtering. While the simplest version of LAP computes only first-order differences, we show that the order of LAP approximation is quadratic, unlike standard optical flow equation based algorithms for which this approximation is only linear. More generally, the order of approximation of the LAP algorithm is twice larger than the differentiation order involved. The key step in the derivation is the use of Padé approximants.