This routine is used to acquire new features in a scene. It selects a relatively uniform scattering, to minimize the probability of missing important obstacles, and chooses distictve areas for unambiguous correlation. This is achieved by returning regions which are local maxima of a directional variance measure. The variance of the window is the minimum of these four sums. The operator is applied to a reduced version of the picture, where the specified window size shrinks to 2 or 3 pixels.
Noise sensitivity is reduced and speed increased.
Partly hand coded, the routine takes 75 ms for a 260x240 image, with 8x8 windows. Given a feature in one picture, the correlator attempts to find the matching region in another image. The search uses a coarse to fine strategy, which begins in reduced versions of the pictures. The order of reduction is chosen to shrink the smaller dimension of the search rectangle to between n and 2n pixels. An n by n window in the shrunken source image, centered on the desired feature, is considered.