Point-extraction algorithm.

<p>(A) Video images have the average background subtracted and are then filtered to amplify and localize the white dots painted on the cockroach's legs. The colored lines approximate the areas shown in greater detail in panel B, though from different images. (B) Proceeding from proximal to distal points on each leg, a 3D ellipsoid is defined based on the last known position of each point, with its short axis along the line between the point and the next-most-proximal point on the same leg. This ellipsoid is projected onto the image from the camera, and the intensity-weighted centroid of the pixels within the resulting ellipse defines the new estimate of that point's 2D position. The blue lines are the ellipsoid axes. The pink ellipses are the 2D projections of the corresponding ellipsoids constraining the search area, and the red Xs show the centroids of the ellipses.