Cipolla and Blake 1990 : The authors develop previous theories of the analysis of deformation of apparent contours under viewer motion. Earlier results showing how surface curvature can be inferred from acceleration of image features are generalized for arbitrary viewer motion and perspective projection. It is shown that relative image acceleration, based on parallax measurements, is robust to uncertainties in robot motion. The theory has been implemented and extensively tested in a real-time (15 frames per second) tracking system based on deformable contours (snakes). It is shown that focusing attention by means of snakes allows rapid, robust computation of surface curvature, including discrimination of extremal and occluding contours.