Structure from motion often refers to the computation of 3D structure from a matched sequence of images. However, a (relative) depth map of a surface may not be a good representation for storage and recognition; a more concise representation seems necessary. The sign of the Gaussian curvature of a surface is one candidate to be a part of a useful representation of the surface. I will show that in order to compute the sign of the Gaussian curvature it is not necessary first to go through the computationally expensive and error sensitive process of recovering the exact function of the surface and the motion parameters. Keywords: Optical flow; Qualitative vision, Stereo; Gaussian curvature; Focus of expansion.