1. A method comprising: receiving, from a video camera, data characterizing a plurality of digital video frames; detecting a plurality of features in each of the plurality of digital video frames; determining, from the detected features, a local scale change and a translational motion of one or more groups of features between at least a pair of the plurality of digital video frames, wherein the local scale change characterizes an inter-frame rate of expansion in an area defined by a convex hull of the one or more groups of features, wherein the translational motion characterizes an inter-frame difference in pixel position of a center of mass of the group of features; calculating a likelihood of collision, wherein the likelihood of collision is a ratio between the local scale change and the translational motion; and providing the likelihood of collision, wherein at least one of the receiving, detecting, determining, calculating, and providing is performed by at least one data processor forming part of at least one computing system.