1. A system comprising: one or more image capture devices that capture a sequence of images that contains a particular type of object; one or more processors; and one or more memories storing instructions that, when executed on the one or more processors, cause the system to: compute movement of individual groups of pixels within the sequence of images; extract motion features based at least in part on the movement of the individual groups of pixels; and determine that the motion features characterize the particular type of object.