1. A method, comprising: performing, by one or more computing devices: obtaining a trimap for a frame in an image sequence comprising a plurality of frames, wherein the trimap indicates a segmentation of an image in the frame into a known foreground region, a known background region, and an unknown region between the foreground region and the background region; propagating the unknown region for the frame to a next frame in the image sequence according to optical flow information that indicates movement of image content between the frame and the next frame; and generating a trimap for the next frame according to the propagated unknown region.