1. A method for controlling a computer system, the method comprising: acquiring video of a subject in front of a background; obtaining from the video a time-resolved sequence of depth maps, each depth map including an array of pixels; fitting a geometric model of the subject to a first depth map in the sequence; registering coordinates of the geometric model to a second depth map in the sequence; selecting from the second depth map a background section lacking coherent motion and located more than a threshold distance from the coordinates of the geometric model; and refitting the geometric model of the subject to the second depth map with the background section excluded, said acquiring obtaining, fitting, registering, selecting and refitting enacted within a computer vision system of the computer system.