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; enacting area-wise motion detection on the sequence of depth maps to select from a depth map an area enclosing a moving contour, excluding a non-moving contour, and thereby targeting motion; defining an axis parallel to an average surface normal of one or more contour gradients within the area; selecting as foreground a section of the depth map bounded by the area and lying in front of a plane normal to that axis, and associating with the background, pixels of the depth map located behind the plane or outside the area; and fitting a geometric model of the subject to the section selected as the foreground.