A computer implemented method for sensing occupancy of a workspace includes creating a difference image that represents luminance differences of pixels in past and current images of the workspace resulting from motion in the workspace, determining motion occurring in regions of the workspace based on the difference image, and altering a workspace environment based at least in part on the determined motion. The method also includes determining which pixels in the difference image represent persistent motion that can be ignored and determining which pixels representing motion in the difference image are invalid because the pixels are isolated from other pixels representing motion.