Visual communication at very low data rates

A study is reported of the extraction of two-level cartoons from moving grey-level images for visual communication in the range 4.8-19.2 kbits/s. It is postulated that perceptually significant features of the human face and hands, at which cartoon lines should be drawn in the image, occur wherever surfaces in object space are approximately tangential to the line of sight of the camera or viewer. The implications of this postulate are analyzed for smooth lambertian reflectors seen against a background of the same luminance factor; it is shown that the combined effects of surface gradient and illumination lead to the formation of luminance valleys at the significant features. An experimental comparison of several detectors is described which confirmed the advantage of valley-based detection for deriving economical but recognizable cartoons.