Abstract: A control signal synchronized to a digital video signal to be processed and defining the position of a rectilinear line (51,52,53,54) relatively to the image frame (50) is generated by providing an initial value corresponding to the perpendicular distance of the origin (0) of the scanning point of the video raster from the line (51,52,53,54) and incrementing or decrementing this value during movement of the scanning point in accordance with the rates of change of this distance in the horizontal and vertical directions. By logically combining the signals corresponding to a plurality of lines (52,52,53,54) a further signal is obtained which is "true" when the scanning point is within a geometric figure defined by the lines. This signal can provide a "key" signal controlling the display within the figure of image information to be combined with the video signal.