RASTER SCAN DISPLAY UNIT

Digital display data is stored in first and second memories which are accessed together to provide respective streams of picture element data groups for display on a raster scan display device. A comparator compares each data group from a selected one of the streams with a data group representing a particular color. The comparator output is applied as a control input to a multiplexer which also receives the data streams. When no equality is detected, the multiplexer passes the data groups in the compared stream through to the display device. Whenever equality is detected the multiplexer passes the corresponding data group in the non-compared stream through to the display device. The result is that the displayed image corresponding to the compared stream is made transparent at areas corresponding to the compared color, and there areas are filled in with an image corresponding to data from the non-compared stream of data.