A set of overlay images are established within a stack of memory maps for superpositioning over a host image on a raster display. The overlay images include legends, grids, cursors, graphs, formats, etc. which are primarily empty background with few actual image pixels. Overlay conflict between corresponding image pixels from different images is resolved by priority logic in accordance with the order of the image planes within the stack. The background pixels maintain a video switch in the host mode for continuing the flow of host data. The absence of background pixels in any plane (that is the presence of overlay image pixels in any plane) keys the video switch to the insert mode for substituting the overlay pixels for corresponding pixels of host data. The resulting composite data stream is decoded and presented to a D/A for display on a CRT. One overlay code is dedicated to background pixels in the overlay memory map, for indicating the absence of overlay pixels to control the status of the video switch. The background code is not decoded and therefore does not pass through the D/As to produce a visible display characteristic. The corresponding code in the host data may therefore represent a visible characteristic unique to the host image, such as keying a window within the host display to incorporate a secondary host image.