Abstract: An interactive, computer-based system for combining on a common display, as overlaid images, video signals from a source of video images (such as a video disc) with graphics and text from a computer, particularly for use in computer-aided instruction, computer-based information retrieval and visual annotation or supplementation of video images. The video signals are converted to RGB format (or some other non-phase-modulated format) if not already so encoded. They are then supplied to a switch which receives the text and graphic-signals, also in RGB format, as another input. The output of the switch is supplied to a video display. The switch is controlled by an attribute of the text and graphics signals, for controlling the selection of the video source on a pixel-by-pixel basis.