An image display control signal is developed from a plurality of bit-plane memories having digital image data stored therein, by applying bits corresponding to the same location that are simultaneously read out from the plurality of bit-plane memories to a control input of a respective one of a corresponding plurality of switches. A digital signal representative of a display characteristic (such as intensity) for each of the bit-plane images is applied to a D/A converter for generating an analog display characteristic signal. The analog display characteristic signal for each bit-plane is applied to the signal input of a respective one of the switches. The signal output of each switch is coupled to a processing circuit which processes the analog display characteristic signals for developing the image display control signal. In a preferred embodiment, the processing comprises selecting the analog display characteristic signal corresponding to the greatest image intensity as the image display control signal.

A scanning display signal generating system for a plurality of planes includes a first look up table addressed by a first set of the planes, and a second look up table addressed by a second set of planes. A logic unit receives the outputs of the tables and provides a display signal which is a selected logical combination of the outputs. A function control unit provides a control signal to the logic unit to select the desired logical combination. A large number of planes are thus displayed using small LUT memory components, providing display values in real time to the scanner. The output of one look up table may be provided as a control signal to the logic unit. In one embodiment the first look up table is addressed by text planes, and an output therefrom provides the control signal for suppressing the output of the second look up table.

An information processing apparatus and method are provided to determine whether to display line drawings according to the size and number of pixels of the display screen. The screen may be divided into nine areas and the images are displayed in each area. When line-drawing information related to the displayed images exists, a determination is made as to whether to display those line drawings superimposed on the images based on the size of the divided areas and the number of pixels therein. When it is anticipated that it is not possible to discriminate the line drawings when displayed, display of the line drawings is not performed and marks (M) indicating that the line-drawing information exists are instead displayed.

A method and apparatus for automatically changing the display in overlapping rectangular viewport areas of a display screen of a digital display apparatus and in which each viewport area is assigned a different priority level. The method includes the steps of (a) storing in a random access store indications of the position and size of each viewport area, together with an indication of the priority level of the viewport area and (b) constructing a first matrix of (2n+1).sup.2 elements, where n is equal to the number of viewport areas, by assigning a vertical component to each vertical coordinate of each viewport area and a horizontal component to each horizontal coordinate of each viewport area, and for each element so formed storing an indication of the highest priority level of the viewports falling within the boundary formed by the coordinates defining the element.

A VGA controller having an overlay mode for displaying images formed from selective components of multiple image planes. A first image and a second image overlaying the first image are interleaved into three of four bit planes of a dynamic random access memory. When displaying the image, the controller will simultaneously read from the first and second images. If the value of the second image is non-zero, then the second image is displayed. Otherwise, the first image is displayed.