Data output through graphic devices on computer systems is made possible through techniques that use video generation modules to display images. This differs from text mode output, for which the computer generates horizontal lines of alphanumeric symbols. Although the technical requirements of both systems overlap, graphic devices use an approach that assumes that every dot on the screen is separately accessible. By contrast, in text mode, the smallest screen element is actually a group of points that together all define a character—a letter, a numeral, or a punctuation mark.

A graphic display is composed of a screen or panel that is made up of a large number of small cells or dots that are called pixels. These pixels emit light when they are struck by a beam of electrons and switched on. At any one instant, the computer hardware can switch some pixels...