Differential Evaluation: a Cahe-based Technique for Incremental Update of Graphical Displays of Structures

Many applications of graphical man–machine interfaces require incremental update of an image, with controllable time granularity, having multiple views of the information, generated by modular, maintainable software having minimal resource overhead. A technique called differential evaluation is presented that addresses these needs. Instead of driving the display from a data structure, it uses an ad-hoc display procedure in conjunction with a FIFO cache to generate and maintain the display. Maintaining displays of variable structure is accomplished through the use of conditional mechanisms in the display procedure. The technique has been extended to meet many different needs, such as the use of graphic contexts, display of overlapping objects, double-buffering, and a variety of user input schemes. The efficiency of the technique, in time and storage, permits its use with modest equipment. It has been used for several years in a number of industrial user interfaces.