A library for incremental update of bitmap images

To achieve the maximum performance from bitmap displays, the screen must be used not just as an output device, but as a data structure that may cache computed images. In an interactive text or picture editor, that may mean converting the internal representation of what's being edited into a set of rectangles that tile the screen. Incremental updates of the image may then be done by rearranging some subset of the tiling using bitmap operations, independently of how the tiling was derived. We have taken the ideas used in the screen update algorithms for the sam text editor and generalized them so they may be applied to more structured documents than the simple character stream sam edits. The ideas have been tested by building a library arid a simple interactive document editor that treat a document as a hierarchical structure that may include text, pictures, and variable spacing. The core of the library is operators to make incremental changes to the display while maintaining the hierarchical data structure that describes it.