The edge flag algorithm — A fill method for raster scan displays

Contour (polygon) filling is a primitive required in many application areas of raster scan graphics. The bit-map memory in a frame-store display is computationally well suited to this task, as it provides a large scratch pad working space. In this paper, a number of contour filling algorithms based on the read/write properties of the frame-store memory are compared with the classical `ordered-edge-list´ approach. Performance is evaluated on a microcomputer controlled frame-store display system in terms of ability to fill correctly, execution speed and processor memory requirements. A new algorithm, based on a more exact definition of an object edge, is presented. This algorithm, denoted edge flag algorithm, is implemented within the frame-store memory. It features high speed, in conjunction with minimal CPU memory requirements, making it ideally suited to hardware or microcode (firmware) implementation.