A more flexible image generation environment

A supervisory process is used to distribute picture-generation tasks to heterogeneous subprocesses. Significant advantages accrue by tailoring the subprocesses to their tasks. In particular, scan conversion algorithms tailored to different surface types may be used in the same image, a changing mixture of processors is possible, and, by multiprogramming, a single processor may be used more effectively. A two-level shape data structure supports this execution environment, allowing top-level priority decisions which avoid comparisons between surface elements from non-interfering objects during image construction.