A potential customer examining computer graphics systems including a random positioning and refreshed CRT needs a lot of time and effort to form an opinion of the various marketed systems. To him not only systems appear very different, but also manuals are often cryptic and equivocal. In addition, graphic packages supplied by manufactures naturally take advantage of specific capabilities of the hardware and try to bypass their deficiencies. As a consequence, it is nearly impossible to run on a display system application programs which are written for another system.