Interactive graphics on intelligent terminals in a time-sharing environment

Summary

Interactive graphics in a time-sharing environment should be organized in such a way that the user's activities are locally processed in order to avoid unacceptably long response times — especially if only low-speed communication links are available. On the other hand, the host computer must be kept informed about the user's actions and, conversely, the display file in the terminal has to be updated whenever the execution of the application program cases a change in the visual representation. In order to avoid the transmission of redundancy, we decompose the display file into two intersecting parts such that the part in the host computer and the other in the terminal contains only the locally required information. The necessary communication between both parts is maintained by an information module generated on the base of a low-low-level intermediate language (L⁴) and exchanged between computer and terminal. This leads to the notion of an abstract terminal whose “machine language” is L⁴, facilitating the implementation and portability of graphic programming systems. The L⁴ concept and our experiences with its implementation is discussed.