In the opinion of the authors, a fully portable interactive graphical program should be able to take advantage of the object display terminal's hardware features where these are beneficial to the efficiency of man-machine interaction. By analysing the FORTRAN code of an interactive graphical program designed to operate on both refresh displays and storage tubes, a program design approach has evolved which attempts to reduce the effort required to achieve this aim.
The advantages of portable computer software have been recognized for some time and have resulted in substantial research into methods of achieving portable compilers and operating systems 1 so that the final application program will yield consistent results on whatever computer it is run. It is not, however, generally expected that the results will be identical in all cases as numerical applications will clearly be affected by machine word length. However, the objective is that the results obtained should be as good as possible with the particular hardware in use.
This aim is particularly difficult to achieve with an interactive graphics program, mainly because of the wide and increasing range of hardware that is available. Not only are there display screens of various shapes and sizes, but also several different principles of image generation, all of which have repercussions on the application program. Furthermore, there is a wide range of input devices that can be attached to a display terminal and a fully portable program would have to cater for a multitude of different combinations. Whilst the application programmer could write programs specifically for the most basic graphics terminal and claim with some justification that the resultant application software will run on a wide range of devices, this does not meet the objective of portability as defined above. It is important to emphasize that the program must use the display terminal facilities that are available where this would enhance performance of the man-machine combination.
The two most distinct interactive display types that are currently marketed in the UK are the direct-view storage and the beam-driven refresh (or random-scan) display. In the CAD Group at Leicester University, several engineering programs have been written that run on both types of display. The programs were written according to a particular design philsophy, one of the aims of which was to reduce the concern of the engineering programmer with portability. By analysing the code of one such program, taken from the field of structural