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# Displays Supported

It is important for a CAD system to have a certain degree of independence from any single display device in order to provide longevity of the software and freedom from a single equipment supplier. The MGED editor supports serial use of multiple displays by way of an object-oriented programmatic interface between the editor proper and the display-specific code. All display-specific code for each type of hardware is isolated in a separate display manager module. High performance of the display manager was an important design goal. Existing graphics libraries were considered, but no well established standard existed with the necessary performance and 3-dimensional constructs. By having the display manager modules incorporated as a direct part of the MGED editor, the high rates of display update necessary to deliver true interactive response are possible, even when using CPUs of modest power.

An arbitrary number of display managers may be included in a copy of MGED, allowing the user to rapidly and conveniently move his editing session from display to display. This is useful for switching between several displays, each of which may have unique benefits: one might have color capability, and another might have depth cueing. The release command closes out MGED's use of the current display, and does an implicit attach to the ``null'' display manager. This can be useful to allow another user to briefly examine an image on the same display hardware without having to lose the state of the MGED editing session. The attach command is used to attach to a new display via it's appropriate display manager routines. If another display is already attached, it is released first. The null display manager also allows the MGED editor to be run from a normal alphanumeric terminal with no graphic display at all. This can be useful when the only tasks at hand involve viewing or changing database structures, or entering or adjusting geometry parameters in numerical form.

Creation of a new display manager module in the ``C'' language [RJLK78] generally takes an experienced programmer from one to three days. The uniform interface to the display manager provides two levels of interactive support. The first level of display support includes the Tektronix 4014, 4016, and compatible displays, including the Teletype 5620 bit-mapped displays. However, while storage-tube style display devices allow MGED to deliver the correct functionality, they lack the rate of screen refresh needed for productive interaction. The second level of support, including real-time interaction, is provided by the Vector General 3300 displays, the Megatek 7250 and 7255 displays, the Raster Technologies Model One/180 display, the Evans and Sutherland PS300 displays with either serial, parallel, or Ethernet attachment, the Sun workstations, and the Silicon Graphics IRIS workstation family.

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