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# INTRODUCTION

Computer graphics is one of the fastest growing fields in the computer industry. Computer graphics has applications in many diverse areas, from electronic games to medicine; from cartoons to the space industry. Just what is interactive computer graphics and why is it so versatile? Human visual perception is quite keen and communicating with a computer is generally faster and easier with images, rather than with numbers. Furthermore, by having the computer continuously updating a display, the display itself becomes the communications medium. The user converses with the computer through the display using devices such as light pens, mice, data tablets, buttons, and knobs. The response of the computer is immediately reflected on the display, providing a fast communication channel between person and machine. This technology is called interactive computer graphics.

As the Army's lead laboratory for vulnerability technology, the Ballistic Research Laboratory (BRL) constantly performs analyses for a wide variety of military systems. Three dimensional computer models of the physical characteristics of these systems are vital to these studies. Since the mid-1960's, BRL has used a solid modeling technique called Combinatorial Solid Geometry (CSG or COMGEOM) for representing these models. The COMGEOM technique uses Boolean logic operations to combine basic geometric shapes or primitives to produce complex three-dimensional objects. The COMGEOM geometric models are processed by the Geometric Information For Targets (GIFT) [KBR79][BR75] and LIBRT [Muu87] for use in follow-on engineering analysis.

Geometric models are large collections of numerical data which have traditionally been created and edited manually, and analyzed in a batch environment. The production and modification of geometric models has been a slow, labor-intensive process. In 1980, BRL initiated an effort to improve the response time of the geometric modeling process by applying interactive computer graphics techniques. As a result of this work, BRL developed the Multi-device Graphics EDitor (MGED), an interactive editor for solid models based on the COMGEOM technique. Using MGED, a designer can build, view, and modify model descriptions interactively by manipulating the graphical representation, receiving immediate visual feedback on a graphics display. MGED replaces the manual method for the production and modification of geometric models.

Before MGED was built, existing packages were evaluated with respect to their utility for the geometric modeling process. Quite an exhaustive search of commercially available systems was conducted and none were found which met the BRL requirements. A study was initiated to examine the feasibility of producing the required capability in-house; a preliminary version of MGED which quite convincingly demonstrated the feasibility of such an undertaking [WM80]. It was then decided to develop MGED into a full production code. Production MGED code has been used since January 1982 to build models interactively at BRL.

This report is intended to serve as a user manual for the MGED program. The process of viewing and editing a description using MGED is covered in detail. The internal data structure is also covered, as it is an important part in the overall design. All the commands will be discussed and a command summary table presented. Also, a section will be devoted to the hardware interfaces for each major class of workstations which MGED supports.

- Philosophy
- Displays Supported
- Portability
- Object-Oriented Design

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