1. Introduction

1. Background, Purpose, and Scope

Since 1979, the U.S. Army Research Laboratory ^[1] has been developing and distributing the Ballistic Research Laboratory - Computer-Aided Design (BRL-CAD) three-dimensional (3-D) solid modeling package to support combat vehicle vulnerability studies and various other military and industrial applications. The software, which is now in its third generation, includes a large collection of tools and utilities, including an interactive geometry editor, raytracing and generic framebuffer libraries, network-distributed image-processing/signal-processing capabilities, and an embedded scripting language.

In support of the package, a multivolume tutorial series is being written to assist users with the many features and functionality of BRL-CAD. Three volumes have been published thus far. Volume I provides an overview of the package contents and installation (Butler and Edwards, 2002). Volume II addresses the basic functionality of the package’s Multi-Device Geometry Editor (MGED) and offers a comprehensive list of the user commands available (Butler et al., 2001). Volume III discusses the modeling process as well as principles and techniques to help maximize BRL-CAD’s effectiveness (Butler et al., 2003). All of these documents are available for download at http://ftp.arl.army.mil/brlcad/ (U.S. Army Research Laboratory, 2003).

The purpose of Volume IV is to discuss issues of compatibility and conversion between the BRL-CAD geometry file format and the formats of various other computer-aided design (CAD), computer-aided manufacturing (CAM), and computer-aided engineering (CAE) packages. Conversion is increasingly important for BRL-CAD users who must interact with a growing number of Government and commercial organizations involved in the research, development, testing, and evaluation of today’s combat systems.

Note that this document addresses BRL-CAD geometry converters, not BRL-CAD image converters (e.g., pix-bw, pix-ps, pix-rle, bw-ps, and pl-ps). For further information on image conversion, see the man page on the utility of interest.

Also, because of the many file formats in existence today and the rapidly changing nature of computer software and software companies, it would be impractical to try to address all of the conversion paths and methods that are currently or potentially possible. Many times, conversion from one file format to another is not a one-to-one process. Depending on the amount of time and effort the BRL-CAD user is willing to invest, seemingly incompatible formats can be "forced" to convert via another CAD format or via a standardized CAD format (e.g., the Initial Graphics Exchange Specification [IGES] or the Standard for the Exchange of Product Model Data [STEP]). In fact, when it comes to converting from BRL-CAD, the widely accepted Stereo Lithography Tessellation Language (STL) format offers a crude path to convert BRL-CAD objects to virtually any commercial CAD system. The user is reminded, however, that such forced conversions can sometimes produce geometry of such poor quality (e.g., low-resolution or lossy output formats) or poor performance characteristics (e.g., large or computationally intensive output files) that completely rebuilding a model from scratch might be a preferable alternative.

In any case, the focus of this document is on the primary formats that convert to and from BRL-CAD. 2. Pertinent Characteristics of the BRL-CAD Format discusses pertinent characteristics specific to the BRL-CAD format. 3. Converting From/to ASCII addresses general conversion to/from the American Standard Code for Information Interchange (ASCII) format. 4. Converting to BRL-CAD addresses the primary formats that convert to BRL-CAD. 5. Converting From BRL-CAD addresses the primary formats that convert from BRL-CAD. 6. Building a New Converter provides guidance for those users who desire to create their own customized converters. And 7. Postconversion Issues addresses postconversion issues. In addition, the user is encouraged to consult the web sites and other resources cited at the end of each converter discussion to obtain the latest information on each format.

2. The Need for Conversion

Since its inception, BRL-CAD has proven itself to be a particularly effective tool for producing high-resolution and physically realistic geometry for ballistic penetration, radar signature, and other types of related analyses. However, several commercial CAD packages have also gained popularity, especially within organizations that design and manufacture military systems. Although these packages are not designed for vulnerability studies per se, their widespread use throughout military circles necessitates that BRL-CAD users be able to convert to and/or from them.

There are numerous benefits associated with the use of commercial packages in vulnerability studies. The Survivability/Vulnerability Information Analysis Center (SURVIAC) identified some of the most common benefits in its 2002 State-of-the-Art Report (SOAR) on geometric modeling. They include the following (SURVICE Engineering Company, 2002):

Reduced Modeling Time and Effort — Manufacturers often spend hundreds of hours constructing detailed CAD models to streamline their design, production, and assembly processes (e.g., through computer numerical control equipment). It therefore makes economic sense-and is consistent with the military’s ongoing commitment to leverage commercial off-the-shelf technology-to take advantage of existing geometric models and data where possible and avoid the significant cost and effort of building new models from scratch.
Increased Funding and Support — Because commercial packages must maintain a larger user base to stay competitive in the open market, the most popular packages typically possess ample funding and personnel to continuously develop, support, and improve them.
Compatibility With Standardized Formats — Most commercial packages possess the direct or indirect (through third-party vendors) capability to convert to standard or intermediary geometry formats, making the packages compatible (at least to some degree) with a wide range of other CAD formats.
Third-Party Add-On Support — Large commercial packages typically offer a variety of plug-ins for other packages/utilities.

Of course, commercial CAD packages also have some common liabilities when used for vulnerability studies. They include the following (SURVICE Engineering Company, 2002):

Incompatible/Inaccessible File Formats — As discussed in 2. Pertinent Characteristics of the BRL-CAD Format, some CAD formats use the boundary representation (BREP) approach to solid modeling, which is largely incompatible with the constructive solid geometry (CSG) approach that BRL-CAD uses. In addition, although most commercial packages have some capability for data exchange conversion, data storage is often in a proprietary (and therefore inaccessible) native format. Moreover, when target descriptions are converted to a format designed for vulnerability assessment (i.e., BRL-CAD or FASTGEN), they often require manual checking, adjustment, and additional modeling (see 7. Postconversion Issues). Typical problems that must be addressed include the translation of curved and irregular surfaces, the representation of solids of rotation, tolerancing, and interference handling.
Too Much Detail — Commercial geometry files often contain too much of a good thing for vulnerability analysis. That is to say, commercial CAD packages often model geometry all the way to the "nuts and bolts" level, whereas vulnerability analyses are typically concerned only with details down to the level of shielding and critical components. Unfortunately, added detail produces unnecessarily large and complex input files and thus longer processing times for vulnerability assessments.
Too Little Detail — In addition to providing too much detail, commercial packages sometimes provide too little detail for vulnerability studies. Vulnerability analysts and the applications designed to interrogate geometry rely on geometric measurements and material properties not always present in commercial CAD formats.
Package-Specific Naming Conventions — Some organizations and CAD packages use unique object naming schemes that make geometry difficult to organize and work with when converted to or from BRL-CAD format.
Relatively Slow Raytracing Capability — Commercial CAD packages typically have relatively slow raytracing speeds, and raytracing is the primary means of geometry interrogation in vulnerability assessment.
Limited Integrated Vulnerability Assessment Support — Commercial CAD packages are designed for engineering analysis, not ballistic analysis and therefore offer few, if any, "shotlining" tools and limited integrated vulnerability assessment support.
High Cost — In addition to facing the typically high cost of commercial CAD software, the user often faces the decision of whether or not to invest in non-PC hardware (e.g., UNIX workstations) to obtain maximum performance, especially with large, complex geometry. In addition, users may be required to pay continuing licensing and maintenance fees, often on a per-seat basis (although recent developments have offered PC-based implementations and short-term leasing "seats" to make these packages more affordable).

Not surprisingly, when considering the benefits and liabilities of using commercial packages in vulnerability assessment, one can see that the conversion of these formats to/from BRL-CAD is one of the most needed as well as one of the most challenging tasks the BRL-CAD user faces today.

1. On 30 September 1992, the U.S. Army Ballistic Research Laboratory (BRL) was deactivated and subsequently became part of the U.S. Army Research Laboratory (ARL) on 1 October 1992.