BRL-CAD Tutorial Series: 

Volume IV – Converting Geometry Between BRL-CAD 

and Other Formats 

by John R. Anderson and Eric W. Edwards 

ARL-SR-121 May 2004 

Approved for public release; distribution is unlimited. 
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Destroy this report when it is no longer needed. Do not return it to the originator. 

Note that the name BRL-CAD and the BRL-CAD eagle logo are trademarks of the U.S. Army. 
Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5068 

ARL-SR-121 May 2004 

BRL-CAD Tutorial Series: 

Volume IV – Converting Geometry Between BRL-CAD 

and Other Formats 

John R. Anderson 

Survivability/Lethality Analysis Directorate, ARL 

Eric W. Edwards 

SURVICE Engineering Company 

Approved for public release; distribution is unlimited. 
ii

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14. ABSTRACT 

Since 1979, the U.S. Army Research Laboratory has been developing and distributing the BRL-CAD constructive solid 

geometry (CSG) modeling package for a wide range of military and industrial applications. The package includes a large 

collection of tools and utilities, including an interactive three-dimensional geometry editor, raytracing and generic framebuffer 

libraries, network-distributed image-processing and signal-processing capabilities, and an embedded scripting language. 

As part of this effort, a multivolume tutorial series is being developed to assist users in the many features of the BRL-CAD 

package. “Converting Geometry Between BRL-CAD and Other Formats,” which is the fourth volume in the series, discusses 

pertinent characteristics of the BRL-CAD file format and provides specific instruction on converting to/from various other 

modeling file formats. Also discussed are the steps involved in creating a new BRL-CAD converter and postconversion issues. 

15. SUBJECT TERMS

BRL-CAD, geometry conversion, CAD, constructive solid geometry, boundary representation 

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John R. Anderson 

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iii

Contents 

List of Figures v

List of Tables v

Acknowledgments vi

1. Introduction 1

1.1 Background, Purpose, and Scope....................................................................................1 

1.2 The Need for Conversion ................................................................................................2 

2. Pertinent Characteristics of the BRL-CAD Format 4

2.1 CSG vs. BREP.................................................................................................................4 

2.2 Prerelease 6.0 Database vs. Current Format....................................................................7 

2.3 Converters Currently Available in BRL-CAD................................................................8 

3. Converting From/to ASCII 8

3.1 Converting From ASCII..................................................................................................9 

3.2 Converting to ASCII .......................................................................................................9

4. Converting to BRL-CAD 10

4.1 General Information About Converting to BRL-CAD..................................................10 

4.2 Converting From AutoCAD DXF Format ....................................................................10 

4.3 Converting From Elysium Neutral Facetted Format.....................................................11 

4.4 Converting From EUCLID Format ...............................................................................12 

4.5 Converting From FASTGEN Format............................................................................13 

4.5.1 FASTGEN4.........................................................................................................14 

4.5.2 Preprocessed FASTGEN Version 3/PATCH......................................................14 

4.6 Converting From IGES Format.....................................................................................15 

4.7 Converting From Jack ...................................................................................................16 

4.8 Converting From NASTRAN Format...........................................................................16 

4.9 Converting From Pro/E Format.....................................................................................17 
iv

4.10 Converting From STL Format.......................................................................................21 

4.11 Converting From TANKILL Format ............................................................................22 

4.12 Converting From Unigraphics Format ..........................................................................22 

4.13 Converting From Viewpoint Datalabs Format..............................................................24 

5. Converting From BRL-CAD 25

5.1 General Information About Converting From BRL-CAD............................................25 

5.2 Converting to ACAD Format........................................................................................26 

5.3 Converting to AutoCAD DXF Format..........................................................................27 

5.4 Converting to EUCLID Format.....................................................................................27 

5.5 Converting to IGES Format ..........................................................................................28 

5.6 Converting to Jack.........................................................................................................28 

5.7 Converting to STL Format ............................................................................................29 

5.8 Converting to TANKILL Format..................................................................................29 

5.9 Converting to VRML Format........................................................................................30 

5.10 Converting to Wavefront Format ..................................................................................31 

5.11 Converting to X3D Format............................................................................................31 

6. Building a New Converter 32

6.1 General Information About Building a New Converter................................................32 

6.2 Converting From BRL-CAD.........................................................................................33 

6.3 Converting to BRL-CAD ..............................................................................................33 

7. Postconversion Issues 35

8. References 38

Distribution List 41 
v

List of Figures 

Figure 1. CSG and BREP approaches to representing an extrusion die (Tanenbaum, 2001).........5 

Figure 2. Approximating a smooth, curved surface (left) with a BOT (right) (Tanenbaum, 

2001). .........................................................................................................................................6 

Figure 3. Pro/E to BRL-CAD converter dialog box. ....................................................................20 

List of Tables 

Table 1. BRL-CAD conversion capabilities. ..................................................................................8 

Table 2. BRL-CAD import converters..........................................................................................10

Table 3. BRL-CAD export converters..........................................................................................25
vi

Acknowledgments 

The authors would like to thank Messrs. Lee Butler, Edwin Davisson, William Landis, and 

David Davis for technically reviewing this document in a timely manner and for making 

numerous suggestions to improve its content and presentation. The authors also thank Dr. Paul 

Tanenbaum for the use of his graphics for figures 1 and 2 and Ms. TraNese Christy for her help 

in adapting them for presentation in this document. Finally, the authors would like to 

posthumously acknowledge Mr. Michael Muuss, the original architect of BRL-CAD. Without 

his vision, intellect, and diligence, this work would not have been possible. Thus, the BRL-CAD 

Tutorial Series is dedicated to his memory. 
vii

Supplementary Tutorial Boxes 

Note that tutorial boxes have been used throughout 

this document to supplement the information 

presented in text. Each of these boxes is designated 

by a graduation hat icon. 
viii

INTENTIONALLY LEFT BLANK. 
1

1. Introduction 

1.1 Background, Purpose, and Scope 

Since 1979, the U.S. Army Research Laboratory*

 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 

 *

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. 
2

“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 BRLCAD. Section 2 discusses pertinent characteristics specific to the BRL-CAD format. Section 3 

addresses general conversion to/from the American Standard Code for Information Interchange 

(ASCII) format. Section 4 addresses the primary formats that convert to BRL-CAD. Section 5 

addresses the primary formats that convert from BRL-CAD. Section 6 provides guidance for 

those users who desire to create their own customized converters. And section 7 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. 

1.2 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. 
3

• 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 section 2, 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 section 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.
4

• 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. 

2. Pertinent Characteristics of the BRL-CAD Format 

2.1 CSG vs. BREP 

As mentioned previously, there are two basic approaches to solid modeling: CSG and BREP. In 

CSG modeling, an object is represented as a Boolean combination of simple primitive shapes 

(e.g., spheres, cylinders, cones, etc.). In BREP modeling, an object is represented by a set of 

surfaces (e.g., facets, triangles, and splines) that are “stitched” together to completely enclose the 

object. 

For example, as shown in figure 1, to geometrically represent a simple extrusion die used in 

material manufacturing, a CSG model might subtract a cone from a cylinder to achieve the 

desired shape. Alternatively, a BREP model might achieve the same shape by joining several 

surfaces (Tanenbaum, 2001). 

Most of the major commercial solid modeling packages currently in distribution (e.g., 

Pro/ENGINEER [commonly known as Pro/E], Unigraphics, etc.) use the BREP approach, 

although they do have some CSG capability. BRL-CAD, on the other hand, is primarily a CSG 

modeling system with some BREP capability. 

Conversions that occur between CSG and BREP typically involve their “lowest common 

denominator.” 
5

Rendered image of an extrusion die. 

CSG approach in which a cone is 

subtracted from a cylinder. BREP approach in which four 

surfaces are stitched together. 

Figure 1. CSG and BREP approaches to representing an extrusion die (Tanenbaum, 2001). 

For BRL-CAD, a simple type of BREP is available in the form of triangles; it is called the Bag of 

Triangles (BOT) primitive. If the BREP object consists of all triangles, the BRL-CAD 

representation can be an exact duplicate. But more often, a BREP object consists of smooth, 

curved surfaces, meaning the BRL-CAD triangle representation will be an approximation of the 

object. Note that this approximation may be made as close to the true surface as desired, but at 

the cost of more, smaller triangles (see figure 2) (Tanenbaum, 2001). Thus, the higher degree of 

smoothness means the greater the impact on file size and performance of any application trying 

to employ all the triangles. 

Because most commercial CAD systems have the capability to produce tessellated 

approximations of their BREP objects, some converters to BRL-CAD (e.g., those with Pro/E and 

Unigraphics) take advantage of these capabilities. In addition, the previously mentioned STL 

format, which represents solid objects entirely with triangles, offers the community a universal 

(albeit crude) way to convert BRL-CAD geometry (via the g-stl converter) to nearly any 

commercial CAD system.
6

Figure 2. Approximating a smooth, curved surface (left) with a BOT (right) (Tanenbaum, 2001). 

BREP Terminology 

Boundary representations can take several forms. Although the 

terms for these forms are sometimes used interchangeably, the 

following list provides some key distinctions: 

• Polygonal (or Facetized) Geometry - A type of BREP 

geometry that represents objects with a collection of regions 

of the plane bounded by a closed path of line segments 

(polygons). 

• Triangle - A special type of polygon that has exactly three 

sides. 

• Non-Uniform Rational B-Spline (NURB) Surface - A 

piecewise polynomial representation of a smooth, curved 

surface (e.g., an airplane wing). 

• BOT (Bag of Triangles) - A BRL-CAD primitive that 

consists of a collection of triangles and that gives the 

package limited BREP capability by approximating solid 

geometry. 

In the end, although there are benefits to using the BREP methodology (e.g., ease of use, 

software availability/compatibility, etc.), BRL-CAD developers and vulnerability analysts have 

found CSG modeling to be the best approach in terms of model accuracy, storage efficiency, 

precision, and analysis speed. 

More, smaller 

triangles can 

give a closer 

approximation 

of the curved 

surface, but at 

the cost of 

storage space 

and program 

performance. 
7

CSG or BREP? 

Depending on one’s point of view, there are advantages and 

disadvantages to using CSG vs. BREP. Although “the right” 

approach for a given project ultimately depends on many specifics 

(e.g., model purpose, hardware/software availability, and 

compatibility with other users), the following areas should be 

considered when choosing between CSG and BREP: 

• Realism - Does the model need to approximate real material 

densities, thicknesses, and volumes, or is the realistic 

appearance of object surfaces more important? 

• Resolution - How geometrically precise must the model be? 

For example, radar cross-section studies are sensitive to 

surface detail; ballistic penetration analyses usually are not. 

• Resources - How much disk space and memory will be 

needed to store the geometry and data structures? 

• Time - How much CPU time is needed to read, write, and 

process the model, and how much user time will be needed 

to understand, modify, and conduct analyses with it? 

2.2 Prerelease 6.0 Database vs. Current Format 

With the release of BRL-CAD 6.0, an entirely new BRL-CAD database format was introduced. 

Additional capabilities provided by the new format include unlimited name lengths, object 

attributes, machine independence (for IEEE floating point architectures), reduced disk space 

requirements, opaque binary objects, and the capability to hide objects. (For more information 

on the new database format, see Butler et al. [2003]). 

Older BRL-CAD databases may be upgraded to the current database (db) format using the 

dbupgrade utility. To execute this command, the user types the following command at the 

command-line prompt: 

dbupgrade oldformat.g currentformat.g 

This command reads an existing input.g database (in the older format) and writes a new 

output.g in the latest format. Remember that all BRL-CAD geometry files are designated by a 

“.g” suffix. Thus, the conversion commands discussed in the following sections use a “g” to 

represent the BRL-CAD file format (e.g., asc2g, dxf-g, g-acad, etc.). 
8

What About Databases in the Old Format? 

Although databases created with BRL-CAD release 6.0 or later are 

in the new format by default, BRL-CAD retains the ability to work 

in the old format. Thus, new-database users need not be concerned 

about losing databases in the old format. Also, whenever a new 

database format is implemented in the future (typically a rare event), 

the dbupgrade utility will be modified so that it will always be able 

to upgrade databases to the current format. 

2.3 Converters Currently Available in BRL-CAD 

Table 1 shows the CAD formats for which BRL-CAD currently converts from (imports) or to 

(exports). 

Table 1. BRL-CAD conversion capabilities. 

CAD Format Import Capability? Export Capability? 

ACAD — 9

ASCII 9 9

AutoCAD DXF 9 9

Elysium Neutral Facetted 9 — 

EUCLIDa 9 9

FASTGEN 9 — 

IGES 9 9

Jack 9 9

NASTRAN 9 — 

Pro/E 9 — 

STL 9 9

TANKILL 9 9

Unigraphics 9 — 

Viewpoint 9 — 

VRML — 9

Wavefront (.obj) — 9

X3D — 9 a Conversion is to/from the ASCII EUCLID decoded format, not native format. 

3. Converting From/to ASCII 

With the implementation of BRL-CAD’s machine-independent database format, the need for 

converting to or from ASCII format has rapidly diminished. Nonetheless, BRL-CAD still 

supports ASCII conversions, and release 6.0 introduced a new ASCII format. This format is 
9

simply a Tcl* script that (when executed by a properly empowered Tcl interpreter) rebuilds the 

database from which it was derived. In contrast, the old ASCII format consisted of a sequence of 

one-line records that corresponded to the old database format records. 

3.1 Converting From ASCII 

The asc2g utility is used to convert ASCII format databases into the new binary format. This 

utility will always create the new binary format, but it will accept either of the two ASCII 

formats. 

The usage for this utility is as follows: 

asc2g input.asc output.g 

In this command, input.asc is a file previously created using g2asc and output.g is the name 

where the resulting binary database should be stored. 

3.2 Converting to ASCII 

The converse of the asc2g utility is the g2asc utility, which is used to convert from either the 

current database format or the prerelease 6.0 database format to an ASCII format. Databases in 

the new format will be converted to the new (Tcl script) ASCII format. Databases in the old 

format will be converted to the old ASCII format. 

The command for this utility is as follows: 

g2asc input.g output.asc 

In this command, the input.g is a BRL-CAD database (old or new format) and output.asc is the 

name where the ASCII output is placed. As mentioned previously, the form of the ASCII output 

depends on which database format is used by the input database. If it is known that the input 

database is in the older BRL-CAD format, then the g2asc may be executed using redirected input 

and output, as follows: 

g2asc < input.g > output.g 

If this is attempted using the newer database format, however, an error message will be displayed 

and no conversion will be performed. 

 *

Tcl/Tk is an interpreted programming language that can be used to extend/customize BRL-CAD functionality. Users are 

encouraged to consult one of the many texts on Tcl/Tk syntax and use, including Practical Programming in Tcl and Tk (Welch, 

1999) and TCL/TK in a Nutshell (Raines and Tranter, 1999). 
10

4. Converting to BRL-CAD 

4.1 General Information About Converting to BRL-CAD 

Table 2 contains the primary file formats (other than ASCII) that currently convert to BRL-CAD. 

Note that, as discussed previously, the “-g” designation at the end of each converter name 

indicates that the converted file will be in BRL-CAD’s standard geometry (.g) format. 

Table 2. BRL-CAD import converters. 

CAD Format Import Converter 

AutoCAD DXF dxf-g 

Elysium Neutral Facetted enf-g 

EUCLIDa euclid-g 

FASTGEN 

(FASTGEN4 and PATCH) fast4-g/patch-g 

IGES iges-g 

Jack jack-g 

NASTRAN nastran-g 

Pro/E —b

STL stl-g 

TANKILL tankill-g 

Unigraphics ug-g 

Viewpoint datalabs viewpoint-g a Conversion is from the ASCII EUCLID decoded format, not native format. b Converter is part of the Pro/E GUI. 

4.2 Converting From AutoCAD DXF Format 

DXF is Autodesk Inc.’s proprietary data specification that has been developed to support links 

with AutoCAD, the company’s popular CAD software. The format, which has been 

implemented in many CAD systems (particularly those that work on PCs), is simple and 

relatively limited; however, it has become a well-established means of exchanging engineering 

drawings. 

The DXF format allows for the specification of the units used in the DXF file; however, not all 

DXF files include this information. Thus, if units information is not found in the DXF file, 

millimeters (which is also the default and underlying unit of measure in BRL-CAD) is the 

assumed unit of measure. Of course, if millimeters is not the right choice for a particular user or 

application, the s (scale) option can be used. This option and the other available options for the 

dxf-g converter are discussed in the text that follows. 

When using this converter, polygons and meshes are converted to BOT primitives. Each layer 

becomes a region. Lines, circles, and arcs become n-manifold geometry (NMG) objects, and 

points become spheres.
11

The NMG primitive represents geometry that is defined by points, 

lines, polygonal facets, and collections thereof. It is the primary 

BRL-CAD primitive for encoding one- or two-dimensional (2-D) 

data. For more information on the NMG, see Muuss and Butler 

(1991) and Weiler (1987). 

The command for the dxf-g converter is as follows: 

dxf-g [options] input.dxf output.g 

The options for the command are as follows: 

• c - specifies that only the first color encountered for a layer in the DXF file is used; 

additional colors for the same layer are ignored. 

• d - debug; produces core dump on failure. 

• s - applies the specified scale factor to the DXF data (takes an argument). 

• t - sets tolerance distance (in millimeters) (default is 0.005) (takes an argument). 

• v - verbose; prints out progress information. 

For more information on the DXF file format and import possibilities, see the on-line 

documentation on the Autodesk web site at http://adeskftp.autodesk.com/ 

prodsupp/downloads/dxf.pdf (Autodesk, 2003). 

4.3 Converting From Elysium Neutral Facetted Format 

Elysium Inc. was founded in 1984 to provide product structure synchronization and 

compatibility between different CAD systems. The company claims to hold the industry’s 

highest translation success rate and relies on three of its products (CADporter, CADdoctor, and 

CADserver) to convert geometry between a large number of formats, including Pro/E, 

Unigraphics, ABAQUS/CAE, ACIS, CATIA, Inventor, I-deas, Metrix Build!IT, One Space 

Designer, Parasolid, and SolidWorks (Elysium, 2003). 

The enf-g converter was originally written when BRL-CAD developers needed an intermediate 

format—the Elysium neutral facetted file format—to allow the conversion of Unigraphics 

geometry to BRL-CAD. The converter converts each part to a BRL-CAD region consisting of 

one BOT primitive. Ident numbers are incremented for each region. If a part name mapping file 

is provided, part names in the input file will be output using the corresponding names from the 

mapping file. The part name mapping file may look similar to the following: 
12

 01a3-011-03 washer, flat 

 01a2-011-02 nut, flex 

. 

. 

.

The syntax for the enf-g command is as follows: 

enf.g input_file output.g 

The options for the command are as follows: 

• i starting_ident - sets the starting ident number (default is 1000). 

• n part_name_mapping - sets the mapping from input names to output names. 

• t tolerance_distance - sets distance calculation tolerance (in millimeters) 

(default is 0.005). 

For more information on the Elysium file format and import possibilities, see the Elysium web 

site at http://www.elysiuminc.com (Elysium, 2003). 

4.4 Converting From EUCLID Format 

EUCLID is one of Europe’s primary product design, manufacturing, and engineering CAD 

packages (though it is quickly being superceded by CATIA). Formerly distributed by the 

European Aeronautic Defence and Space Company’s Matra Datavision subsidiary and now a 

product of the IBM subsidiary MDTVISION, the package was developed for the design and 

manufacture of complex models and has been used on major systems such as the Airbus, 

Eurocopter, Ariane, Eurofighter, Astrium, and Euromissile. 

Package features include specialized applications for design, styling, drafting, analysis, 

machining, and product data management. Recent enhancements have focused on automatic 

creation of 2-D drawings from 3-D models, milling cycles, two- to four-axis wire cutting, sheet 

metal design, mold design, and standard data exchange format interfaces. 

EUCLID offers several data translation interfaces, including those for DXF, IGES, VDA-FS, 

STL, and SET formats. In addition, other modules are available to help refine and customize 

data transfer, with direct connectivity at two levels available for CATIA (V4/V5) data transfer 

using BREPs to handle surface data. EUCLID geometry can be output to standard graphic (e.g., 

PostScript, Encapsulated PostScript, and Interleaf) and plotting (e.g., CalComp, HP, HP/GL2, 

OCE, and Versatec) formats as well as to an STL file. 

The euclid-g converter converts an ASCII EUCLID “decoded” format file to BRL-CAD. Each 

part is converted to a BRL-CAD region consisting of a single BOT primitive. 
13

The syntax for the converter is as follows: 

euclid-g [options]

The options for the command are as follows: 

• v - verbose; prints out progress information. 

• i input_euclid_db - sets input file name (default is stdin). 

• o output_brlcad_db - sets output file name (default is stdout). 

• d tolerance_distance - sets distance calculation tolerance (in millimeters) 

(default is 0.005). 

• n - produces NMG primitives (default is BOT primitives). 

• x lvl - sets librt debug flag (see raytrace.h in the binary distribution for definitions of 

DEBUG_xxx). 

• X lvl - sets NMG library debug flag (see nmg.h in the binary distribution for 

definitions of DEBUG_xxx).

For more information about the EUCLID file format and import possibilities, see the 

MDTVISION web site at http://support.mdtvision.com (MDTVISION, 2003). 

Note that all header (h) files—including raytrace.h, nmg.h, and 

bu.h—are accessible in both the source and binary distributions of 

BRL-CAD. In the binary distribution, these files are installed in 

the /user/brlcad/include/brlcad directory. 

4.5 Converting From FASTGEN Format 

Developed by the Falcon Research and Development Company over 30 years ago, FASTGEN 

has been widely used in the Department of Defense air combat system assessment community 

(e.g., the Air Force Research Laboratory, the Joint Technical Coordinating Group for Munitions 

Effectiveness, and the Joint Technical Coordinating Group on Aircraft Survivability. Now 

distributed by the Survivability/Vulnerability Information Analysis Center (SURVIAC) as part 

of the Vulnerability Modeling Tool Set, the FASTGEN format uses geometry BREP based on 

NASTRAN, with data presented in a generic, open text-based file format. 

Components of a target are represented in FASTGEN by triangles, quadrilaterals, cones, 

cylinders, spheres, and hexahedrons. These basic elements are designated as either plate 

(hollow) or volume (solid) mode and combined using a hierarchical structure for the formation 

of components and groups (SURVICE Engineering Company, 2002). 

Notable aerospace firms and support organizations that use FASTGEN include Lockheed Martin, 

Boeing, Bell Helicopter, Northrop Grumman, Pratt & Whitney, General Electric, KETRON, ITT, 

BAH, SURVICE Engineering, and ASI. 
14

There are several versions of FASTGEN currently in use. FASTGEN converters include 

FASTGEN4 and preprocessed FASTGEN Version 3 (also referred to as PATCH). 

4.5.1 FASTGEN4 

The fast4-g converter converts FASTGEN4 to BRL-CAD. FASTGEN4 entities are converted to 

corresponding equivalent BRL-CAD objects. 

The syntax for the fast4-g command is as follows: 

fast4-g [options] fastgen4_input output.g 

The options for the command are as follows: 

• d - prints verbose debugging output.

• q - prints nothing except errors. 

• w - prints warnings about creating default names. 

• plot_file_name - creates a UNIX plot file of all CTRI and CQUAD elements 

processed. 

• c component_list - processes only the listed region ids; may be a list (e.g., 

3001,4082,5347) or a range (e.g., 2314–3527). 

• m muves_file_name - creates a MUVES warnings file containing CHGCOMP and 

CBACKING elements. 

• b lvl - sets libbu debug flag (see bu.h in the binary distribution for definitions of 

BU_DEBUG_xxx). 

• x lvl - sets librt debug flag (see raytrace.h in the binary distribution for definitions of 

DEBUG_xxx). 

4.5.2 Preprocessed FASTGEN Version 3/PATCH 

The patch-g converter converts preprocessed FASTGEN version 3 files to BRL-CAD format. 

The syntax for the command is as follows: 

patch-g [options] output.g 

The options for the command are as follows: 

• f fastgen.rp - specifies preprocessed FASTGEN file (default is stdin). 

• a - processes phantom armor. 

• n - processes volume mode as plate mode. 

• u # - specifies the number of union operations per region (default is five). 

• c ”x y z“ - specifies the center of the object (in inches) (for some surface normal 

calculations). 

• t title - specifies optional title (default is “Untitled MGED database”). 

• o object_name - specifies optional top-level name (default is “all”). 

• p - writes volume and plate mode components as BOTs. 

• 6 - processes plate mode triangles as ARB6 solids (overrides p option for triangles). 

• i group.file - specifies group labels source file. 

• m mat.file - specifies materials information source file.
15

• r - reverses normals for plate mode triangles. 

• d lvl - sets debug level. 

• x lvl - sets librt debug flag (see raytrace.h in the binary distribution for definitions of 

DEBUG_xxx). 

• X lvl - sets librt NMG debug flag (see nmg.h in the binary distribution for definitions 

of DEBUG_xxx). 

• T tolerance_distance - sets distance tolerance (in inches). 

• A parallel_tolerance - sets parallel tolerance (sine). 

For more information about the FASTGEN file formats and import possibilities, see the 

SURVIAC web site at http://www.bahdayton.com/surviac/fastgen.htm (SURVIAC, 2003). 

4.6 Converting From IGES Format 

IGES was developed in 1979 by a consortium of government, industry, and academia 

representatives. Originally intended to provide a means of exchanging graphics and engineering 

drawings, IGES was extended to include solid models as well. This specification includes so 

many different implementations within itself that conversion between IGES flavors has become a 

small industry. As a result, conversion to/from IGES is a “hit-or-miss” proposition (NIST, 

2003). 

The Product Data Exchange using STEP specification is intended to replace IGES and correct 

the aforementioned deficiency by explicitly identifying different types of conversion and 

requiring converters to conform to those types. BRL-CAD supports conversion of two 

implementations of IGES, entirely facetted BREP and CSG with facetted BREP primitives. 

The syntax for the iges-g converter is as follows: 

iges-g [options] –o output.g input.igs 

The options available for this command are as follows: 

• n - combines all the rational B-spline surfaces in the file into one BRL-CAD spline 

primitive. 

• d - converts drawings in the IGES file to 2-D BRL-CAD NMG primitives. 

• 3 - same as the d option, but the final projection to 2-D is not performed. This can 

produce 3-D drawings in some cases. 

• t - converts all trimmed surfaces in the IGES file to a single BRL-CAD NMG primitive. 

• N primitive_name - specifies a name for single primitive created using the t or n

options. 

• p - converts all BREP entities to BRL-CAD NMG primitives rather than the default BOT 

primitives. 

• x lvl - sets librt debug flag (see raytrace.h in the binary distribution for definitions of 

DEBUG_xxx). 

• X lvl - sets librt NMG debug flag (see nmg.h in the binary distribution for definitions 

of DEBUG_xxx). 
16

The n, d, t, and 3 options are mutually exclusive. If none of these four options is provided, the 

default action is to convert only IGES solid model entities (CSG and planar face BREP) to 

BRL-CAD. 

For more information about the IGES file format and import possibilities, see the National 

Institute of Standards and Technology (NIST) web site at http://www.nist.gov/iges (NIST, 2003). 

4.7 Converting From Jack 

Jack is a 3-D interactive ergonomics and human factors CAD package developed by the 

University of Pennsylvania’s Center for Human Modeling and Simulation. Now maintained and 

distributed by Electronic Data Systems (EDS) (the company that now also distributes 

Unigraphics and NASTRAN), the package enables users to study and improve the ergonomics of 

product design and workplace tasks through the positioning of biomechanically accurate digital 

humans of various sizes in virtual environments. Jack and Jill digital “humans” can tell 

engineers what they can see and reach, how comfortable they are, when and why they’re getting 

hurt, when they’re getting tired, and other important ergonomics information. The package’s 

principal features include a detailed human model, realistic behavioral controls, anthropometric 

scaling, task animation and evaluation systems, view analysis, automatic reach and grasp, and 

collision detection and avoidance (The University of Pennsylvania, 2001; EDS, 2003a). 

The jack-g converter creates a single region consisting of a single BOT primitive. The syntax 

for the converter is as follows: 

jack-g [options] input.jack output.g 

The options for this command are: 

• r region_name - specifies a name for the BRL-CAD region created. If this option is 

not specified, the input file name will be used to construct a region name. 

• g group_name - specifies the name of a combination to create to hold the BRL-CAD 

region created. If this option is not specified, the combination will not be created. 

For more information on the Jack file format and import possibilities, see the EDS web page at 

http://www.eds.com/products/plm/efactory/jack/ (EDS, 2003a). 

4.8 Converting From NASTRAN Format 

Originally developed under National Aeronautics and Space Administration (NASA) 

sponsorship in the mid-1960s, the NASA Structural Analysis (NASTRAN) program was one of 

the first efforts to consolidate structural mechanics into a single computer program. It has since 

been used as a general-purpose software tool in numerous industries, including aerospace, 

automotive, medical, heavy machinery, electronic devices, and consumer products. The program 

is developed and distributed by the MSC.Software Corporation (MSC) and (as of June 2003) the 

EDS Corporation. It employs advanced finite element analysis computational techniques to 
17

analyze material strength/performance and evaluate static structures and the dynamic motion of 

structures (SURVICE Engineering Company, 2002; MSC.Software Corporation, 2003). 

NASTRAN’s nonlinear analysis capabilities can address a wide range of static and dynamic 

problems exhibiting both material and geometric nonlinear behavior. Heat transfer problems can 

also be solved using conduction, convection, and radiation methods under a variety of applied 

loads and boundary conditions. 

The NASTRAN finite element modeling program is one of the general-purpose structural 

analysis programs used worldwide. Even though it was originally intended for structural 

analysis problems, its current applications include aeroelasticity, heat transfer, fluid structure 

interaction, acoustics, electromagnetics, and many other applications. 

NASTRAN includes a file specification for describing geometric data. NASTRAN’s wide use 

and adoption by CAD vendors make it well suited as a file standard. 

The nastran-g converter currently only converts CBAR, CROD, CTRIA3, and CQUAD4 

elements of NASTRAN files to BRL-CAD format. CBAR and CROD elements become 

cylinders in BRL-CAD. CTRIA3 and CQUAD4 elements become BOT facets. 

The syntax for the converter is as follows: 

nastran-g [options] 

The options for the command are as follows: 

• i NASTRAN_input_file - sets input NASTRAN file (default is stdin). 

• o output.g - sets output file name (default is “nastran.g”). 

• n - produces NMG primitives (default is BOT primitives). 

• x lvl - sets librt debug flag (see raytrace.h in the binary distribution for definitions of 

DEBUG_xxx). 

• X lvl - sets NMG library debug flag (see nmg.h in the binary distribution for 

definitions of DEBUG_xxx). 

• t tolerance_distance - sets distance calculation tolerance (in millimeters) 

(default is 0.005). 

• m - sets input units to millimeters (default is inches). 

For more on the NASTRAN file format and import possibilities, see the MSC web site at 

http://www.mscsoftware.com (MSC.Software Corporation, 2003) and the EDS web site at 

http://www.eds.com/products/plm/nastran/ (EDS, 2003b). 

4.9 Converting From Pro/E Format 

Distributed by the Parametric Technology Corporation (PTC), Pro/E is one of the most widely 

used commercial CAD packages for designing, engineering, and manufacturing products. The 

long list of major corporations that use Pro/E software for Product Lifecycle Management 

includes Boeing, Rolex, Audi, Dell, Nike, Maytag, Braun, and Hewlett-Packard (PTC, 2003). 
18

Because of Pro/E’s popularity in the Defense community, the Pro/E-to-BRL-CAD converter is 

one of the most important conversion utilities that BRL-CAD offers. Note that unlike the 

converters for other formats, the Pro/E converter is no longer command-line activated. This 

converter was written using the Pro/Toolkit module of Pro/E and therefore runs as part of the 

Pro/E program and GUI. 

Accordingly, in order to use the converter, the user must have a seat of Pro/E as well as the 

BRL-CAD distribution. Currently, the converter is only supported on Silicon Graphics (SGI) 

machines with MIPS processors running the Irix operating system. The source code for this 

converter is included in the binary distribution, so users can compile it for different platforms if 

they have the Pro/Toolkit module for that platform. 

Pro/E models are made up of two elements: parts and assemblies. Part files (which are 

designated by a .prt extension) are the basic building blocks of Pro/E geometry. Assembly files 

(which are designated by a .asm extension) are composed of parts and/or other assemblies. The 

converter produces a BRL-CAD region for each Pro/E part that is converted and a BRL-CAD 

combination for each Pro/E assembly that is converted. Each of these regions will consist of a 

single BOT primitive. 

The conversion of geometry from Pro/E to BRL-CAD is a two-stage process. This converter 

first produces the ASCII form of BRL-CAD databases. The user then converts these databases 

to binary form using the asc2g utility. 

In addition, because Pro/E files for most combat vehicles are so large (often requiring a stack of 

CDs to store them), entire geometries typically cannot be loaded all at once. Thus, the BRLCAD user often has to convert geometry system by system (e.g., engine, transmission, and 

suspension) and then concatenate (i.e., join) them together in a single BRL-CAD geometry file. 

For more detailed information about this process, see the discussion on dbconcat in Volume III 

of this tutorial series. 

Pro/E makes extensive use of referenced geometry. As discussed in the previous volume of this 

tutorial series (see section 5 of Butler et al. [2003]), referencing is the method by which multiple 

occurrences of objects are created by referring to a single object numerous times with different 

orientations and locations for each reference. These references are duplicated in BRL-CAD 

using combinations and transformation matrices. In some cases—such as when geometry is used 

with vulnerability codes that require each region to have a unique ident number—users may need 

to use the xpush command in MGED after the conversion is complete to replace the references 

with real geometry. For more information about this procedure, users should consult MGED’s 

on-line help or the xpush entry in volume II of this tutorial series (see appendix A of Butler et al. 

[2001]). 
19

Keys to Converting Pro/E Geometry 

To maximize the efficiency and effectiveness of converting Pro/E 

geometry, the BRL-CAD user should first acquire the following 

information from the Pro/E designer: 

• A list of top-level assemblies - This information is vital in 

helping the BRL-CAD user gain an understanding of the 

overall model structure and know where to begin the 

conversion process. 

• A mapping of part numbers to part names - Although part 

numbers can be an important aspect of design and 

manufacturing, they have little meaning for vulnerability 

analysts, who are more concerned with the descriptive 

names of the components than the numbers represent.

The command to start the Pro/E program is specified by each installer. When Pro/E is started, 

the program looks for a file named protk.dat in a few specific places, one of which is in the 

current directory. This file informs Pro/E about Pro/TOOLKIT modules it should load. There is 

a protk.dat file for the Pro/E-to-BRL-CAD converter, and it is included in the distribution under 

the pro-engineer directory. Users should copy this file to the directory where they will be 

starting Pro/E. After that file is in place, Pro/E will load the converter at startup. If the loading 

succeeds, users will see a message saying “Installation of Proe-BRL converter succeeded.” With 

Pro/E started and the converter module loaded, the user can open any Pro/E model he wants to 

convert. 

The conversion process is started by selecting the ProE-BRL item in the Pro/E drop-down File 

menu. The converter dialog box, shown in figure 3, will then appear. 
20

Figure 3. Pro/E to BRL-CAD converter dialog box. 

The following list provides a description of the use and functionality of the primary elements in 

the dialog box. Note that the box is preloaded with reasonable defaults for many of the inputs. 

In addition, if the user has a question about any of the entry windows, check boxes, or buttons, 

he can move the mouse over them to see a brief explanation of their use. 

• Output File Name - This is the name of the file to receive the ASCII output. 

• Log File Name - If provided, verbose status logging will be written to that file. 

• Part Name File - If provided, the converter will use that information to map Pro/E part 

and assembly names into the specified BRL-CAD object names. This file is simply a text 

file with each line specifying a Pro/E part or assembly name and a BRL-CAD object 

name separated by white space. Note that the BRL-CAD names should not include any 

special characters such as “/”, “[“, “]”, or white space. 

• Starting Ident Number - Ident numbers will be assigned to the resulting BRL-CAD 

regions sequentially starting with the number that appears in the window. 

• Max Tessellation Error - This value (expressed in millimeters) is used to control the 

coarseness of the tessellation. It is the maximum distance between the actual surface and 

its tessellated approximation. Smaller values here will result in finer tessellations and 

more triangles. The default value is reasonable for ballistic vulnerability analysis 

purposes. 

• Angle Control Value - This is a number between 0 and 1 that provides additional control 

over the coarseness of the tessellation. The exact relationship between the tessellation 

and this value is not specified in the Pro/E documentation, but the default value of 0.5 

seems to work well. 

For more information about the Pro/E file format and import possibilities, see the Parametric 

Technology Corporation (PTC) web site at http://www.ptc.com (PTC, 2003). 
21

4.10 Converting From STL Format 

The STL format was developed by 3D Systems, Inc., in the 1980s for use with its 

StereoLithography Apparatus (SLA). The SLA device produces a physical 3-D model based on 

an STL format file. Because of its simplicity, the STL format has become an industry standard 

for exchanging 3-D models. Unfortunately, this simplicity also presents some limitations. 

The format consists only of triangles, and each triangle is represented by three vertices and a 

surface normal vector. Because the vertices for each triangle are explicitly listed, rather than 

indexed from a list, the topology must be inferred by the receiving system, which can sometimes 

lead to incorrect geometry. 

STL files may be either ASCII or binary. The ASCII format includes the capability of including 

more than one solid part and an optional name for each part, while the binary format can only 

support a single solid part with no naming. 

The stl-g converter converts STL format to BRL-CAD. The STL format is entirely triangles. 

The resulting BRL-CAD database will consist of one or more regions and a top-level 

combination named “all,” which contains all the regions produced. Each region will consist of a 

single BOT primitive. 

Note that the ASCII format STL file includes a capability to contain more than one solid part. 

The regions created will be named according to the name specified in the STL file unless a name 

is provided on the command line. If the STL file does not specify any name, and the user does 

not provide a name, then the regions produced in the BRL-CAD database will be constructed 

from the name of the STL file. 

The syntax for the stl-g converter is as follows: 

stl-g [options] input.stl output.g 

where input.stl is the STL file to be converted and output.g is the name of the BRL-CAD 

database to receive the converted output. 

The options for this command are as follows: 

• b - designates that the input STL file is in binary format (the default is ASCII). 

• c units - specifies the units used in the STL file. Choices include “cm,” “m,” “in,” 

“ft,” and many others (the default is millimeters). 

• N name - specifies a name for the resulting BRL-CAD region. If more than one region 

is produced, unique region names will be constructed by adding a suffix consisting of an 

underscore and an integer. 

• d - designates that additional debugging information be printed during the conversion. 

• i ident - specifies the ident number assigned to the first region created during the 

conversion. Additional regions will be assigned sequential ident numbers. 

• I ident - specifies the ident number to assign to all the BRL-CAD regions created 

during this conversion. (This option and the i ident option are mutually exclusive.) 
22

• m material_code - specifies the integer material code to be assigned to each 

BRL-CAD region created during this conversion. 

• t tolerance_distance - specifies the minimum distance (in millimeters) allowed 

between distinct vertices. Vertices closer than this minimum will be considered to be the 

same vertex (the default value is 0.005 mm). 

• x librt_debug_flag - specifies a flag for the raytracing library that will result in 

additional debug log messages (see librt/debug.h in the binary distribution for details). 

For more information on the STL file format and import possibilities, see the 3D Systems web 

site at http://www.3dsystems.com (3D Systems, 2003). 

4.11 Converting From TANKILL Format 

Distributed by the Advantage Business Group, a contractor to the British Ministry of Defence, 

the TANKILL format is used with the TANKILL vulnerability and lethality assessment code. 

This format is another purely triangulated representation of solid objects. 

The syntax for the tankill-g converter is as follows: 

tankill-g [options] 

The options for the command are as follows: 

• v - verbose; prints out progress information. 

• n - produces NMG primitives (default is BOT primitives). 

• i input.tkl - specifies the input TANKILL file (default is stdin). 

• o output.g - specifies the output BRL-CAD file (default is tankill.g). 

• k - keeps components with id = 1001 (normally skipped). 

• x lvl - sets librt debug flag (see raytrace.h in the binary distribution for definitions of 

DEBUG_xxx). 

• X lvl - sets NMG library debug flag (see nmg.h in the binary distribution for 

definitions of DEBUG_xxx). 

For more information on the TANKILL file format and import possibilities, contact the 

Advantage Business Group at the Barbician, East Street, Farnham, Surrey GU9 7TB or visit the 

web site at http://www.advantage-business.co.uk (Advantage Business Group, 2003). 

4.12 Converting From Unigraphics Format 

Like Pro/E, Unigraphics is a widely popular CAD format used by thousands of companies in the 

United States and abroad, including General Motors, Ford, Kodak, General Electric, Pratt & 

Whitney, Boeing, and Samsung. Now distributed by EDS (the company that also distributes 

Jack and a version of NASTRAN), the Unigraphics toolset addresses traditional 

CAD/CAM/CAE, conceptual and industrial design, knowledge-based engineering, real-time 

design collaboration, and process automation (EDS, 2003c). 
23

There are three modeling methodologies offered with Unigraphics. First, explicit (or traditional) 

modeling uses points, curves, and surfaces with no associativity or history. Second, historybased modeling uses associative geometric entities. Third, direct modeling uses a combination 

of both explicit and history-based modeling and also allows the global application of geometric 

rules and constraints across geometry of all origins (SURVICE Engineering Company, 2002). 

Unigraphics bases its component geometric modeling capabilities on the Parasolid geometry 

engine (developed by EDS in Cambridge, England) and related XT file format. This enables 

Parasolid-based systems (e.g., Unigraphics, Solid Edge, and systems by Parametric Technology, 

SolidWorks, Bentley Systems, CADKEY, ANSYS, Mechanical Dynamics, and MSC.Software) 

to share and exchange geometric data without translation via an interoperable data pipeline 

(SURVICE Engineering Company, 2002). 

Because the Unigraphics-to-BRL-CAD converter, ug-g, was written using the Unigraphics 

UG/Open API library, users must have a Unigraphics UG/Open execute or development license 

in order to run it. This converter is compiled for SGI Irix machines running on MIPS processors; 

however, users can compile it for other platforms by obtaining the BRL-CAD source distribution 

and a UG/Open development license from Unigraphics. 

This converter creates a BRL-CAD region consisting of a single BOT primitive for each 

Unigraphics part and a combination for each Unigraphics assembly. Each instance of a 

Unigraphics part is converted independently, so there are no transformation matrices created in 

the resulting BRL-CAD model. The BRL-CAD regions are given the same name as the parts are 

assigned in the Unigraphics model, unless a part-name mapping file is provided. Region names 

are made unique, if necessary, by adding a suffix consisting of a dot and an integer number. 

The syntax for this converter is as follows: 

ug-g [options] o output.g UG_part_file [subpart1 subpart2 ...] 

where the UG_part_file is a Unigraphics part file. If subparts are listed on the command line, 

only those named parts in the specified part file will be converted. 

The available options are as follows: 

• d level - specifies a debug level for additional log messages. Currently, any nonzero 

value here provides additional logging. 

• i initial_ident - specifies the ident number for the first region created by the 

conversion. Subsequent regions are assigned sequential ident numbers. 

• n part_name_file - specifies a file containing a mapping of Unigraphics part and/or 

assembly names to BRL-CAD object names. If this file is provided, it will be used to 

create object names in the BRL-CAD model. The format of this file is simply a line per 

part, with the Unigraphics part/assembly name followed by the desired BRL-CAD object 

name, separated by white space. Note that the usual restrictions for BRL-CAD object 

names apply (e.g., no spaces, no special symbols, etc.) For a detailed discussion of 
24

recommended naming schemes and restrictions, see section 4 of Volume III (Butler et al., 

2003). 

• R refset_name - specifies a desired reference set (which Unigraphics uses to provide 

additional control over assembly components), overriding the reference set specified in 

the Unigraphics model. 

• f - facetizes all the geometry. If this option is not specified, the converter will attempt to 

create CSG equivalent geometry wherever possible. (Note that this option is 

experimental.) 

• s - lists all features that were attempted to convert to CSG. (This option and the f option 

are mutually exclusive.) 

• u - extracts surface normals from the Unigraphics model. Using this option will result in 

a BRL-CAD model that is significantly larger than when not using it, but raytraced 

objects will appear much smoother. 

• o output.g - specifies the name of the file to receive the BRL-CAD model. 

• t tolerance_distance - specifies the minimum distance (in millimeters) allowed 

between distinct vertices. Vertices closer than this minimum will be considered to be the 

same vertex (the default value is 0.005 mm). 

• a surface_normal_tolerance - specifies a surface normal error tolerance (in 

degrees) for the facetization process. By default, the surface normal is not considered 

during facetization. 

• c min_chamfer - specifies that any chamfer with dimensions less than the provided 

minimum (in millimeters) will be ignored. By default, no features are ignored. 

• r min_round - specifies that any round (or fillet) with a radius less than the specified 

minimum (in millimeters) will be ignored. By default, no features are ignored. 

For more information on the Unigraphics file format and conversion potential, see the EDS web 

site at http://www.eds.com/products/plm/unigraphics_nx/ (EDS, 2003c). 

4.13 Converting From Viewpoint Datalabs Format 

Viewpoint Datalabs started out as a commercial supplier of 3-D models, maintaining a large 

repository of facetted models of many objects. The company has since grown to provide more 

services than models, and its model repository is now maintained by Digimation, Inc. 

The viewpoint-g converter converts the Viewpoint Datalabs coor/elem format to BRL-CAD 

format. Objects in the input files are converted to regions, each consisting of a single BOT 

primitive. The converter will assign vertex normals if they are present in the input files. Two 

files are expected, one that contains vertex coordinates (and optional normals) and one that lists 

the vertex numbers for each polygonal face. This format was used by the original Viewpoint 

Datalabs model repository. The current repository uses more common formats such as DXF and 

VRML. 

The syntax for the converter is as follows: 

viewpoint-g [options] 
25

The options for the command are as follows: 

• c coord_file_name - sets the input vertex coordinates file name (required). 

• e elements_file_name - sets the input faces file name (required). 

• o output_file_name - sets the output BRL-CAD database name (default is 

viewpoint.g). 

• t tolerance_distance - sets distance calculation tolerance (in millimeters) 

(default is 0.005). 

For more information about the Datalabs format and conversion potential, see the Digimation 

web site at http://www.digimation.com (Digimation, 2003). 

5. Converting From BRL-CAD 

5.1 General Information About Converting From BRL-CAD 

Table 3 contains the primary file formats (other than ASCII) to which BRL-CAD currently 

converts. Once again, the “g-“ designation at the beginning of each converter name indicates 

that the conversion process begins with BRL-CAD’s standard geometry (.g) format. 

Note that most of the BRL-CAD export converters operate by tessellating all the primitive 

shapes in the BRL-CAD model, then evaluating the Boolean formula for each region. The result 

of this is a facetted representation of each region. The tessellation tolerances are used to 

determine the coarseness of the tessellation of curved surfaces. Smaller tolerances result in 

more, smaller facets and a larger output file. The calculation tolerance is the minimum distance 

between any two vertices in a region. Any vertices closer than this are fused into a single vertex. 

Table 3. BRL-CAD export converters. 

CAD Format Export Converter 

ACAD g-acad 

AutoCAD DXF g-dxf 

EUCLID a g-euclid 

IGES g-iges 

Jack g-jack 

STL g-stl 

TANKILL g-tankill 

VRML g-vrml 

Wavefront (.obj) g-wave 

X3D g-x3d a Conversion is to the ASCII EUCLID decoded format, not 

native format. 

Occasionally, the Boolean evaluation process will fail. When this happens, an error message 

specifying the region that failed to convert will be generated. The converter will then proceed to 

work on the next region to be converted. 
26

5.2 Converting to ACAD Format 

The Advanced Computer Aided Design (ACAD) format is the format for the in-house CAD 

package developed in the early 1980s by Lockheed Martin Tactical Aircraft Systems (then 

General Dynamics - Fort Worth) and distributed since 1995 by the Electromagnetic Code 

Consortium. The package was developed to improve the product design process by eliminating 

manual drawing boards and reducing design iteration cycle times. Notable aircraft programs that 

have used ACAD include the Joint Strike Fighter (F-35), the Raptor (F-22), the Advanced 

Technology Fighter, the National Aerospace Plane (X-30), and the Attack/Fighter-Experimental 

(A/F-X) (SURVICE Engineering Company, 2002). 

A notable characteristic of ACAD is the layers by which modeled parts are separated and 

managed (e.g., by subsystem). The package stores geometric data in a relational database that 

uses parent-child dependencies to enable automatic and rapid geometric modification. For 

example, changing a control spline of a fuselage will automatically regenerate any surface(s) 

built with that spline. Any geometry that is associated to the fuselage surface (i.e., plane/curve 

and surface intersections, fillets, etc.) will then regenerate. 

One of ACAD’s strengths is its bidirectional IGES translator, which provides compatibility with 

other IGES-supported formats, including CADAM, CATIA, COMPUTERVISION, AutoCAD, 

and Pro/E. 

Is it ACAD or AutoCAD? 

Although these packages are often confused, ACAD was developed 

by Lockheed Martin and is not connected with AutoCAD, which 

was developed by Autodesk Inc. 

As its name implies, the g-acad utility converts a file in BRL-CAD format to a file in facetted 

ACAD format. The form of the command is as follows: 

g-acad [options] –o output_file input.g object(s) 

The options for the g-acad command are as follows: 

• i - designates that the output be in inches (the default is millimeters). 

• x lvl - sets librt debug flag (see raytrace.h in the binary distribution for definitions of 

DEBUG_xxx). 

• X lvl - sets the NMG library debug flag (see nmg.h in the binary distribution for 

definitions of DEBUG_xxx). 

• e error_file - sends error messages to specified file (default is standard error 

[stderr]). 

• v - verbose; prints out progress information. 
27

• D tolerance_distance - sets distance calculation tolerance (in millimeters) 

(default is 0.005). 

• r rel_tess_tol - sets relative tessellation tolerance (normally 0.0 to 1.0) (default is 

0.1). 

• a abs_tess_tol - sets absolute tessellation tolerance to specified value (in 

millimeters) (overrides r option). 

• n norm_tess_tol - sets surface normal tessellation tolerance (angle in radians) 

(overrides r option). 

For more information on the ACAD file format and conversion potential, contact Lockheed 

Martin Tactical Aircraft Systems at http://www.lockheedmartin.com (Lockheed Martin, 2003). 

5.3 Converting to AutoCAD DXF Format 

The g-dxf command converts BRL-CAD objects to the previously mentioned AutoCAD DXF 

format. The syntax for this command is as follows: 

g-dxf [options] input.g object(s) 

The options for the g-dxf command are as follows: 

• i - requests the output DXF file to be in inches (default is millimeters). 

• o output.dxf - specifies the file to receive the DXF output (default is stdout). 

• p - requests that the output DXF file consist of POLYFACE MESH entities (the default 

is 3DFACE entities). 

The command also accepts the v, r, a, n, x, and X options, which have been discussed in the 

import converters portion of this document (section 4). 

For more information on the DXF file format and export possibilities, see the on-line 

documentation on the Autodesk web site at http://www.autodesk.com (Autodesk, Inc., 2003). 

5.4 Converting to EUCLID Format 

The g-euclid command converts BRL-CAD objects to a EUCLID “decoded” facetted format. 

Note that, as previously mentioned, this format is not native EUCLID format. The form of the 

command is as follows: 

g-euclid [options] input.g objects(s) 

The options for the g-euclid command are as follows: 

• u units - sets the output units (the default is millimeters). 

• o output_file_name - sets the output file name (the default is stdout). 

For more information about the EUCLID file format and export possibilities, see the 

MDTVISION web site at http://support.mdtvision.com (MDTVISION, 2003). 
28

5.5 Converting to IGES Format 

The syntax for the BRL-CAD to IGES converter is as follows: 

g-iges [options] input.g object(s) 

The options for the g-iges command are as follows: 

• o output_name - indicates the name of the resulting IGES file. If the m option is 

specified, then this is the name of a directory where resulting IGES files will be placed 

(the directory must already exist). (The default is stdout.) 

• f - designates that the resulting IGES file will be entirely facetted BREP entities. The 

f and t options (the next option discussed) are mutually exclusive. 

• t - designates that the resulting IGES file will be entirely trimmed NURB entities similar 

to the f option, but each facet is represented as a trimmed NURB. The f and t options are 

mutually exclusive. 

• m - Each region in the specified objects is output in an individual IGES file and placed in 

the directory specified with the o option. This option implies the t option. 

• s - Used in conjunction with the f or t options to indicate that the facets should all be 

expressed as planar NURB surfaces rather than the default simple planar surface. 

With the t and f options, the converter will employ Boolean evaluation of each region. If neither 

option is specified, then a CSG IGES file will be produced. The command also accepts the 

previously mentioned v, r, a, n, x, and X options. 

Package-Preferred IGES Converter Options 

Note that different CAD packages seem to “prefer” certain options 

for the IGES converter. In particular, Pro/E works well with IGES 

files that use the m option, and Unigraphics works well with IGES 

files that use the s option. 

For more information about the IGES file format and export possibilities, see the NIST web site 

at http://www.nist.gov/iges (NIST, 2003). 

5.6 Converting to Jack 

The syntax for the jack-g converter is as follows: 

jack-g [options] input.jack output.g 
29

The options for the jack-g command are as follows: 

• r region_name - specifies the name of the region to create in the BRL-CAD output (by 

default, this name is constructed from the input file name). 

• g combination_name - specifies the name of a combination to be created to hold the 

region produced. If not specified, no combination will be created. 

For more information about the Jack file format and export possibilities, see the EDS web page 

at http://www.eds.com/products/plm/efactory/jack/ (EDS, 2003a). 

5.7 Converting to STL Format 

The syntax for the BRL-CAD to STL converter is as follows: 

g-stl [options] input.g object(s) 

The options for the g-stl command are as follows: 

• o output_name - specifies the name of the file to receive the STL format output. 

This option is mutually exclusive with the m option. (The default is stdout.) 

• m directory_name - specifies the name of an existing directory where output STL 

files will be placed. This option is mutually exclusive with the o option. When this 

option is exercised, each region in the specified object(s) is output in a separate file 

written to the specified directory. The file names will be based on the BRL-CAD 

database path to the region, with “/” characters replaced by “@” and “.” characters 

replaced by “_”. 

• b - produces binary format STL files. The combination of this option and the o option 

produces a binary STL file containing one solid object representing all the regions in the 

specified object(s). (The default output format is ASCII.) 

• D calculation_tolerance - sets the distance calculation tolerance (in 

millimeters) (the default is 0.005). 

• i - produces an STL file in units of inches (the default is millimeters). 

The command also accepts the previously mentioned v, r, a, n, x, and X options. 

For more information on the STL file format and export possibilities, see the 3D Systems web 

site at http://www.3dsystems.com (3D Systems, 2003). 

5.8 Converting to TANKILL Format 

As discussed previously, the TANKILL format is another purely triangulated representation of 

solid objects. 

The syntax for the g-tankill command is as follows: 

g-tankill [options] input.g object(s) 
30

The options for the g-tankill command are as follows: 

• i idents_file - specifies that the ident numbers in the output file should be assigned 

sequentially as BRL-CAD regions are encountered (rather than using the ident numbers 

assigned in the BRL-CAD file). A mapping of the assigned ident numbers and the BRLCAD regions is written to the specified idents_file. 

• s surroundings_code - specifies the surroundings code, which is a code 

TANKILL uses to indicate the type of volume that surrounds an object (the default is 

1000). 

• o output_name - specifies the name of a file to receive the output (default it stdout). 

The command also accepts the previously mentioned v, r, a, n, x, and X options. 

For more information on the TANKILL file format and export possibilities, contact the 

Advantage Business Group at the Barbician, East Street, Farnham, Surrey GU9 7TB or visit the 

web site at http://www.advantage-business.co.uk (Advantage Business Group, 2003). 

5.9 Converting to VRML Format 

The Virtual Reality Modeling Language (VRML) began in 1994 at the first World Wide Web 

Conference. The format—which is maintained by the nonprofit, vendor-neutral Web3D 

Consortium—was designed to be a Web-interoperable standard for specifying multiparticipant 

3-D graphical visualization tools. When a user selects a hyperlink to a VRML document from 

within a correctly configured WWW browser, a VRML viewer is launched to navigate through a 

given geometry (e.g., a model of a building). Future versions of VRML are planned to include 

more advanced features, including animations, motion physics, and real-time multiuser 

interaction (Web3D Consortium, 2003b). 

The g-vrml command converts BRL-CAD objects to the VRML 2.0 format. It takes the 

following form: 

g-vrml [options] input.g object(s) 

The options for the command are as follows: 

• d tolerance_distance - sets distance calculation tolerance in millimeters (the 

default is 0.005). 

• u units - sets the desired output units (the default is millimeters). 

• o output_file_name - sets output file name (the default is stdout). 

The command also accepts the previously mentioned v, r, a, n, x, and X options. 

For more information on VRML, see the Web3D Consortium web site at http://www.web3d.org 

/fs_technicalinfo.htm (Web3D Consortium, 2003b). 
31

The Web3D Consortium 

The Web3D Consortium is a nonprofit organization promoting 

open standards for 3-D Web and broadcast applications. Members 

include leading corporate and educational institutions, including 

3Dlabs, ATI Technologies, blaxxun, Nexternet, OpenWorlds, Sony, 

SGDL Systems, ParallelGraphics, the Naval Postgraduate School, 

and the Army Simulation, Training, and Instrumentation Command 

(STRICOM). Notable efforts include developing VRML 

specifications and an improved, componentized new-generation 

specification for Web3D, including X3D (see subsection 5.11) 

(SURVICE Engineering Company, 2002). 

5.10 Converting to Wavefront Format 

The Wavefront OBJ format was developed in 1984 by Wavefront Technologies (now Alias, a 

company that merged with Wavefront under SGI). The format, which was intended for use with 

the Wavefront Advanced Visualizer, consists of lines, polygons, and smooth surfaces. 

This g-wave converter produces an OBJ file that consists entirely of polygons (triangles). The 

command takes the following form: 

g-wave [options] input.g object(s) 

The options for the command are as follows: 

• m - includes “usemtl” statements in the output (encoded aircode, los, and material 

number). 

• i - sets the output units to inches (the default is millimeters). 

• d tolerance_distance - sets distance calculation tolerance in millimeters (the 

default is 0.005). 

• u - includes vertexuse normals in the output. 

• o output.obj - specifies the output file name (default is stdout). 

The command also accepts the previously mentioned v, r, a, n, x, and X options. 

For more information on the Wavefront file format and export possibilities, see the Alias web 

site at http://www.alias.com/eng/index.shtml (Alias, 2003). 

5.11 Converting to X3D Format 

The successor to the Web3D Consortium’s VRML format, the Extensible 3D (X3D) format was 

designed to be a standard for interactive web- and broadcast-based 3-D content. Its intended 

uses include engineering and scientific visualization, multimedia presentations, entertainment 

and educational applications, web pages, and shared virtual worlds. It is also intended to serve 

as a universal interchange format for integrated 3-D graphics and multimedia. X3D possesses 
32

several improvements over VRML, including advanced APIs, additional data encoding formats, 

stricter conformance, and a componentized architecture that allows for a modular approach 

(Web3D Consortium, 2003a). 

The g-x3d command converts BRL-CAD objects to the X3D format. It takes the following 

form: 

g-x3d [options] input.g object(s) 

The options for the command are as follows: 

• d tolerance_distance - sets distance calculation tolerance in millimeters (the 

default is 0.005). 

• u units - sets the desired output units (the default is millimeters). 

• o output_file_name - sets output file name (the default is stdout). 

The command also accepts the previously mentioned v, r, a, n, x, and X options. 

For more information on the X3D file format and export possibilities, see the Web3D 

Consortium web site at http://www.web3d.org/technicalinfo/specifications/ISO_IEC_19775 

(Web3D Consortium, 2003a). 

6. Building a New Converter 

6.1 General Information About Building a New Converter 

If the previously mentioned import/export converters do not meet the user’s needs, a custom 

converter can be built. The effort required to accomplish this, of course, depends on the user’s 

coding ability, the intended model use, and the available resources. If one needs to convert from 

a simple ASCII format consisting entirely of triangles to BRL-CAD, a few days of effort will 

probably suffice. On the other hand, if one wants to develop a converter from a proprietary 

commercial CAD system to BRL-CAD, several months of effort and training may be required. 

For example, Pro/E, Unigraphics, and similar commercial CAD systems provide libraries and 

APIs that a developer can use to access their models. Typically, training is required in order to 

understand what these libraries can do and how they work, and training in the use of the CAD 

system itself may be required as a prerequisite to understand the package environment and 

terminology. The purchase of expensive licenses is also a typical requirement, and, in some 

cases, the use of the code developed will require the purchase of another license. 

But before one begins developing a new converter, one should first consider the possibility of 

using combinations of existing converters. For example, if one needs to convert from a 

commercial CAD system to BRL-CAD and no converter exists for that system, he might 

consider importing his model into Pro/E or Unigraphics and converting to BRL-CAD from there. 
33

Understandably, this may result in the loss of some information (and repeated conversions 

usually mean greater losses), but restoring that information manually may be more cost effective 

than building a new converter from scratch. Another possibility might be to convert the 

geometry to STL format and then convert the STL to BRL-CAD using stl-g (see section 4.10). 

6.2 Converting From BRL-CAD 

Converters from BRL-CAD to another format generally come in one of two varieties. The first 

type converts BRL-CAD objects to their CSG counterparts in the receiving system. This type of 

conversion rarely happens as most commercial CAD systems have extremely limited CSG 

capabilities. If this is the case, however, the converter merely needs to visit every object in the 

BRL-CAD model and output it in the form required by the receiving system. A sample code 

(named g-xxx.c) to accomplish this is included in the conv directory in the BRL-CAD source 

distribution. The only missing piece of this code is the actual output of the receiving format. 

The more common type of conversion is when the desired output format is not CSG. Currently, 

the best choice in this situation is to produce a triangular facetted approximation of the BRLCAD regions and output that approximation in the receiving format. A sample code (named 

g-xxx_facets.c) for this approach is also included in the conv directory in the BRL-CAD source 

distribution. Again, only the part where the facets are output in the receiving format is missing. 

The sample code performs Boolean evaluation of the regions using tessellated versions of the 

CSG primitives. This technique is not always successful, so the sample code catches the failures, 

notifies the user, and continues when a failure occurs. 

6.3 Converting to BRL-CAD 

A similar situation exists for converting to BRL-CAD. If one is fortunate enough to be 

converting from a CSG system, then just converting everything to its corresponding BRL-CAD 

representation is all that is required. The libwdb library distributed with BRL-CAD contains all 

the routines needed to build primitives, regions, and combinations in BRL-CAD. 

To create an empty BRL-CAD model, use the following code snippet: 

struct rt_wdb *wdbp; 

if( (wdbp=wdb_fopen( new_file_name ) ) == RT_WDB_NULL ) { 

bu_log( “Failed to open output file (%s)\n”, new_file_name ); 

exit( 1 ) 

}

Then use the wdbp pointer to pass to all the routines in libwdb. 

For example, to build a sphere centered at coordinates (1.0, 2.0, and 3.0) with a radius of 5.0 mm 

in BRL-CAD, the following call to mk_sphere will suffice: 

char *sphere_name=”s1”; 

point_t center; 

fastf_t radius;
34

/* set the center and radius values */ 

VSET( center, 1, 2, 3 ); 

radius = 5.0; 

/* actually build the sphere */ 

if( mk_sph( wdbp, sphere_name, center, radius ) ) { 

bu_log( “Failed to make a sphere (%s)\n”, sphere_name ); 

exit( 1 ); 

}

To build a region or combination in BRL-CAD, a call to mk_comb will do the job. This is a bit 

more complicated as it requires building a list of members prior to the call to mk_comb. In the 

example that follows, a region consisting of primitive “s2” subtracted from primitive “s1” is 

constructed: 

struct bu_list head; 

struct wmember *wmp; 

/* initialize the list of members for this combination */ 

BU_LIST_INIT( &head ); 

/* add the first member (use the UNION operator here) */ 

if( (wmp=mk_addmember( “s1”, &headp, NULL, WMOP_UNION) ) == WMEMBER_NULL 

) { 

bu_log( “Failed to add member(s1)\n” ); 

exit( 1 ); 

} 

/* subtract the second member */ 

if( (wmp=mk_addmember( “s2”, &headp, NULL, WMOP_SUBTRACT) ) == 

WMEMBER_NULL ) { 

bu_log( “Failed to add member(s2)\n” ); 

exit( 1 ); 

} 

/* build the actual region */q 

if( mk_comb( wdbp, “region_name”, &head, 1, NULL, NULL, NULL, 1000, 0, 

1, 100, 0, 0, 0 ) ) { 

bu_log( “Error building region\n” ); 

exit( 1 ); 

}

If the system you are converting from is not a CSG system (which is the most likely case), then 

you will need to do some processing on the originating system to get the data in a form that 

BRL-CAD can use. If you have the API module for the originating system, you may be able to 

use one of those routines to create a triangulated representation of the model. This triangle data 

can then be used to build BOT primitives in the BRL-CAD model using the mk_bot routine. 

Each BOT primitive can be placed in a region of its own (as shown previously). 

A hybrid approach is also possible where simple shapes that can be identified as BRL-CAD 

primitive shapes are converted as BRL-CAD primitives, and everything else is converted as BOT 

primitives. 

If the converter requires building BOT primitives, you might want to consider a converter that 

produces a BRL-CAD ASCII format database. The “C” mk_bot routine is a very thin wrapper 

for the actual internal format of the BOT primitive and requires an intimate knowledge of its 
35

structure. The ASCII format is considerably simpler and easier to work with in this situation. 

As mentioned previously, the current ASCII format is simply a Tcl script. For example, the 

following is the ASCII form of a BRL-CAD database containing a single BOT: 

title {example of a simple BOT primitive} 

units mm 

put {tetra} bot mode volume orient rh flags {} V { { 0 0 1000 } { -500 0 

-1000 } { -500 -500 -1000 } { 500 -500 -1000 }} F { { 0 1 3 } { 0 1 2 } 

{ 0 2 3 } { 1 2 3 }}

The aforementioned example creates a BRL-CAD database with the title of “example of a simple 

BOT primitive,” with preferred units of millimeters and a single tetrahedron-shaped BOT 

primitive. The line that begins with “put” creates the BOT primitive (note that there are actually 

only three lines in this example; the last line is wrapped in this report for clarity). This line is a 

command to the Tcl interpreter. The “put” is the command, “tetra” is the name of the object to 

be created, and “bot” is the type of object to create. The remainder of the line consists of 

key/value pairs. This line creates a BOT primitive named “tetra,” using volume mode (meaning 

this BOT encloses space), with right-hand (rh) rule triangle orientation, and no flags. 

The V key introduces the list of vertices for this BOT primitive, and the F key introduces the list 

of triangle faces (each integer is an index into the list of vertices). Additional put commands 

build more objects in the database. For example, to create a region using the tetra object, the 

user would append the following line to the aforementioned file: 

put {aregion} comb region yes tree {l tetra} 

This line creates a BRL-CAD combination named aregion, which is a region and consists only 

of the tetra object created earlier. The resulting file is converted to BRL-CAD binary format 

using asc2g. 

For an example of code that produces this type of output, see the g2asc.c file in the conv

directory of the source distribution. 

7. Postconversion Issues 

Before converted geometry can be used in a given application or analysis (e.g., a vulnerability 

study), there are several tasks that often need to be performed. They include the following: 

• Modeling Objects That Failed to Convert – Even in the best conversion processes, 

there can be individual objects that, for some reason, do not successfully convert to or 

from the BRL-CAD format. When this happens, the converter reports an error message, 

and the modeler/analyst must either manually rebuild the objects or—if they are not 

critical to the model—omit them (see the following bulleted item).
36

• Remodeling or Eliminating Excessively Detailed, Complex, or Unnecessary Objects - 

In some cases, the problem with converted geometry is not about objects that failed to 

convert but rather about objects that did convert. For example, highly detailed objects 

such as screens, splined grids, and geometrically complex exterior components—which 

are often not needed for vulnerability studies—may unnecessarily enlarge a model’s file 

size and slow its performance speed. Thus, the modeler/analyst has to check for these 

objects and either remodel them in a simpler manner or eliminate them altogether. 

• Modeling Air - Some applications that evaluate ballistic penetration, fire, vaporific 

effects, etc., require that the interior space within a given geometry contain continuous 

regions of air. Therefore, it is sometimes necessary to take a converted model and model 

various air compartments within them (e.g., the crew, passenger, and engine 

compartments). In addition, in cases where one air region contacts another (e.g., in an 

open turret hatch, a gun turret, or an exhaust grill), a thin layer of “phantom armor” is 

sometimes needed to separate the two. For more information about air components and 

codes, see Winner et al. (2002). 

• Assigning Material Codes and Line-of-Sight (LOS) Densities - To more realistically 

approximate actual material weights and properties, modelers often have to assign 

material codes to converted geometry regions. In addition, if a region is not solid 

material, a density percentage is assigned. For a list of standardized material codes and 

densities, see Winner et al. (2002). 

• Assigning Optical Shaders and Colors - Although most engineering analysis 

applications focus on what geometry is made of, rather than what it looks like, it is often 

necessary for viewing, publication, or presentation purposes to assign visual properties to 

converted objects in order to better simulate material characteristics (e.g., mirrored or 

transparent effects) or to match standard system colors (e.g., electrical systems are forest 

green [50 145 20]). For a list of standardized system color values, see Volume III of this 

tutorial series (Butler et al., 2003) or Winner et al. (2002). For more information on 

coloring and shading in MGED, see Volume II of this tutorial series (Butler et al., 2001). 

• Assigning Component Identification Numbers - Some vulnerability interrogation 

applications rely on components being grouped into common regions. For example, the 

tank track ident number needs to be assigned to each track link region. 

• Assigning Meaningful Names to the Converted Objects - As mentioned previously, 

some packages (e.g., Pro/E) typically label their geometry with part numbers instead of 

part names. Although this practice can be effective for manufacturing and other 

production purposes, when the objects are converted, the modeler/analyst often has to 

assign meaningful part names so that the model can be more easily organized, analyzed, 

and manipulated. Accordingly, it is important that complete mappings of part numbers to 

part names are supplied during the conversion process. 

• Combining Multiple Converted Files Into a Single BRL-CAD Model - Files from 

packages such as Pro/E and Unigraphics are often so large that entire models cannot be 

loaded all at once, and thus the BRL-CAD import converters for these formats must 

convert the geometry file by file and system by system. As a result, the BRL-CAD 

modeler/analyst often has to combine all the files of converted geometry so that they can 

be displayed and evaluated as a single, integrated model (see the discussion on dbconcat

in Volume III of this tutorial series [Butler et al., 2003]).
37

• Combining or Breaking Down Regions to Match Analyst Requirements - In some 

cases, converted geometry does not match the “grouping” requirements of an analyst or 

application and therefore must be regrouped. For example, the converted objects of an 

engine might need to be grouped into a single engine region. Conversely, a single engine 

region might need to be broken down into separate regions. 

• Reorganizing or Creating a Model Hierarchy - Some geometry (e.g., an STL file) is 

converted with little to no tree structure or organization. In such cases, the 

modeler/analyst often needs to set up a basic tree hierarchy of parts and assemblies so 

that the model can be more easily handled. For more information about model 

structuring, see Volume III of this tutorial series (Butler et al., 2003). 
38

8. References 

3D Systems, Inc. http://www.3dsystems.com (accessed October 2003). 

Advantage Business Group. http://www.advantage-business.co.uk/ (accessed October 2003). 

Alias. http://www.alias.com/eng/index.shtml (accessed October 2003). 

Autodesk, Inc. AutoCAD 2004 DXF. http://adeskftp.autodesk.com/prodsupp/downloads 

/dxf.pdf (accessed February 2003). 

Butler, L. A.; Edwards, E. W. BRL-CAD Tutorial Series: Volume I – Overview and Installation;

ARL-SR-113; U.S. Army Research Laboratory: Aberdeen Proving Ground, MD, February 

2002. 

Butler, L. A.; Edwards, E. W.; Schueler, B. J.; Parker, R. G.; Anderson, J. R. BRL-CAD Tutorial 

Series: Volume II – Introduction to MGED; ARL-SR-102; U.S Army Research Laboratory: 

Aberdeen Proving Ground, MD, April 2001. 

Butler, L. A.; Edwards, E. W.; Kregel, D. L. BRL-CAD Tutorial Series: Volume III – Principles 

of Effective Modeling; ARL-SR-119; U.S. Army Research Laboratory: Aberdeen Proving 

Ground, MD, September 2003. 

Butler, L. A.; Muuss, M. J.; Tanenbaum, P. J.; Anderson, J. R.; Parker, R. G.; Bowers, R. A.; 

Johnson, C. T.; Edwards, E. W. New BRL-CAD Database Format, Version 5, Draft, U.S. 

Army Research Laboratory: Aberdeen Proving Ground, MD, December 2003. 

Digimation, Inc. http://www.digimation.com (accessed October 2003). 

Electronic Data Systems (EDS). Jack. http://www.eds.com/products/plm/efactory/jack/ 

(accessed October 2003a). 

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NO. OF NO. OF 

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 AT AUSTIN 

 3925 W BRAKER LN STE 400 

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 2800 POWDER MILL RD 

 ADELPHI MD 20783-1197 

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 US ARMY RESEARCH LAB 

 AMSRD ARL CI OK TL 

 2800 POWDER MILL RD 

 ADELPHI MD 20783-1197 

 3 DIRECTOR 

 US ARMY RESEARCH LAB 

 AMSRD ARL CS IS T 

 2800 POWDER MILL RD 

 ADELPHI MD 20783-1197 

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42

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 R MUTZELBURG 

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 WSMR NM 88002-5502 

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 J PALOMO 

 WSMR NM 88002 5513 

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 R FLORES 

 WSMR NM 88002 5513 

 1 USARL 

 AMSRD ARL SL EI 

 J NOWAK 

 FT MONMOUTH NJ 07703-5601 

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 APG MD 21005 5055 

 1 US ARMY EVALUATION CTR 

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 R BOWEN 

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 R LAUGHMAN 

 4120 SUSQUEHANNA AVE 

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 13 DIR USARL 

 AMSRD ARL SL 

 J BEILFUSS 

 P DEITZ 

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 J FRANZ 

 M PERRY 

 P TANENBAUM 

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 S JUARASCIO 

 M RITONDO 

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 R GROTE 
NO. OF 

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43

 AMSRD ARL SL BE 

 L ROACH 

 AMSRD ARL SL E 

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 E PANUSKA 
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 US ARMY RESEARCH LAB 

 AMSRD ARL D 

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 US ARMY RESEARCH LAB 

 AMSRD ARL CI AP 

 2800 POWDER MILL RD 

 ADELPHI MD 20783-1197 

 1 DIRECTOR 

 US ARMY RESEARCH LAB 

 AMSRD ARL SL 

 J PALOMO 

 WSMR NM 88002-5513 

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 US ARMY RESEARCH LAB 

 AMSRD ARL SL E (25 CPS) 

 WSMR NM 88002-5513 

 1 NAWC 

 WEAPONS DIVISION 

 CODE 418300D A WEARNER 

 BLDG 91073 

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 AAC/ENMA 

 D MCCOWN 

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 B THORN 

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 EGLIN AFB FL 32542-5600 

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 A LAGRANGE 

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 6 THE SURVICE ENGNRG CO 

 E EDWARDS 

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 AMSRD ARL SL BD (15 CPS) 

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