4. Converting to BRL-CAD

BRL-CAD import converters 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.

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.

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:

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).

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:

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

enf-g input_file output.g

The options for the command are as follows:

For more information on the Elysium file format and import possibilities, see the Elysium web site at http://www.elysiuminc.com (Elysium, 2003).

EUCLID is one of Europe’s primary product design, manufacturing, and engineering CAD packages (though it is quickly being superseded 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.

The syntax for the converter is as follows:

euclid-g [options]

The options for the command are as follows:

For more information about the EUCLID file format and import possibilities, see the MDTVISION web site at http://support.mdtvision.com (MDTVISION, 2003).

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 .

There are several versions of FASTGEN currently in use. FASTGEN converters include FASTGEN4 and preprocessed FASTGEN Version 3 (also referred to as PATCH).

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. Thus, 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.iges

The options available for this command are as follows:

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).

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:

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).

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 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:

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).

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).

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 vehicles are so large (often several GBs in size), entire geometries typically cannot be loaded all at once. Thus, the BRL-CAD 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]).

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 File  ProE-BRL item in the Pro/E drop-down menu. The converter dialog box, shown in Converter dialog box, will then appear.

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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.

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).

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:

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).

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:

For more information on the TANKILL file format and import possibilities, contact the Advantage Business Group at

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

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).

There are three modeling methodologies offered with Unigraphics. First, explicit (or traditional) modeling uses points, curves, and surfaces with no associativity or history. Second, history-based 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:

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).

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]

The options for the command are as follows: