5. Converting From BRL-CAD

BRL-CAD export converters. 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.

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.

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.

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:

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).

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:

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 (Converting to BRL-CAD).

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).

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:

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

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:

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.

For more information about the IGES file format and export possibilities, see the NIST web site at http://www.nist.gov/iges (NIST, 2003).

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

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

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

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).

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:

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).

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)

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

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)

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:

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).

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:

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).

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 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:

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).