7. Postconversion Issues

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).

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]).

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).