We believe that the two IGES CSG solids that are approximated by the translator can be more accurately represented by either improving the approximation methods or by implementing and supporting a solid of revolution and a solid of linear extrusion into BRL-CAD. The introduction of these solids as full-fledged BRL-CAD primitives would eliminate any need for approximation. Short of adding new solids, improvements to these approximations are possible. For example, a circular arc in a solid of revolution can be accurately modeled by a portion of a torus, or an elliptical section of curve in a solid of linear extrusion can be modeled as a section of a BRL-CAD gentgc. These types of improvements require considerable programming and cannot completely eliminate the need for a last resort approximation similar to the current method. However, because both of these solid types are based on two-dimensional curves and have simple generation schemes, they are both candidates for the application of the BRL-CAD spline solid. To apply a spline approach, each section of the defining curve would be converted to a spline curve of appropriate order. Then the curves would be used to generate BRL-CAD spline surfaces, which, when combined into a single BRL-CAD spline solid, would create an extremely accurate model of the intended solid.
We hope to begin development of a BRL-CAD-to-IGES translator. Development of a complete IGES translator package would position BRL-CAD for easy introduction into any established CAD oriented work place. The capability to transfer designs back and forth between a designer's commercial CAD systems and BRL-CAD would allow him access to BRL-CAD's applications codes without disrupting his normal design process. This would also allow the designer to quickly and easily prepare a BRL-CAD model of his design for submission in response to an Army Request for Proposals (RFP) or Source Selection Evaluation Board (SSEB), resulting in a cost saving for the Army. Such submissions have been required of Army materiel providers in the past and will likely continue to be required.
There are two inherent difficulties in developing a BRL-CAD-to-IGES translator. First, BRL-CAD has very flexible definitions for its primitive solids, allowing a wide range of possible shapes for the gentgc and the genarb8 solids, for example. The IGES entities include special cases of these BRL-CAD solids, but are not general enough to handle all cases. One approach to overcoming this problem is to persuade the IGES community to modify the IGES specification. The mechanism for accomplishing just that starts with a cooperative development and use of proposed, new IGES entities by two or more independent organizations. After a demonstrated implementation and exchange of data, the proposed entities may then be submitted to the IGES committee for review with the intent of inclusion in the specification (ultimately, inclusion in the ANSI standard). Another possible approach, which does not require modifying the specification, is to convert each unsupported (by IGES) BRL-CAD CSG primitive into a BREP object. This approach also appears to be a viable solution, but actually leads us to the second difficulty.
BRL-CAD is, for all practical purposes, a hybrid CAD system. Although BRL-CAD primarily acts as a CSG system, it allows BREP objects to be intermingled among the CSG primitives. The IGES specification maintains a definite separation between CSG and BREP systems and does not allow the two to mix. The distinction is drawn in the definition of the IGES boolean tree, allowing only CSG primitives, solid instances, and other boolean trees as operands. No BREP entities may occur in an IGES boolean tree. Again, there are two possible solutions to the general incompatibility of BRL-CAD trees containing BREPs and the IGES trees that cannot contain BREPS. Since IGES does not allow a mingling of CSG and BREP objects, one solution is to convert all the BRL-CAD objects to BREP objects prior to translation. This may be possible in the near future as the BRL-CAD NMG technology is perfected. In the near term, software can be written to tesselate all BRL-CAD objects to form a type of BREP object with all flat, faceted faces. This approach will solve the problem, but the entire model would then be reduced to approximations. However, the reader should be aware that many CAD systems reduce their objects to faceted BREPs as the final form and usually permit the level of approximation (such as approximating a circle by a series of straight line segments) to be set by the user. Therefore, one can argue that although the resulting approximations might not be rigorously correct from a perfectionist's point of view, the faceted BREPs are usually acceptable for most applications except for some signature applications. Eventually, the NMG objects are expected to be capable of supporting nonplanar surfaces such as spline surfaces; thus, more accurate versions of CSG objects could be constructed using the NMG solid. Even so, the conversion of the BRL-CAD model to all NMG objects for the purpose of translation to IGES BREP will cause the loss of the original architecture of the model because only the BREP counterpart to the boolean tree (not the boolean tree itself) will be translated. Also, translation of the IGES BREP (formerly NMG) back to BRL-CAD will not produce a model that is as easily edited and modified as the original boolean tree form of the CSG model. For example, a simple object that has a hole through it (in boolean tree form) is easily edited to change the size of the hole. However, the same object, after conversion to BREP and translation to/from IGES, presents some challenge to modify the surfaces that define the object's hole if the user requires the hole's size to be changed. But it should be noted that such a scenario would only occur if a BRL-CAD model must be handed off to another CAD system and modified, then for some reason had to be translated back into BRL-CAD. However one could make the assumption that once received from the other CAD system, the existing object(s) need no further modification; thus, the loss of the boolean tree in the process is not a significant shortcoming.
The second approach to this problem is, again, to influence the specification itself. Here, a small modification of just the IGES definition of the boolean tree would allow the faithful representation of any BRL-CAD object in an IGES file. The complete and unaltered structure of a BRL-CAD model could be preserved through translation to IGES and back to BRL-CAD if the IGES boolean tree allowed BREP objects as operands. This is clearly the optimal approach and one that the authors support.
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