CMGC: a CAD to Monte Carlo geometry conversion code

Automatic conversion from a computer-aided design (CAD) model to Monte Carlo geometry is one of the most effective methods for large-scale and detailed Monte Carlo modeling. The CAD to Monte Carlo geometry converter (CMGC) is a newly developed conversion code based on the boundary representation to constructive solid geometry (BRep -> CSG) conversion method. The goal of the conversion process in the CMGC is to generate an appropriate CSG representation to achieve highly efficient Monte Carlo simulations. We designed a complete solid decomposition scheme to split a complex solid into as few nonoverlapping simple sub-solids as possible. In the complete solid decomposition scheme, the complex solid is successively split by so-called direct, indirect, and auxiliary splitting surfaces. We defined the splitting edge and designed a method for determining the direct splitting surface based on the splitting edge, then provided a method for determining indirect and auxiliary splitting surfaces based on solid vertices. Only the sub-solids that contain concave boundary faces need to be supplemented with auxiliary surfaces because the solid is completely decomposed, which will reduce the redundancy in the CSG expression. After decomposition, these sub-solids are located on only one side of their natural and auxiliary surfaces; thus, each sub-solid can be described by the intersections of a series of half-spaces or geometrical primitives. The CMGC has a friendly graphical user interface and can convert a CAD model into geometry input files for several Monte Carlo codes. The reliability of the CMGC was evaluated by converting several complex models and calculating the relative volume errors. Moreover, JMCT was used to test the efficiency of the Monte Carlo simulation. The results showed that the converted models performed well in particle transport calculations.

从计算机辅助设计（CAD）模型到蒙特卡罗几何的自动转换是大规模和详细蒙特卡罗建模的最有效方法之一。CAD到蒙特卡罗几何转换器（CMGC）是一种基于边界表示到构造实体几何（BRep -> CSG）转换方法新开发的转换代码。CMGC中转换过程的目标是生成合适的CSG表示，以实现高效的蒙特卡罗模拟。我们设计了一种完整的实体分解方案，将一个复杂实体尽可能少地分解为不重叠的简单子实体。在完整的实体分解方案中，复杂实体依次被所谓的直接、间接和辅助分割面分割。我们定义了分割边，并设计了一种基于分割边确定直接分割面的方法，然后提供了一种基于实体顶点确定间接和辅助分割面的方法。由于实体已完全分解，只有包含凹边界面的子实体才需要补充辅助面，这将减少CSG表达式中的冗余。分解后，这些子实体仅位于其自然面和辅助面的一侧；因此，每个子实体都可以通过一系列半空间或几何基元的交集来描述。CMGC具有友好的图形用户界面，可以将CAD模型转换为几种蒙特卡罗代码的几何输入文件。通过转换几个复杂模型并计算相对体积误差来评估CMGC的可靠性。此外，使用JMCT来测试蒙特卡罗模拟的效率。结果表明，转换后的模型在粒子输运计算中表现良好。