Geometry Exchange

If you are one of the lucky few that creates native geometry within a standalone Computer-Aided Engineering (CAE) analysis tool or within a single Computer-Aided Design (CAD) system with embedded CAE analysis tools, then stop reading now. Just a moment though, even if you have embedded CAE tools within your CAD system, but you import geometry for analysis you will still face geometry exchange challenges.

CAE tools are not islands. In many instances CAE tools have to perform analysis on non-native geometry. Decisions on what geometry exchange formats to use are critical in ensuring a relatively smooth flow through the analysis process. The accurate re-creation of geometry (shape definition) and topology (how shapes connect to form other shapes, e.g., how faces connect to form a solid) from one system to another is the essence of geometry exchange.

Native Formats

The best format for preserving geometry and topology is the native format (e.g., .sym for Caedium) for a particular CAE system and should be preferred above all other formats - but you already know that. You can also extend this thinking to the underlying geometry engine formats used by CAE systems. Popular geometry engines such as Parasolid, ACIS and Open Cascade are at the heart of many CAD and CAE tools. Each supports the reading and writing of a native geometry engine format that preserves geometry and topology just as well as a native format. The formats are:

  • Parasolid - .xt
  • ACIS - .sat
  • Open Cascade - .brep

Native Filters

Many CAE systems offer filters that read popular native geometry formats and convert them into the receiving system's geometry format. In theory having access to the original geometry file from another system means translation losses are minimal, but this is not always the case.

Exchange Formats

STEP (.stp) and IGES (.igs) are 2 formats that were specifically designed for geometry exchange between different CAE systems. IGES, the older format, is typically used to transfer surface geometry descriptions. IGES has a special solid (or volume) format, but it is rarely used. IGES also doesn't preserve shared topology (e.g., a common edge shared by 2 faces) consistently.

STEP, being the newer format, addresses many of the shortcomings of IGES , with good geometry and topology preservation. STEP also has support for solids and shells without using a special mode. So, when given the option, I would suggest using STEP in preference to IGES.

Facet Formats

Faceted formats represent geometry as collections of polygons (typically planar triangles and quadrilaterals). Clearly, this compromises the accuracy of the geometry shape definition. Whereas analytically defined geometry (NURBS, etc.) can represent smooth surfaces, facets cannot. This is a major issue for CAE tools that typically convert geometry into discrete elements by meshing. When the mesh is created upon a faceted geometry, it can cause anomalies due to the difference between the facets and the true smooth geometry, especially in areas of high curvature, such as the leading edge of an airfoil. Given the option, the analytic geometry formats should be preferred over faceted geometry formats.

Another issue that facets pose is that once in the CAE tool they are not usually capable of participating in operations with analytic geometry. This means you can't use them in subsequent geometry creation and modification operations such as booleans.

For all the issues with facets, they are a common form of geometry exchange. Some faceted formats, such as COLLADA (.dae) and Alias/Wavefront (.obj), provide rudimentary topology through facet groups that are helpful in specifying boundary conditions in a CAE system.

Another popular faceted format is STL (.stl), which gained prominence as the preferred format for rapid prototyping machines. Unfortunately STL is a poor choice for geometry exchange as it has no topology information (other than the facets are supposed to form a closed solid) and duplicate nodes for each facet.

Summary

To summarize - all geometry exchange is not equal. Clearly some formats are better at preserving geometry and topology than others. A preferred hierarchy of geometry exchange formats goes something like:

Native Formats > Native Filters > Exchange Formats (STEP > IGES) > Facets (almost anything > STL)

where > means better than.

Comments

3d Geometric Modelers

I'm considering purchasing a license for Google SketchUp Pro 8 for geometric modeling. It can output 3ds and dxf/dwg file formats. Can you recommend a converter from these formats to step or IGES? Also, do you have a view on SketchUp as a 3d modeler?

Polygon Modeling vs CAD (NURBS) Modeling

SketchUp is what I would call a faceted (or polygon) modeler, i.e., it represents geometry as collections of polygons (e.g., triangles, quadrilaterals). It is relatively easy to create boxy shapes with SketchUp, but the output is constrained to be polygons, not smooth spline-based surfaces created by typical CAD/NURBS modelers, such as TurboCAD, SolidWorks, AutoCAD, Alibre, or Rhino.

Also faceted modelers tend to store incomplete topology (geometry connections and associations). Often in faceted formats you'll find that the feature edges that should be present between connected faces are missing.

My guess is that in the conversion from polygons to STEP or IGES you are looking to recover smooth spline-based surfaces. This is not trivial and I'm not aware of any such converters. If this is your goal then you'd be better off directly using a typical CAD/NURBS modeler that will create smooth surfaces from the get go and, just as important for CAE analysis tools, preserve the topology of your models.

Analytic vs Discrete Geometry

For more on analytic vs facet geometry see "Analytic vs. Discrete Geometry" from Pointwise.

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