topic Re: Exporting triangulation data in AutoCAD Forum

Exporting triangulation data

bostjan_gabr — Thu, 16 Feb 2023 13:34:40 GMT

I am new to AutoCAD and I would need to export the triangulation data of a 3D object in AutoCAD.

That is: list of 3D coordinates of points/vertices and list of triangles (either coordinates of the vertices or reference to points).

Ideally I would also have data of tetrahedra if the object is filled, but this is not necessary. Is something like this possible? If not, what software could give me this data?

Re: Exporting triangulation data

Ed__Jobe — Fri, 17 Feb 2023 16:37:10 GMT

What is your end goal? Are you trying to import into another software for generating something like G code? If so, then there are better tools than AutoCAD.

Re: Exporting triangulation data

leeminardi — Fri, 17 Feb 2023 18:06:25 GMT

There are several options for storing and retrieving 3D triangle data with AutoCAD. The 3DFACE object is often used. It stores the x,y,z world coordinates for up to 4 vertices. The fourth vertex is often ignored as there it may not lie on the same plane defined by the first three vertecies. 3DFACE data can be exported in an ASCII text file as well as other methods. An AutoCAD solid model can be exported in an stl format. An stl file is often used for 3D printing. It is a tesselated model of the solid which is essentially a collection of triangles in 3D space. Each of these methods outputs the 3D coordinates for each triangle and therefore the vertex coordinates are duplicated for adjacent 3Dfaces. They do not create a single unique list of vertices and a list referencing the vertices.

Civil 3D includes a TIN (Triangulated Irregular Network). It is a collection of 3D triangles used to define a surface for civil engineering applications.

As @Ed__Jobe noted, it wold help to know more about your application.

Re: Exporting triangulation data

bostjan_gabr — Fri, 17 Feb 2023 22:05:59 GMT

My end goal is purely academical/reserach. I am testing some algorithms for computing topological properties from large triangulations of bodies and want to obtain a examples from CAD models.

Re: Exporting triangulation data

tboehler — Fri, 17 Feb 2023 22:29:00 GMT

You might want to explore Autodesk Civil 3D. The TIN can be simple to complex depending on your data and site configuration/size.

Civil 3D allows for the use of Points, LiDAR, DEMs, breaklines (3d polylines), contours (polylines), feature lines, grading objects and more. Often models from other programs that have already optimized the ground data are imported in Civil 3D.

Re: Exporting triangulation data

Ed__Jobe — Fri, 17 Feb 2023 22:37:46 GMT

Since you are doing research, I assume you can code. You could explode the 3D solid, which would give you a bunch of region objects. Select them and run explode again to get lines and arcs. Then run the OVERKILL command to remove duplicate lines. Now code a sub that iterates all the objects in modelspace and logs the startpoint and endpoint of each entity.

Re: Exporting triangulation data

WeTanks — Fri, 17 Feb 2023 23:13:28 GMT

Please refer to the attached documnet.

Re: Exporting triangulation data

leeminardi — Sat, 18 Feb 2023 02:07:27 GMT

If you have the object as a solid object in AutoCAD you can export it as an STL binary file. YOu can then use a program like Paraview (free) to convert the file to ASCII.

For example, here's a torus with a cylinder subtracted from it as modeled in AutoCAD.

You can display it in AutoCAD as a tessellated model by changing the visualstyle to 2D Wireframe and then give the hide command.

You can change the density of triangles via Options, Rendered object smoothness.

Here's a sample of the STL ASCII file as viewed in an ASCII editor (e.g., Notepas++).

As mentioned previously, vertices are duplicated where adjacent triangles are coincident. The only software I know that creates a list of vertices and a list to triangles that reference the vertex list are FEA programs. If you really need that format for your data you might be able to find an FEA mesh generator that will mesh an object with triangular elements.

Re: Exporting triangulation data

bostjan_gabr — Wed, 22 Feb 2023 08:30:46 GMT

Thanks. I have the triangles.

Also another way: export to stl and then open the file in Python using numpy.stl