Generating a mesh for a 3D solid using AutoCADs Brep API from .NET

Thanks for Balaji Ramamoorthy – who recently joined our team in India – and Adam Nagy for helping generate the code for this post.

There are lots of reasons people might want to tessellate a 3D solid in AutoCAD to generate a mesh. The code in today’s post uses the Boundary Representation (Brep) API in AutoCAD to do just that, generating a set of 3D faces.

A few points about the implementation:

I’ve only made a small number of settings to control the mesh generation: more are available for you to experiment with.
It should be simple enough to generate a SubDMesh, rather than a set of faces, but that’s left as an exercise for the reader.
I only tested with a very simple solid (a sphere). The settings may need tweaking depending on the complexity of the solid you’re meshing.

Here’s the C# code:

using Autodesk.AutoCAD.ApplicationServices;
using Autodesk.AutoCAD.BoundaryRepresentation;
using Autodesk.AutoCAD.DatabaseServices;
using Autodesk.AutoCAD.EditorInput;
using Autodesk.AutoCAD.Geometry;
using Autodesk.AutoCAD.Runtime;
using AcDb = Autodesk.AutoCAD.DatabaseServices;
 
namespace MeshSolid
{
  public class MeshCreator
  {
    // Mesh a selected solid
 
    [CommandMethod("SOLMESH")]
    static public void MeshFromSolid()
    {
      Document doc =
        Application.DocumentManager.MdiActiveDocument;
      Database db = doc.Database;
      Editor ed = doc.Editor;
 
      // Ask the user to select a solid
 
      PromptEntityOptions peo =
        new PromptEntityOptions("Select a 3D solid");
      peo.SetRejectMessage("\nA 3D solid must be selected.");
      peo.AddAllowedClass(typeof(Solid3d), true);
      PromptEntityResult per = ed.GetEntity(peo);
 
      if (per.Status != PromptStatus.OK)
        return;
 
      Transaction tr =
        db.TransactionManager.StartTransaction();
      using (tr)
      {
        BlockTable bt =
          (BlockTable)tr.GetObject(
            db.BlockTableId,
            OpenMode.ForRead,
            false
          );
        BlockTableRecord btr =
          (BlockTableRecord)tr.GetObject(
            bt[BlockTableRecord.ModelSpace],
            OpenMode.ForWrite,
            false
          );
 
        Solid3d sol =
          tr.GetObject(
            per.ObjectId,
            OpenMode.ForRead
          ) as Solid3d;
 
        // Calculate the approximate size of our solid
 
        double length =
          sol.GeometricExtents.MinPoint.GetVectorTo(
            sol.GeometricExtents.MaxPoint
          ).Length;
 
        try
        {
          using (Brep brp = new Brep(sol))
          {
            // Create and set our mesh control object
 
            using (Mesh2dControl mc = new Mesh2dControl())
            {
              // These settings seem extreme, but only result
              // in ~500 faces for a sphere (during my testing,
              // anyway). Other control settings are available
 
              mc.MaxNodeSpacing = length / 10000;
              mc.MaxSubdivisions = 100000000;
 
              // Create a mesh filter object
 
              using (Mesh2dFilter mf = new Mesh2dFilter())
              {
                // Use it to map our control settings to the Brep
 
                mf.Insert(brp, mc);
 
                // Generate a mesh using the filter
 
                using (Mesh2d m = new Mesh2d(mf))
                {
                  // Extract individual faces from the mesh data
 
                  foreach (Element2d e in m.Element2ds)
                  {
                    Point3dCollection pts = new Point3dCollection();
                    foreach (Node n in e.Nodes)
                    {
                      pts.Add(n.Point);
                      n.Dispose();
                    }
                    e.Dispose();
 
                    // A face could be a triangle or a quadrilateral
                    // (the Booleans indicate the edge visibility)
 
                    AcDb.Face face = null;
                    if (pts.Count == 3)
                      face =
                        new AcDb.Face(
                          pts[0], pts[1], pts[2],
                          true, true, true, true
                        );
                    else if (pts.Count == 4)
                      face =
                        new AcDb.Face(
                          pts[0], pts[1], pts[2], pts[3],
                          true, true, true, true
                        );
 
                    // If we have a valid face, add it to the
                    // database and the transaction
 
                    if (face != null)
                    {
                      // Make each face yellow for visibility
 
                      face.ColorIndex = 2;
 
                      btr.AppendEntity(face);
                      tr.AddNewlyCreatedDBObject(face, true);
                    }
                  }
                }
              }
            }
          }
          tr.Commit();
        }
        catch (System.Exception ex)
        {
          ed.WriteMessage("Exception: {0}", ex.Message);
        }
      }
    }
  }
}
 

Here’s what happens when we run the SOLMESH command, selecting a simple sphere (which I later moved to the side, for comparison):

And now with the realistic visual style set, just to show we have opacity:

Update:

As mentioned in the comments, the code was randomly crashing on larger models. Viru Aithal and Balaji Ramamoorthy tracked down the problem, which is due to a common issue with AutoCAD objects: the enumerators for Mesh2dElement2dCollection and Element2dNodeCollection both return newly-created objects (a Node and an Element2d, respectively) which need to be disposed of before the .NET Finalizer (which works on a background thread) attempts to do so for you. Two lines (with calls to n.Dispose() and e.Dispose()) have been added to the above code to address this issue.