Drawing a box with meshes in AutoCAD (C#)

Question

I've been playing around with AutoCAD API's PolygonMesh class. I want to draw a simple box using meshes. Here is a simple method I wrote to see how PolygonMesh behaves

[CommandMethod("TESTSIMPLEMESH")]
public void TestSimpleMesh()
{
    // Get the current document and database, and start a transaction
    Document acDoc = Autodesk.AutoCAD.ApplicationServices.Application.DocumentManager.MdiActiveDocument;
    Database acCurDb = acDoc.Database;

    using (Transaction acTrans = acCurDb.TransactionManager.StartTransaction())
    {
        BlockTable acBlkTbl = acTrans.GetObject(_database.BlockTableId, OpenMode.ForRead) as BlockTable; // Open the Block table record for read
        BlockTableRecord acBlkTblRec = acTrans.GetObject(acBlkTbl[BlockTableRecord.ModelSpace], OpenMode.ForWrite) as BlockTableRecord; // Open the Block table record Model space for write

        // Create a polygon mesh
        PolygonMesh acPolyMesh = new PolygonMesh();
        acPolyMesh.MSize = 4;
        acPolyMesh.NSize = 4;
        acPolyMesh.MakeNClosed(); //What is N???
        acPolyMesh.MakeMClosed(); //What is M???

        // Add the new object to the block table record and the transaction
        acBlkTblRec.AppendEntity(acPolyMesh);
        acTrans.AddNewlyCreatedDBObject(acPolyMesh, true);

        //Creating collection of points to add to the mesh
        Point3dCollection acPts3dPMesh = new Point3dCollection();
        acPts3dPMesh.Add(new Point3d(100, 100, 0));
        acPts3dPMesh.Add(new Point3d(200, 100, 0));
        acPts3dPMesh.Add(new Point3d(200, 200, 0));
        acPts3dPMesh.Add(new Point3d(100, 200, 0));
        acPts3dPMesh.Add(new Point3d(100, 100, 100));
        acPts3dPMesh.Add(new Point3d(200, 100, 100));
        acPts3dPMesh.Add(new Point3d(200, 200, 100));
        acPts3dPMesh.Add(new Point3d(100, 200, 100));

        //Converting those points to PolygonMeshVertecies and appending them to the PolygonMesh
        foreach (Point3d acPt3d in acPts3dPMesh)
        {
            PolygonMeshVertex acPMeshVer = new PolygonMeshVertex(acPt3d);
            acPolyMesh.AppendVertex(acPMeshVer);
            acTrans.AddNewlyCreatedDBObject(acPMeshVer, true);
        }

        // Save the new objects to the database
        acTrans.Commit();
    }
}

What I expect the method to do is draw a simple block. Instead I get a block but there are lines going back to the origin:

How do I change this method so that it just draws the box instead?

Also if someone could explain the values M and N relative to meshes that would be awesome.

Answer 1

From the AutoCAD ObjectARX help file:

PolygonMesh.MSize

Accesses the vertex count in the M direction. This is the number of vertices that will be used to make up an M row in the PolygonMesh if the PolyMeshType is SimpleMesh. For any other PolyMeshType, the M surface density value will be used as the row size.

PolygonMesh.NSize

Accesses the vertex count in the N direction. This is the number of vertices that will be used to make up an N column in the PolygonMesh if the PolyMeshType is SimpleMesh. For any other PolyMeshType, the N surface density value will be used as the column size.

So if you change your original code to M=2/N=4, you should get a better result.

PolygonMesh acPolyMesh = new PolygonMesh();
acPolyMesh.MSize = 2;
acPolyMesh.NSize = 4;

And the M x N should give the number of vertices added via AppendVertex. On your original code, 4x4=16, but you only added 8, so all the remaining points were left as 0,0,0 (origin), causing the problem you reported.