Writing out PolyfaceMesh seems to produce corrupted results.

[IainS1986]

I'm trying to write out some data to a PolyfaceMesh object. I'm replacing another Cad library so I'm porting over logic that previously worked in one library, and trying to work out the equivalent properties in ACad, so this 100% could just be my logic is wrong, but I'm not sure if its that or an internal issue.

Here are two example files, one was exported in a different library and shows the PolyfaceMesh how it should look, the other is from ACad.

(Hope these AutoDesk viewer links work, let me know if you need something different - they are only live for 27 days)

Correct: https://autode.sk/47N5WGg
Incorrect: https://autode.sk/3VRJ3Kn

Part of the confusion on my side is the VertexRecord within PolyfaceMesh. It requires 4 indices, when 3 is also valid for a PolyfaceMesh. I've read conflicting documentation (imagine that!) to what the 4th index is meant to be if you only have 3 points (ive tried leaving the index to 0, or the same as the 3rd point or the same as the 1st point, all produce different messes).

Here's some rough psuedo code of how I'm making a PolyfaceMesh

            var polyFaceMesh = new PolyfaceMesh();
            polyFaceMesh.LineWeight = LineweightType.W100;
            polyFaceMesh.Color = Color.Red;
            // polyFaceMesh.BlendCrease = false;
            polyFaceMesh.Layer = layer;
            // polyFaceMesh.Vertices.AddRange(points.Select(x => Generate3D_Point(unit, x)));
            polyFaceMesh.Vertices.AddRange(points.Select(GenerateCad_Vertex3D));
            for (short p = 0; p < points.Length; p += 3)
            {
                polyFaceMesh.Faces.Add(new VertexFaceRecord()
                {
                    Index1 = p,
                    Index2 = (short)(p + 1),
                    Index3 = (short)(p + 2),
                    Index4 = 0,
                });
            }
            cadDocument.ModelSpace.Entities.Add(polyFaceMesh);

I have a list of points, where the list of points are in sequeence of 3 where each group of 3 is a triangle.

This is very similar to a previous library, except that let me add a "MeshFace" with only 3 points, so I'm not sure what it 'did' under the hood.

Some confusion as well...

• VertexFaceRecord is itself, a Vertex - is that just weird Cad logic?
• PolyfaceMesh clone logic seems odd -

  ACadSharp/src/ACadSharp/Entities/PolyfaceMesh.cs

  Line 49 in 8a4aed3

  clone.Vertices.Add((VertexFaceRecord)v.Clone());

  • Its going through the Faces, then cloning these as VertexRecords into the Vertices
  • Is that right? Because when I add to the Faces (in the above psuedo code), i'm also adding Vertices but not the same ones as the Faces property.

Basically, anyone got an example of making a PolyfaceMesh from a list of points in groups of 3! Is it just my logic or is there a bug?

---

Release needed

• Yes, using the Nuget package.
• No, using the github repo.

[IainS1986]

Also, I tried adding the Vertices as clones of the VertexRecord in the Faces list (same as how Clone is implemented) and I get an exception,

Wrong vertex type AcDbFaceRecord for AcDbPolyFaceMesh

So not sure if the Clone function is correct.

This is all I tried on my pseudocode above,

                polyFaceMesh.Faces.Add(vertexRecord);
                polyFaceMesh.Vertices.Add((Vertex) vertexRecord.Clone());

Clone i think is trying to add VertexFaceRecords into the vertices that should be VertexFaceMesh

[DomCR]

Hi @IainS1986,

Here's some rough psuedo code of how I'm making a PolyfaceMesh
The vertices seem to be in the right place, the problem is the triangle/quad generation, in your code you are assuming that the points are sorted by triangle and which for the looks of the second file it doesn't seem to be the case.

I haven't work a lot with the polyface mesh, I'll take a look and do some testing on my end and see what I can find.

I'll suggest to try to create a simple cube and see how Autodesk creates the mesh and how the triangles relate to each other.

PolyfaceMesh clone logic seems odd -

Yep, there is an issue there, I guess a typo for C&P, I'll open a branch to fix it, thanks!

Fix:

• Polyface mesh clone fix #795

[IainS1986]

Our points are defintely in triangle order - we use the same points in various other thigns (OpenGL, Metal, and other CAD libs were looking to replace). So I know it goes [a, b, c, a2, b2, c2, a3, b3, c3 ..] and that shared points will be repeated between triangles.

So I know the triangles are in order, but could the vertices get reordered without me realising?

[IainS1986]

The data in the first file was generated in a similar fashion with another lib, so we went through the points in groups of 3

This was the code from the 'other' lib jsut for reference - so you can see how i'm basically done a line by line port

            var points = surface.Mesh;
            var polyFaceMesh = new DxfPolyfaceMesh();
            polyFaceMesh.Layer = layer;
            polyFaceMesh.Vertices.AddRange(points.Select(x => GenerateDXF_Vertex3D(unit, CreateDoubleArray(x))));
            for (short p = 0; p < points.Length; p += 3)
                polyFaceMesh.Faces.Add(new DxfMeshFace(EntityColor.ByLayer, p, (short)(p + 1), (short)(p + 2)));

            dxfModel.Entities.Add(polyFaceMesh);

Annoyingly the library is not open source and somwhat obfuscated (and although things are called "DxfPolyfaceMesh" etc, its not just Dxf, it outputs to DWG or DXF so I guess they just named the objects after Dxf first)

[DomCR]

So I know the triangles are in order, but could the vertices get reordered without me realising?
The library doesn't change that.

I've created a simple mesh, 4 vertices and 2 triangles, the indexes start at 1, so maybe changing the for iteration and add 1 to the first will have some effect:

triangles: [1,2,3,-4] [-1,4,5,0]

Not sure about the negative values yet.

[DomCR]

Could you share the valid file by compressing it in a zip file, I want to try to recreate the mesh on my end.

[IainS1986]

I think this is the correct file (sorry i've got DWG/DXF files coming out my ears and trying to make sure i don't just give you one that I made with ACadSharp)

Volume - copy (26).dwg.zip

I'm going to try just adding 1 to everything, could well be I'm wrongly assuming its zero index based, so all triangles are off by 1 when getting their corners

[IainS1986]

Perfect! https://autode.sk/3K4iZJD

It was just an off by 1 issue.

        ```
        var polyFaceMesh = new PolyfaceMesh();
        polyFaceMesh.LineWeight = LineweightType.W100;
        polyFaceMesh.Color = Color.Red;
        polyFaceMesh.Layer = layer;
        polyFaceMesh.Vertices.AddRange(points.Select(x => new VertexFaceMesh()
        {
            Location = x.ToPoint3D(unit)
        }));
        for (short p = 0; p < points.Length; p += 3)
        {
            var vertexRecord = new VertexFaceRecord()
            {
                Index1 = (short)(p + 1),
                Index2 = (short)(p + 2),
                Index3 = (short)(p + 3),
                // Index4 = (short)(p + 2),
            };
            polyFaceMesh.Faces.Add(vertexRecord);
        }
        cadDocument.Entities.Add(polyFaceMesh);

That looks all correct my side (so i'm leaving Index4 as 0)

[DomCR]

I'll add some notes in the index documentation so it's clear.

I'll investigate the negative numbers and see what's what.

[IainS1986]

In case you ever want it for refrence, here's the Vector3 export (to csv) of the points list I'm using above

Volume.csv

[woutware]

Annoyingly the library is not open source and somwhat obfuscated

Guilty! CadLib was indeed originally a DXF only library.

@IainS1986 Could you shoot me an e-mail, I would find some feedback valuable.