XbimGeometry producing flipped triangles

[Kuraperunat]

Here's yet another case of trying to create triangulated meshes using Xbim. It's working perfectly except for some, seemingly random ifc objects the triangle indices are always flipped. I've tried looking at all the existing code and fiddled around with the parameters and the transformations but I can't figure out what's causing it.

Here's an example of the issue: https://i.imgur.com/GrwMAv9.jpg
On the left you see a mesh generated by another program, and on the right you see the result of Xbim. The triangle indices of Xbim are flipped (e.g. 0,1,2 is for some reason 2,1,0). This happens with many different ifc files and in seemingly totally random places.

Here's the relevant part of my code where I triangulate the model.

using (var model = IfcStore.Open(path)) {

    var ctx = new Xbim3DModelContext(model);
    ctx.CreateContext();

    using (var geomRead = model.GeometryStore.BeginRead()) {

        var scale = model.ModelFactors.OneMetre;

        var prodIds = new HashSet<int>();
        foreach (var product in model.Instances.OfType<IIfcProduct>()) {
            if (product is IIfcFeatureElement) continue;
            prodIds.Add(product.EntityLabel);
        }

        var toIgnore = new short[4];
        toIgnore[0] = model.Metadata.ExpressTypeId("IFCOPENINGELEMENT");
        toIgnore[1] = model.Metadata.ExpressTypeId("IFCPROJECTIONELEMENT");
        if (model.IfcSchemaVersion == Xbim.Common.Step21.IfcSchemaVersion.Ifc4) {
            toIgnore[2] = model.Metadata.ExpressTypeId("IFCVOIDINGFEATURE");
            toIgnore[3] = model.Metadata.ExpressTypeId("IFCSURFACEFEATURE");
        }

        foreach (var geometry in geomRead.ShapeGeometries) {

            if (geometry.ShapeData.Length <= 0) //no geometry to display so don't write out any products for it
                continue;
            var instances = geomRead.ShapeInstancesOfGeometry(geometry.ShapeLabel);

            var xbimShapeInstances = instances.Where(
                si => !toIgnore.Contains(si.IfcTypeId) &&
                si.RepresentationType ==
                XbimGeometryRepresentationType.OpeningsAndAdditionsIncluded &&
                prodIds.Contains(si.IfcProductLabel)).ToList();

            if (!xbimShapeInstances.Any()) continue;

            XbimShapeTriangulation tr;
            using (var ms = new MemoryStream(((IXbimShapeGeometryData)geometry).ShapeData))
                using (var br = new BinaryReader(ms))
                    tr = br.ReadShapeTriangulation();

            foreach (IXbimShapeInstanceData xbimShapeInstance in xbimShapeInstances) {

                var styleId = xbimShapeInstance.StyleLabel > 0
                    ? xbimShapeInstance.StyleLabel
                    : xbimShapeInstance.IfcTypeId * -1;

                var instanceTransform = ((XbimShapeInstance)xbimShapeInstance).Transformation;
                var trTransformed = tr.Transform(instanceTransform);

                var offset = vertices.Count;

                for (int k = 0; k < trTransformed.Vertices.Count; k++) {
                    var v = trTransformed.Vertices[k];
                    vertices.Add(new Vector3((float)(v.X / scale), (float)(v.Y / scale), (float)(v.Z / scale)));
                }

                for (int k = 0; k < trTransformed.Faces.Count; k++) {
                    var face = trTransformed.Faces[k];
                    var indices = face.Indices;
                    for (int z = 0; z < indices.Count; z += 3) {
                        int t0 = indices[z + 0], t1 = indices[z + 1], t2 = indices[z + 2];
                        triangles.Add(offset + t0);
                        triangles.Add(offset + t1);
                        triangles.Add(offset + t2);
                    }
                }
            }
        }
    }
}

[shole]

i can reproduce/illustrate this in XbimXplorer without third party code by changing SurfaceLayerStyler.cs method GetNewStyleMesh() to be

protected static WpfMeshGeometry3D GetNewStyleMesh(WpfMaterial wpfMaterial, Model3DGroup >tmpTransparentsGroup,
       Model3DGroup tmpOpaquesGroup) {
       XbimColour col = new XbimColour("fff", 1f, 0, 0, 1f); // make a red backface material
       WpfMaterial backmat = new WpfMaterial();
       backmat.CreateMaterial(col);
       if (wpfMaterial.IsTransparent) { // but let's ignore transparencies
           backmat = wpfMaterial;
       }
       var mg = new WpfMeshGeometry3D(wpfMaterial, backmat);
       mg.WpfModel.SetValue(FrameworkElement.TagProperty, mg);
       mg.BeginUpdate();
       if (wpfMaterial.IsTransparent)
           tmpTransparentsGroup.Children.Add(mg);
       else
           tmpOpaquesGroup.Children.Add(mg);
       return mg;
   }

This will paint backfaces in red, like this;
https://i.imgur.com/WnBUjHo.jpg
This is a healthy ifc model which imports through ifcopenshell correctly.

[shole]

After a full day of compiling, I tested this with the latest common develop branch code (Geometry 4.0.15-V0030) and the issue remains.
https://i.imgur.com/oifQZYj.jpg
As you can see the flipped triangles are not the same, so it's an issue in the meshing somewhere.
(Separate issue; The develop version also adds a nonexistent down-facing square in this IFC which does not exist in the model. Some other IFCs do not add this.)

[shole]

After studying a bit I think this issue is twofold;

1. Inverted transforms are not resolved
   This is often used to mirror elements
   Well illustrated by the chairs in the test model SampleHouse.ifc
   https://i.imgur.com/12Pp3NY.jpg
   This should be a relatively easy fix; just flip faces if transform scale sum is negative.
   ( if ( sign(scale.x)*sign(scale.y)*sign(scale.z) < 0 ) flipTriangles=true; )

2. Something goes awry in non-primitive 3d meshes.
   The roofing on my example model in previous posts is not primitives but triangle mesh.
   It's split into multiple ceiling elements which are solid boxes in shape, but it starts flipping the faces at seemingly random positions - not the whole elements like in case 1 above.
   It's not a thing with the model as this happens with other non-primitive meshes.
   Here's some random letter flipping https://i.imgur.com/JlNRamT.jpg - the primitive based building in this model is almost entirely correct.
   Opening the files give a number of these warnings from XbimGeometryCreator in the log, so I think the issue might be in the ifc format parser somewhere;

GeomEngine: #8999221=IfcCompositeCurve [Invalid part of a composite curve found. It has been discarded]
GeomEngine: #6537656=IfcPolygonalBoundedHalfSpace [Incorrectly defined PolygonalBoundary #8999221. It has been ignored]

I know at some point these models were processed with Solibri IFC optimizer (which is standard practice), if that makes a difference.
But essentially all industry standard ifc readers still open them correctly.
By contrast opening a model without freeflowing non-primitive triangles does not give these warnings and i only see issues caused by case 1 with inverted transforms.

[SteveLockley]

Hi Kuraperunat, I am aware of this problem, its been long standing I am afraid, but I am trying to resolve it now. Do you have any Ifc files that I could use to reproduce you scenarios please? Snippets can be extracted using the Xbim.Extract Utility if you cannot share a whole model

[SBRK]

I'm having the same problem. With Xbim.Essentials and Xbim.Geometry v4, no problem. With v5, I'm seeing random flipped faces.

[Wolgraph]

Hi.
I've had the same problem with an ifc project of mine.
I couldn't fix the issue withing the code, so I managed to write a class to check and unify the normals and fix the face orientation.
Give it a look.
https://github.com/Wolgraph/GeometryEditor

[bekraft]

Just found a candidate for solution of this. I've had the same trouble while importing meshes to Unity. The problem seems to be related to planar meshing algorithm and orientation alignment. It wasn't stable enough.