WPF 3D Triangle Overlap Problem

Question

I represent a scene with WPF 3D by creating MeshGeometry3D and adding vertices and normals to it. Everything looks good in the rendering scene (there is light, the material looks great), but where the grid triangles intersect, the triangle closer to the camera is not necessarily displayed on top. They seem to be drawn in random order. Is there a way to ensure that the mesh triangles appear in the “correct” order?

Just in case this helps, here is my XAML:

<Viewport3D> <ModelVisual3D> <ModelVisual3D.Content> <Model3DGroup> <AmbientLight Color="#FF5A5A5A" /> <GeometryModel3D x:Name="geometryModel"> <GeometryModel3D.Material> <DiffuseMaterial Brush="DarkRed"/> </GeometryModel3D.Material> </GeometryModel3D> </Model3DGroup> </ModelVisual3D.Content> </ModelVisual3D> </Viewport3D> 

and in the code I create a grid something like this:

 var mesh = new MeshGeometry3D(); foreach (var item in objectsToTriangulate) { var triangles = item.Triangulate(); foreach (var triangle in triangles) { mesh.Positions.Add(triangle.Vertices[0]); mesh.Positions.Add(triangle.Vertices[1]); mesh.Positions.Add(triangle.Vertices[2]); mesh.Normals.Add(triangle.Normal); mesh.Normals.Add(triangle.Normal); mesh.Normals.Add(triangle.Normal); } } geometryModel.Geometry = mesh; 

EDIT: None of the triangles intersect (except the edges), and sometimes the triangle that appears on top is actually WAY after another, so I don’t think this is an ambiguity in 3D sorting of triangles, as Ray Burns suggested.

Another behavior I noticed is that the order in which the triangles are displayed does not change when I move around the scene. That is, if I look at the problem area from the other side, the triangles are displayed in the same way, now the "correct" order.

Answer 1

In WPF, as in most 3D systems, the simplifying assumption is made that any given triangle is supposed to lie completely or completely behind any other given triangle. But in reality this is not always the case. In particular, if two triangles intersect along their interiors (and not just along the edges) and are not visible along their line of incision, accurate rendering will draw each edge of the triangle for part of the viewport.

Because of this assumption, 3D engines sort triangles by looking at its entire triangle at some distance from the camera. He can choose the closest corner of the triangle, the farthest corner, average the angles, or use some other algorithm, but in the end he chooses a representative point to calculate the Z-order.

I assume your triangles are structured in such a way that a representative point is used to calculate the Z-order, causing them to appear in unexpected order.

Edit

From the information that you provide in your editing, I see that my first guess was wrong. I will leave it in case it will be useful to someone else, and give some more ideas that I had. Hope someone else can call back here. I have never had such a problem with depth, so all this is just educated guesswork.

Here are my ideas:

• Perhaps your BackMaterial is not set or transparent, so you only see triangles whose winding order is clockwise from your point of view. Depending on your actual grid, the missing invisible triangles may make themselves appear to be behind them, when in fact they are just visible through them. It can also happen if your material has not been installed or was transparent.

• Something clearly defines the display order of the triangles. Could this be the order from your TriangleIndices array? If you arbitrarily reorder the TriangleIndices array (in sets of three, of course) without any changes, will this change the display? If so, you have learned something about the problem and may have found a workaround (if it uses the TriangleIndices order, you can sort it yourself).

• If you are using ProjectionCamera or OrthoGraphicCamera, are NearPlaneDistance and FarPlaneDistance set? The wrong NearPlaneDistance could especially make the triangles closer to you invisible, making it seem like the triangles are drawn even further down from the top. Incorrect distances can also affect the granularity of the depth buffer, which can give you the effect you are experiencing.

• Is your model extremely large or extremely small? If you scale the model and position of the camera, does it matter? Depth buffers are usually 32-bit integers, so in extremely small models it is possible that two triangles are rounded to the same depth buffer value. This will also lead to a description of the effect.

• You may have encountered an error. You can try some changes to see if they have affected the problem, for example, you can try only for software, different types of lighting (diffuse or mirror, etc.), different types of cameras, a graphics card from another vendor and etc.

I hope some of these ideas help.