WPF 3D triangle overlap problem

Question

I'm rendering a scene with WPF 3D by making a MeshGeometry3D and adding vertices and normals to it. Everything looks good in the rendered scene (lights are there, material looks fine), but where mesh triangles overlap, the triangle closer to the camera is not necessarily rendered on top. It looks like they're being drawn in a random order. Is there any way I can ensure that the mesh triangles are rendered in the "correct" order?

Just in case it helps, here's 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 code, I'm generating the mesh 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 at the edges), and sometimes the triangle that appears on top is actually WAY behind the other one, so I don't think it's an ambiguity with the 3D sorting of the triangles, as Ray Burns has suggested.

The other behavior that I've noticed is that the order the triangles are rendered does not seem to change as I move around the scene. I.e. if I view a problem area from the other side, the triangles are rendered in the same, now "correct", order.

Answer 1

In WPF, as in most 3D systems, a simplifying assumption is made that any given triangle is assumed to lie entirely in front of or entirely behind any other given triangle. But in fact this is not necessarily always the case. Specifically, if two triangles intersect along their interiors (not just at their edges) and are not viewed along their intesection line, an accurate rendering would paint each triangle front for part of the viewport.

Because of this assumption, 3D engines sort triangles by considering he entire triangle to be a certain distance from the camera. It may choose the triangle's nearest corner, the furthest corner, average the corners, or use some other algorithm but in the end it selects a representative point for computing Z Order.

My guess is that your triangles are structured in a way that the representative point is uses for computing Z Order is causing them to display in an unexpected order.

Edit

From the information you provide in your edit, I can see that my first guess was wrong. I'll leave it in case it is useful to someone else, and give a few more ideas I've had. Hopefully someone else can chime in here. I've never had this kind of depth problem myself so these are all just educated guesses.

Here are my ideas:

1. It may be that your BackMaterial is not set or transparent, causing you to see only triangles whose winding order is clockwise from your perspective. Depending on your actual mesh, the missing invisible triangles could make it appear that the ones in the rear are overlapping them when in actual fact they are simply visible through them. This could also happen if your Material was not set or was transparent.

2. Something is clearly determining the order the triangles are displayed in. Could it be the order from your TriangleIndices array? If you randomly reorder the TriangleIndices array (in sets of three of course) without making any other changes, does it change the display? If so, you've learned something about the problem and perhaps found a workaround (if it is using TriangleIndices order, you could do the sorting yourself).

3. If you are using ProjectionCamera or OrthoGraphicCamera, are the NearPlaneDistance and FarPlaneDistance set appropriately? A wrong NearPlaneDistance especially could make triangles closer to you invisible, making it appear that triangles further away are actually being drawn on top. Wrong distances could also affect the granularity of the depth buffer, which could give you the effect you are experiencing.

4. Is your model extremely large or extremely small? If you scale the model and the camera position, does it make a difference? Depth buffers are generally 32 bit integers, so it is possible in extremely tiny models to have two triangles round off to the same depth buffer value. This would also cause the effect you describe.

5. It may be that you are encountering a bug. You can try some changes to see if they affect the problem, for example you might try software-only rendering, different lighting types (diffuse vs specular, etc), different camera types, a graphics card from a different vendor, etc.

I hope some of these ideas help.