I imported a fbx model to blender. It consists of one mesh with 12882 vertices (info from blender: at the top after selecting everything in edit mode it says 12882 verts). When accessing the model in MonoGame (the one ModelMeshPart in the Model) and looking up VertexBuffer.VertexCount I get a different number of 13366 vertices.
The model I used is available here: teapot.fbx
Iām on mobile, so I canāt check the file, but maybe it has to do with normals? Does the file have normals? Do you let MG generate normals?
The model has normals I guess, in blender they show up as expected. I copy the normals from the ModelMeshPart and also calculate them from the positions and I get the same values. I seems odd to me, that I get a different vertex count from the model than displayed in blender (I also opened the fbx model with visual studio 2012 and it displays the same vertex count number as blender).
Blender understands vertexs as a āreal life vertexā. However 3d engines require vertex data to be sent to the GPU in order to render.
This means that if the same āblenderā vertex has different normals for each face that share that vertex, this vertex position must be āduplicatedā because from the GPU standpoint, itās a different vertex.
You canāt say a GPU āi want the vertex #3 with the normal #5ā, so the vertex position is duplicated (but the normal is different)
To test it, just create a cube. Blender will say 8 vertices, MG will say 24. Now smooth the cube so that each normal is shared by each vertex. Blender will say 8 vertices, and MG will say 8 vertices too. (but the cube will not look like a cube)
Actually blender tells you that there are only 8 vertices, but he uses 24 to render too. =)
A teapot should have smooth normals shared by all vertices.
Maybe itās an option on the OpenAsset that duplicates vertices even when they share common Position-Normal-TextureUV.
MeshBuilder has a method to find duplicate vertices.
Does the model importer make use of that? do we need some epsilon when it compares the position/normal values?
That does make sense. But just from looking at the normals in blender it seems to me, that the model has shared normals or smooth normals everywhere. When drawing the model, the normals I get in the effect / shader seem to be right (using the world normal as color output in the pixel shader). Copying the normals from the VertexBuffer (using the provided VertexCount of the VertexBuffer) and using them together with the positions I construct a VertexBuffer to visualize the normals similar to blender. Drawing the constructed normal VertexBuffer does show only some of the expected normals (comapred to the normals in blender). The nozzle / spout and the lid of the teapot have no normals and the handel only partly. Apart from that the normals look exactly like expected.
Iāve had a closed look at the teapot and yes, there should be no āsharp edgesā. Iāve made a small test checking for duplicate vertexs, and found this:
(white vertexs are duplicated)
Differences seem to be in the texture coordinates:
I donāt know who is responsible, blender or assimp. Anyways I donāt feel like itās a bug.
Good work 
Now could you maybe try to visualize the normals? By visualizing I mean to build a VertexBuffer with a LineList (start point: vertex position, endpoint: vertex position + vertex normal). Hopefully this works so I know I have a bug somewhere
Edit: Nevermind, all normals are thereā¦ I mixed up some indices 
