iOS Import .obj file to Model I/O without duplicating vertices

Question

I'm trying to import a .obj file to use in Scene Kit using the Model I/O framework. I initially used the simple MDLAsset initWithURL: function, but after transferring the mesh to a SCNGeometry, I realized this function was triangulizing the mesh, such that each face had 3 unique vertices, and there were separate vertices at the same location for border faces. This was causing some major problems with my other functions, so I tried to fix it by instead using the MDLAsset initWithURL:vertexDescriptor:bufferAllocator:preserveTopology function with preserveTopology set to YES with the descriptor/allocator set to the default with nil. This preserving topology fixed my problem of duplicating vertices, so the faces/edges were all good, but in the process I lost the normals data.

By lost the normals, I don't mean multiple indexing, I mean after setting preserveTopology to YES, the buffer did not contain any normals values at all. Whereas before it was v1/n1/v2/n2... and the stride was 24 bytes (3 dimensions *4 bytes/float * 2 attributes), now the first half of the buffer is v1/v2/... with a stride of 12 and the entire 2nd half of the buffer is just 0.0 floats.

Also something weird with this, when you look at the SCNGeometrySources of the Geometry, there are 2 sources, 1 with semantic kGeometrySourceSemanticVertex, and 1 with semantic kGeometrySourceSemanticNormal. You would think that the semantic vertex source would contain the position data, and the semantic normal source would contain the normal data. However that is not the case. No matter what you set preserveTopology, they are buffers of size to contain both position and normal data with identical values. So when I said before there was no normal data, I mean both of these buffers, semantic vertex AND semantic normal went from being v1/n1/v2/n2... to v1/v2/.../(0.0, 0.0, 0.0)/(0.0, 0.0, 0.0)/... I went into the mdlmesh's buffer (before the transfer to scene kit) at found the same problem, so the problem must be with the initWithURL, not with the model i/o to scenekit bridge.

So I figured there must be something wrong with the default vertex descriptor and buffer allocator (since I was using nil) and went about trying to create my own that matched these 2 possible data formats. Alas after much trying I was unable to get something that worked.

Any ideas on how I should do this? How to give MDLAsset the proper vertexDescriptor and bufferAllocator (I feel like nil should be ok here) for importing a .obj file? Thanks

Answer 1

An obj file with vertices and normals has vertices, indicated by v lines, normals, indicated by vn lines, and faces, indicated by f lines.

The v and vn lines will just be the floating point values you expect, and the f line will be of the form -

f v0//n0 v1//n1 etc

Since OpenGL and Metal don't allow multiple indexing, you'll see the first effect of vertices being duplicated. For example,

f 0//0 1//2 2//0

can't work as a vertex buffer because it would require different indices per vertex. So typical OBJ parsers have to create new vertices that allow the face to become

f 0//0 1//1 2//2

The preserve topology option doesn't help you. It preserves the connectivity and shape of the mesh (no triangulation occurs, shared edges remain shared) but it still enforces a single index per vertex component.

One solution would be to make sure that your tool that is outputting the OBJ files uses single indexing during export, if that is an option.

Another option, and this won't solve the problem immediately, would be file a request that multiple-indexing be supported at the Model I/O level. SceneKit would still have to uniquely-index because it is has to be able to render.

Another option would be to use a format like PLY that doesn't have multiple indexing.