Looks great tchelyzt! I think I’m finally figuring out what’s going on “behind the scenes” with ModelIO and figured I’d summarize here to (1) see if my understanding is correct and (2) maybe provide some insight.
When I save a polyhedron as an OBJ file it does as you say and gives collections of indices like:
f 1/1/1 2/2/1 3/3/1
f 4/4/2 5/5/2 6/6/2
f 7/7/3 8/8/3 9/9/3
f 2/65/22 1/66/22 23/67/22 22/68/22
f 3/61/21 54/62/21 53/63/21 1/64/21
f 30/181/51 23/182/51 1/183/51 53/184/51 13/185/51
Clearly the OBJ format then considers face normals as the normal indices are not the same as the vertex indices. Thus, as shown above, vertex index 1 has four different normal indices (1, 21, 22, 51) depending on which face we’re considering.
As Metal cannot handle this, ModelIO simply adds more vertices and normals. Vertex 1 in the above case will be repeated four(?) times. Instead of 60 vertex coordinates in the OBJ file we get 348 in the Metal buffer. There are 62 normals in the OBJ file and 240 texture coordinates. It’s not clear how we get from 60, 62, and 240 to 348. The indices went from 60 in the OBJ to 116 in the Metal buffer.
ModelIO is not simply repeating a vertex for each face as in this polyhedron all vertices are shared by four faces.
Coloring by normal shows that each face indeed has a constant normal.
