BMesh: How to map vertex-based UV-coordinates to loops?

Question

I have a game model format which stores the UV coordinates for each vertex. To implement seams in the UV mapping, a vertex just appears twice, each having a different UV coordinate (it solves the lighting issues introduced with that by using custom normals stored for each vertex, but sadly I can't teach Blender custom normals correctly yet as I learned... another story).

Basically, my vertex data looks like this (made-up data):

Index   X       Y       Z       U       V
0       1.4235  0.2434  -0.322  0.0     0.5
1       2.3242  0.231   0.323   0.5     0.0
2       1.2242  0.531   -0.323  0.5     0.5
...

So I started writing an importer for this format, and thought it's easy enough to just set an uv property of a BMVert instance - just to find out that BMVert does not have a property for UV mapping at all.

From looking at the samples, it (logically) looks like I have to set the UV coordinates by iterating through loops rather than the vertices. But I'm a little stumped for an answer on how to find out the index of the vertex having the required UV coordinate when iterating through the loops.

The answer here UV Vertex using Python? was suggested, but it does not show how to retrieve the corresponding index of a vertex.

My current code to import the XYZ vertex positions (and normals) is as follows:

# Get the vertices and indices of the most detailled LoD model.
vertices = get_vertices(...)
indices = get_indices(...)
# Create a mesh and a bmesh to represent the polygon.
mesh = bpy.data.meshes.new(fshp.header.name_offset.name)
bm = bmesh.new()
bm.from_mesh(mesh)
# Go through the vertices and add them to the bmesh.
for vertex in vertices:
    bm_vert = bm.verts.new(vertex.p0)
    #bm_vert.normal = vertex.n0 # TODO: Blender does not really support custom normals yet, and they look weird.
bm.verts.ensure_lookup_table() # Required after adding / removing vertices and before accessing them by index.
# Connect the faces, they are organized as a triangle list.
for i in range(0, len(indices), 3):
    bm.faces.new(bm.verts[j] for j in indices[i:i + 3])
# TODO: Set the UV coordinates??
uv_layer = bm.loops.layers.uv.active
for face in bm.faces:
    for loop in face.loops:
        loop[uv_layer].uv = #..?
# Write the bmesh data back to the mesh.
bm.to_mesh(mesh)
bm.free()
mesh_obj = bpy.data.objects.new(mesh.name, mesh)
bpy.context.scene.objects.link(mesh_obj)

Answer 1

It took me some time to realize I could just ask the vert property of a BMLoop when iterating through them, and then ask the index property of the retrieved BMVert to find my index.

It required to call bm.verts.index_update() before, or all the retrieved indices would've been -1.

This is my working code:

vertices = model.get_vertices()
# Add the vertices to the model.
for vertex in vertices:
    bm_vert = bm.verts.new(vertex.p0)
    #bm_vert.normal = vertex.n0 # TODO: Blender does not really support custom normals yet, and they look weird.
bm.verts.ensure_lookup_table() # Required after adding / removing vertices and before accessing them by index.
bm.verts.index_update()  # Required to actually retrieve the indices later on (or they stay -1).
# Connect the faces, they are organized as a triangle list.
for i in range(0, len(indices), 3):
    bm.faces.new(bm.verts[j] for j in indices[i:i + 3])
# Set the UV coordinates by iterating through the face loops.
uv_layer = bm.loops.layers.uv.new()
for face in bm.faces:
    for loop in face.loops:
        # Get the index of the vertex the loop contains.
        loop[uv_layer].uv = vertices[loop.vert.index].u0

Answer 2

You do the mapping like this:

for face in bm.faces:
    for vert, loop in zip(face.verts, face.loops):
        loop[uv_layer].uv = get_uvs(vert.index, face.index,...)

The last line depends on your file format and how are the UVs stored. You should be able to retrieve the appropriate UV coords from vert.index and face.index info.