Blender 2.69 Properly Export UV Maps Using Python

Question

I'm currently working on my custom format exporter form blender and have hit some kind of a speed bump... Through tons of experimentation I have finally found that UV data is stored in:

mesh.uv_layers[layer].data

But I don't think that the internal blender data format agrees with the way I'm going about exporting. First off, my exporter assumes that all faces are either triangles or quads. If a face is a quad then it outputs it as two triangles:

t1 = 0 1 2
t2 = 2 3 0

Now, my exporter just iterates over mesh.polygons and outputs per vertex per triangle, but uv_layers.data obviously doesn't store it this way. So how could I go about exporting it? I need a way to lookup the uv coordinates for a specific vertex and output it in the same way as I do this:

for vert in fac.vertices:
     file.write("+v %g %g %g\n" % (obj.vertices[vert].co.x, obj.vertices[vert].co.y, obj.vertices[vert].co.z))

But using the vertex's UV instead.

Any suggestions are greatly appreciated! Thanks guys!!!

EDIT:

For anyone who runs into a similar problem, here is my working code:

    for fac in obj.polygons:
        if len(fac.vertices) == 3:
            for li in fac.loop_indices:
                file.write("+t %g %g\n" % (uv_layer[li].uv.x, uv_layer[li].uv.y))
        if len(fac.vertices) == 4:
            file.write("+t %g %g\n" % (uv_layer[fac.loop_indices[0]].uv.x, uv_layer[fac.loop_indices[0]].uv.y))
            file.write("+t %g %g\n" % (uv_layer[fac.loop_indices[1]].uv.x, uv_layer[fac.loop_indices[1]].uv.y))
            file.write("+t %g %g\n" % (uv_layer[fac.loop_indices[2]].uv.x, uv_layer[fac.loop_indices[2]].uv.y))
            file.write("+t %g %g\n" % (uv_layer[fac.loop_indices[2]].uv.x, uv_layer[fac.loop_indices[2]].uv.y))
            file.write("+t %g %g\n" % (uv_layer[fac.loop_indices[3]].uv.x, uv_layer[fac.loop_indices[3]].uv.y))
            file.write("+t %g %g\n" % (uv_layer[fac.loop_indices[0]].uv.x, uv_layer[fac.loop_indices[0]].uv.y))

This code will export the uv coordinates for any triangle or quad in the mesh (Quads are broken down into two triangles). The result is a list of the vertex uv coordinates in order of the vertices. So every 3 lines represents a triangle.

obj is the mesh data of the current mesh, which is accessed by bpy.data.meshes['NameOfMesh'].

Answer 1

UV's aren't per vertex in Blender, but per loop (which means per face-vertex). Also see here and here.

For triangle export, use the tessellated faces and split the quads into triangles or triangulate using the bmesh operator. Export the faces together with the UV coordinates. If UVs are the same for a shared vertex, you could simply pick the first loop, otherwise interpolate? (may give weird results...)

Print loop and vertex indices per polygon together with UV coordinate (uv index equals loop index):

import bpy

me = bpy.context.object.data
uv_layer = me.uv_layers.active.data

for poly in me.polygons:
    print("Polygon", poly.index)
    for li in poly.loop_indices:
        vi = me.loops[li].vertex_index
        uv = uv_layer[li].uv
        print("    Loop index %i (Vertex %i) - UV %f %f" % (li, vi, uv.x, uv.y))