Geometry export: use unique names for repeated materials (#67)

When a user assigns the same material to multiple material slots, the
previous logic would use the same name for all of them, and therefore
overwrite the exported mesh file repeatedly. Only the mesh part that
appeared last in the materials slot list would remain.
This PR ensures that unique names are used for the output meshes.

mitsuba-blender/io/exporter/geometry.py

@@ -1,9 +1,10 @@
-from .materials import export_material
-from .export_context import Files
-from mathutils import Matrix
 import os
 import bpy
 
+from .materials import export_material
+from .export_context import Files
+
+
 def convert_mesh(export_ctx, b_mesh, matrix_world, name, mat_nr):
     '''
     This method creates a mitsuba mesh from a blender mesh and returns it.
@@ -86,27 +87,44 @@ def export_object(deg_instance, export_ctx, is_particle):
         else:
             transform = b_object.matrix_world
 
+
         if mat_count == 0: # No assigned material
-            converted_parts.append((-1, convert_mesh(export_ctx,
-                                                    b_mesh,
-                                                    transform,
-                                                    name_clean,
-                                                    0)))
-        for mat_nr in range(mat_count):
-            if b_mesh.materials[mat_nr]:
+            converted_parts.append((
+                name_clean,
+                -1,
+                convert_mesh(export_ctx, b_mesh, transform, name_clean, 0)
+            ))
+        else:
+            refs_per_mat = {}
+            for mat_nr in range(mat_count):
+                mat = b_mesh.materials[mat_nr]
+                if not mat:
+                    continue
+
+                # Ensures that the exported mesh parts have unique names,
+                # even if multiple material slots refer to the same material.
+                n_mat_refs = refs_per_mat.get(mat.name, 0)
+                name = f'{name_clean}-{mat.name}'
+
+                if n_mat_refs >= 1:
+                    name += f'-{n_mat_refs:03d}'
+
                 mts_mesh = convert_mesh(export_ctx,
                                         b_mesh,
                                         transform,
-                                        f"{name_clean}-{b_mesh.materials[mat_nr].name}",
+                                        name,
                                         mat_nr)
                 if mts_mesh is not None and mts_mesh.face_count() > 0:
-                    converted_parts.append((mat_nr, mts_mesh))
-                    export_material(export_ctx, b_mesh.materials[mat_nr])
+                    converted_parts.append((name, mat_nr, mts_mesh))
+                    refs_per_mat[mat.name] = n_mat_refs + 1
+
+                    if n_mat_refs == 0:
+                        # Only export this material once
+                        export_material(export_ctx, mat)
 
         if b_object.type != 'MESH':
             b_object.to_mesh_clear()
 
-        part_count = len(converted_parts)
         # Use a ShapeGroup for instances and split meshes
         use_shapegroup = is_instance or is_instance_emitter or is_particle
         # TODO: Check if shapegroups for split meshes is worth it
@@ -115,12 +133,8 @@ def export_object(deg_instance, export_ctx, is_particle):
                 'type': 'shapegroup'
             }
 
-        for (mat_nr, mts_mesh) in converted_parts:
-            # Determine the file name
-            if part_count == 1:
-                name = f"{name_clean}"
-            else:
-                name = f"{name_clean}-{b_mesh.materials[mat_nr].name}"
+        for (name, mat_nr, mts_mesh) in converted_parts:
+            name = name_clean if len(converted_parts) == 1 else name
             mesh_id = f"mesh-{name}"
 
             # Save as binary ply