glTF exporter: option to export placeholder of materials

* export all materials used by object exported
* not export materials at all (mesh will be exported as a single primitive)
* export placeholder: materials are not exported, but slots are still used to separate mesh into primitives

io_scene_gltf2/__init__.py

@@ -15,7 +15,7 @@
 bl_info = {
     'name': 'glTF 2.0 format',
     'author': 'Julien Duroure, Scurest, Norbert Nopper, Urs Hanselmann, Moritz Becher, Benjamin Schmithüsen, Jim Eckerlein, and many external contributors',
-    "version": (1, 4, 29),
+    "version": (1, 4, 30),
     'blender': (2, 90, 0),
     'location': 'File > Import-Export',
     'description': 'Import-Export as glTF 2.0',
@@ -214,10 +214,16 @@ class ExportGLTF2_Base:
         default=False
     )
 
-    export_materials: BoolProperty(
+    export_materials: EnumProperty(
         name='Materials',
-        description='Export materials',
-        default=True
+        items=(('EXPORT', 'Export',
+        'Export all materials used by included objects'),
+        ('PLACEHOLDER', 'Placeholder',
+        'Do not export materials, but write multiple primitive groups per mesh, keeping material slot information'),
+        ('NONE', 'No export',
+        'Do not export materials, and combine mesh primitive groups, losing material slot information')),
+        description='Export materials ',
+        default='EXPORT'
     )
 
     export_colors: BoolProperty(
@@ -672,7 +678,7 @@ class GLTF_PT_export_geometry(bpy.types.Panel):
         layout.prop(operator, 'export_colors')
         layout.prop(operator, 'export_materials')
         col = layout.column()
-        col.active = operator.export_materials
+        col.active = operator.export_materials == "EXPORT"
         col.prop(operator, 'export_image_format')
 
 

io_scene_gltf2/blender/exp/gltf2_blender_extract.py

@@ -179,7 +179,7 @@ def extract_primitives(glTF, blender_mesh, library, blender_object, blender_vert
 
     prim_indices = {}  # maps material index to TRIANGLES-style indices into dots
 
-    if not use_materials:
+    if use_materials == "NONE": # Only for None. For placeholder and export, keep primitives
         # Put all vertices into one primitive
         prim_indices[-1] = loop_indices
 

io_scene_gltf2/blender/exp/gltf2_blender_gather_primitives.py

@@ -54,15 +54,17 @@ def gather_primitives(
         material_idx = internal_primitive['material']
         double_sided = False
         material = None
-        try:
-            blender_material = bpy.data.materials[material_names[material_idx]]
-            double_sided = not blender_material.use_backface_culling
-            material = gltf2_blender_gather_materials.gather_material(blender_material,
-                                                                  double_sided,
-                                                                  export_settings)
-        except IndexError:
-            # no material at that index
-            pass
+
+        if export_settings['gltf_materials'] == "EXPORT":
+            try:
+                blender_material = bpy.data.materials[material_names[material_idx]]
+                double_sided = not blender_material.use_backface_culling
+                material = gltf2_blender_gather_materials.gather_material(blender_material,
+                                                                      double_sided,
+                                                                      export_settings)
+            except IndexError:
+                # no material at that index
+                pass
 
 
         primitive = gltf2_io.MeshPrimitive(