Improve mesh remeshing to preserve coordinate system and color information (#602)

- Detect and preserve original COLLADA coordinate system (Y_UP/Z_UP)
  by reading up_axis from input DAE file and applying appropriate
  transforms during export
- Extract color information from both vertex colors and materials,
  enabling proper color preservation across multiple remesh operations
- Improve color quantization from 0.02 to 0.01 steps for better
  color accuracy
- Add debug logging for color collection and material creation

This ensures that remeshed meshes maintain the same orientation and
visual appearance as the original, even when remeshing is applied
multiple times consecutively.

Author: iory

skrobot/utils/_blender_remesh_core.py

@@ -31,13 +31,32 @@ def remesh_and_bake_file(input_path, output_path, voxel_size=0.002, export_forma
         Export format ('DAE' or 'STL'). Default is 'DAE'.
     """
     import math
+    import xml.etree.ElementTree as ET
 
     input_path = Path(input_path)
     output_path = Path(output_path)
 
+    # Detect original coordinate system from DAE file
+    original_up_axis = 'Y_UP'  # default
+    if export_format.upper() == 'DAE' and input_path.suffix.lower() == '.dae':
+        try:
+            tree = ET.parse(input_path)
+            root = tree.getroot()
+            # Find up_axis element (handle namespace)
+            ns = {'collada': 'http://www.collada.org/2005/11/COLLADASchema'}
+            up_axis_elem = root.find('.//collada:up_axis', ns)
+            if up_axis_elem is None:
+                up_axis_elem = root.find('.//up_axis')
+            if up_axis_elem is not None and up_axis_elem.text:
+                original_up_axis = up_axis_elem.text.strip()
+                print(f"Detected original coordinate system: {original_up_axis}")
+        except Exception as e:
+            print(f"Warning: Could not detect coordinate system: {e}")
+
     clear_scene()
 
-    # Import with fix_orientation to maintain Blender's Z-up coordinate system
+    # Import with fix_orientation to ensure proper handling in Blender
+    # Blender uses Z-up, so this converts Y-up to Z-up properly
     bpy.ops.wm.collada_import(
         filepath=str(input_path),
         fix_orientation=True,
@@ -53,31 +72,73 @@ def remesh_and_bake_file(input_path, output_path, voxel_size=0.002, export_forma
 
     # Build a map of face center positions to colors
     face_color_data = []
+    objects_with_vertex_colors = 0
+    objects_with_materials = 0
 
     for obj in source_objects:
         world_matrix = obj.matrix_world
 
-        # Get vertex colors if available
-        if obj.data.vertex_colors:
+        # Try to get colors from vertex colors first, then from materials
+        has_vertex_colors = obj.data.vertex_colors and len(obj.data.vertex_colors) > 0
+
+        if has_vertex_colors:
+            objects_with_vertex_colors += 1
             vc = obj.data.vertex_colors.active.data
 
             for poly in obj.data.polygons:
-                # Get average color for this face
+                # Get average color for this face from vertex colors
                 color_sum = [0.0, 0.0, 0.0, 0.0]
                 for loop_index in poly.loop_indices:
                     for i in range(4):
                         color_sum[i] += vc[loop_index].color[i]
-
                 avg_color = tuple(c / len(poly.loop_indices) for c in color_sum)
+                face_color = avg_color[:3]
+
+                # Get face center in world space
+                face_center = world_matrix @ poly.center
+
+                face_color_data.append({
+                    'center': face_center.copy(),
+                    'color': face_color
+                })
+        elif obj.data.materials:
+            objects_with_materials += 1
+
+            for poly in obj.data.polygons:
+                # Get color from material
+                face_color = (0.5, 0.5, 0.5)  # default fallback
+
+                if poly.material_index < len(obj.data.materials):
+                    mat = obj.data.materials[poly.material_index]
+                    if mat and mat.use_nodes:
+                        # Get base color from Principled BSDF
+                        bsdf = mat.node_tree.nodes.get("Principled BSDF")
+                        if bsdf and 'Base Color' in bsdf.inputs:
+                            base_color = bsdf.inputs['Base Color'].default_value
+                            face_color = (base_color[0], base_color[1], base_color[2])
 
                 # Get face center in world space
                 face_center = world_matrix @ poly.center
 
                 face_color_data.append({
                     'center': face_center.copy(),
-                    'color': avg_color[:3]
+                    'color': face_color
                 })
 
+    msg = f"Color collection: {objects_with_vertex_colors} objects with vertex colors"
+    msg += f'{objects_with_materials} with materials'
+    msg += f"Collected {len(face_color_data)} face color samples"
+    print(msg)
+
+    # Print color statistics
+    if face_color_data:
+        unique_colors = set(data['color'] for data in face_color_data)
+        print(f"Unique colors in source: {len(unique_colors)}")
+        # Sample some colors
+        sample_colors = list(unique_colors)[:5]
+        for i, c in enumerate(sample_colors):
+            print(f"  Sample {i + 1}: ({c[0]:.3f}, {c[1]:.3f}, {c[2]:.3f})")
+
     # --- 2. JOIN ALL OBJECTS ---
     bpy.ops.object.select_all(action='DESELECT')
     for obj in source_objects:
@@ -121,7 +182,8 @@ def remesh_and_bake_file(input_path, output_path, voxel_size=0.002, export_forma
             nearest_color = face_color_data[nearest_index]['color']
 
             # Round color to reduce number of materials
-            rounded_color = tuple(round(c * 50) / 50 for c in nearest_color)
+            # Use finer quantization (0.01 steps) to preserve more color detail
+            rounded_color = tuple(round(c * 100) / 100 for c in nearest_color)
 
             # Create material if it doesn't exist
             if rounded_color not in color_to_material:
@@ -137,18 +199,28 @@ def remesh_and_bake_file(input_path, output_path, voxel_size=0.002, export_forma
             # Assign material to face
             poly.material_index = color_to_material[rounded_color]
 
+        print(f"Created {len(color_to_material)} materials from {len(face_color_data)} source colors")
+        print("Material color samples:")
+        for i, (color, mat_idx) in enumerate(list(color_to_material.items())[:5]):
+            print(f"  Material {mat_idx}: ({color[0]:.3f}, {color[1]:.3f}, {color[2]:.3f})")
+
     # Apply smooth shading
     if joined_obj.data.polygons:
         joined_obj.data.polygons.foreach_set('use_smooth', [True] * len(joined_obj.data.polygons))
 
     # --- 5. EXPORT ---
-    # Convert from Blender Z-up to Collada Y-up by rotating X-axis -90 degrees
-    joined_obj.rotation_euler[0] -= math.pi / 2
-
-    # Apply transformation to vertices
+    # Convert back to original coordinate system if needed
+    if export_format.upper() == 'DAE' and original_up_axis == 'Y_UP':
+        # Convert from Blender Z-up back to Y-up
+        # Rotate -90 degrees around X axis
+        joined_obj.rotation_euler[0] -= math.pi / 2
+        bpy.context.view_layer.objects.active = joined_obj
+        joined_obj.select_set(True)
+        bpy.ops.object.transform_apply(location=False, rotation=True, scale=False)
+
+    # Ensure object is selected for export
     bpy.context.view_layer.objects.active = joined_obj
     joined_obj.select_set(True)
-    bpy.ops.object.transform_apply(location=False, rotation=True, scale=False)
 
     if export_format.upper() == 'DAE':
         bpy.ops.wm.collada_export(
@@ -162,6 +234,25 @@ def remesh_and_bake_file(input_path, output_path, voxel_size=0.002, export_forma
             use_object_instantiation=False,
             sort_by_name=False
         )
+
+        # Fix coordinate system in exported DAE file to match original
+        # Use text replacement to preserve all XML formatting and content
+        try:
+            with open(output_path, 'r', encoding='utf-8') as f:
+                content = f.read()
+
+            # Replace Z_UP with original coordinate system
+            content = content.replace('<up_axis>Z_UP</up_axis>', f'<up_axis>{original_up_axis}</up_axis>')
+            # Also handle namespace prefix variations
+            content = content.replace(':up_axis>Z_UP</', f':up_axis>{original_up_axis}</')
+
+            with open(output_path, 'w', encoding='utf-8') as f:
+                f.write(content)
+
+            print(f"Restored coordinate system to: {original_up_axis}")
+        except Exception as e:
+            print(f"Warning: Could not fix coordinate system in output: {e}")
+
     elif export_format.upper() == 'STL':
         bpy.ops.export_mesh.stl(
             filepath=str(output_path),