Add --blender-decimate option to convert-urdf-mesh (#752)

iory · web-flow · commit b2bcbe14104c · 2026-06-02T15:11:04.000+09:00
Reduce mesh triangle count with Blender's decimate (collapse) modifier
while preserving shape and colors, exchanging meshes via glTF.
diff --git a/skrobot/apps/convert_urdf_mesh.py b/skrobot/apps/convert_urdf_mesh.py
@@ -86,6 +86,17 @@ def main():
         '--blender-voxel-size', type=float, default=0.002,
         help='Voxel size for Blender remeshing. Smaller values create more '
         'detailed meshes. Default is 0.002. Only used with --blender-remesh.')
+    parser.add_argument(
+        '--blender-decimate', action='store_true',
+        help='Use Blender decimate (collapse) modifier to reduce the triangle '
+        'count while preserving shape and colors. Unlike --blender-remesh, '
+        'this keeps the original topology and works on Blender 5.x '
+        '(meshes are exchanged via glTF). Requires Blender to be installed.')
+    parser.add_argument(
+        '--blender-decimate-ratio', type=float, default=0.1,
+        help='Collapse ratio for Blender decimation. The resulting triangle '
+        'count is approximately this fraction of the original. Must be in '
+        '(0, 1]. Default is 0.1. Only used with --blender-decimate.')
     parser.add_argument(
         '--blender-executable', type=str, default=None,
         help='Path to Blender executable. If not specified, automatically '
@@ -204,6 +215,8 @@ def nullcontext(enter_result=None):
             collision_mesh_format='.' + args.collision_mesh_format,
             blender_remesh=args.blender_remesh,
             blender_voxel_size=args.blender_voxel_size,
+            blender_decimate=args.blender_decimate,
+            blender_decimate_ratio=args.blender_decimate_ratio,
             blender_executable=args.blender_executable,
             remeshed_suffix=args.remeshed_suffix,
             draco_compression=args.draco), apply_scale(args.scale):
diff --git a/skrobot/utils/_blender_decimate_core.py b/skrobot/utils/_blender_decimate_core.py
@@ -0,0 +1,81 @@
+"""
+Core Blender decimation functions.
+This module is designed to be executed within Blender's Python environment.
+
+Unlike the voxel remesher, decimation (collapse) reduces the triangle count
+while preserving the original shape, materials and vertex colors. The mesh I/O
+is done in glTF/glb format because Blender 5.x no longer ships the Collada
+(``.dae``) importer/exporter; glTF is supported natively on every Blender
+version that includes ``bpy`` and round-trips materials and vertex colors.
+"""
+from pathlib import Path
+
+import bpy
+
+
+def clear_scene():
+    """Clear all objects from the scene."""
+    if bpy.ops.object.mode_set.poll():
+        bpy.ops.object.mode_set(mode='OBJECT')
+    bpy.ops.object.select_all(action='SELECT')
+    bpy.ops.object.delete()
+
+
+def decimate_glb_file(input_path, output_path, ratio=0.1):
+    """Import a glb, apply the decimate (collapse) modifier and re-export glb.
+
+    Parameters
+    ----------
+    input_path : str or pathlib.Path
+        Path to the input ``.glb`` file.
+    output_path : str or pathlib.Path
+        Path to the output ``.glb`` file.
+    ratio : float, optional
+        Collapse ratio passed to Blender's decimate modifier. The resulting
+        triangle count is approximately ``ratio`` times the original. Must be
+        in the range ``(0, 1]``. Default is 0.1.
+    """
+    input_path = Path(input_path)
+    output_path = Path(output_path)
+
+    clear_scene()
+
+    bpy.ops.import_scene.gltf(filepath=str(input_path))
+
+    objs = [o for o in bpy.context.scene.objects if o.type == 'MESH']
+    if not objs:
+        print("Warning: No mesh objects found in input")
+        return
+    before = sum(len(o.data.polygons) for o in objs)
+
+    # Join all mesh objects so a single decimate modifier covers the whole link.
+    bpy.ops.object.select_all(action='DESELECT')
+    for o in objs:
+        o.select_set(True)
+    # Always set the active object explicitly; with a single imported mesh
+    # the active object would otherwise be None and join() is skipped.
+    bpy.context.view_layer.objects.active = objs[0]
+    if len(objs) > 1:
+        bpy.ops.object.join()
+    obj = bpy.context.active_object
+
+    mod = obj.modifiers.new(name="Decimate", type='DECIMATE')
+    mod.decimate_type = 'COLLAPSE'
+    mod.ratio = ratio
+    mod.use_collapse_triangulate = True
+    bpy.ops.object.modifier_apply(modifier=mod.name)
+
+    after = len(obj.data.polygons)
+    print(f"Decimation complete. Faces: {before} -> {after} (ratio={ratio})")
+
+    if obj.data.polygons:
+        obj.data.polygons.foreach_set(
+            'use_smooth', [True] * len(obj.data.polygons))
+
+    bpy.context.view_layer.objects.active = obj
+    obj.select_set(True)
+    bpy.ops.export_scene.gltf(
+        filepath=str(output_path),
+        use_selection=True,
+        export_format='GLB',
+    )
diff --git a/skrobot/utils/blender_mesh.py b/skrobot/utils/blender_mesh.py
@@ -242,6 +242,158 @@ def remesh_with_blender(
         return remeshed_mesh
 
 
+def decimate_with_blender(
+        mesh,
+        ratio=0.1,
+        blender_executable=None,
+        verbose=False):
+    """Decimate a mesh using Blender's decimate (collapse) modifier.
+
+    Unlike :func:`remesh_with_blender`, this reduces the triangle count while
+    preserving the original shape, materials and vertex colors. The mesh is
+    exchanged with Blender in glTF/glb format (Blender 5.x no longer ships the
+    Collada importer/exporter), so colors round-trip as glTF materials/vertex
+    colors.
+
+    Parameters
+    ----------
+    mesh : trimesh.Trimesh or trimesh.Scene or list of trimesh.Trimesh or str or pathlib.Path
+        Mesh to be decimated. Can be a trimesh object, a list of trimesh
+        objects, a :class:`trimesh.Scene`, or a path to a mesh file.
+    ratio : float, optional
+        Collapse ratio passed to Blender's decimate modifier. The resulting
+        triangle count is approximately ``ratio`` times the original. Must be
+        in the range ``(0, 1]``. Default is 0.1.
+    blender_executable : str, optional
+        Path to Blender executable. If None, automatically searches for Blender
+        in common installation locations. Default is None.
+    verbose : bool, optional
+        Whether to print progress information. Default is False.
+
+    Returns
+    -------
+    list of trimesh.Trimesh
+        Decimated meshes with preserved colors.
+
+    Raises
+    ------
+    RuntimeError
+        If Blender decimation fails or the output file is not created.
+    """
+    from pathlib import Path
+    import subprocess
+    import tempfile
+
+    trimesh = _lazy_trimesh()
+
+    if not 0.0 < ratio <= 1.0:
+        raise ValueError(
+            "ratio must be in the range (0, 1], got {}".format(ratio))
+
+    # Auto-detect Blender if not specified
+    if blender_executable is None:
+        blender_executable = _find_blender_executable()
+        if blender_executable is None:
+            raise RuntimeError(
+                "Blender executable not found. Please install Blender or "
+                "specify the path using the blender_executable parameter."
+            )
+        if verbose:
+            print(f"Found Blender at: {blender_executable}")
+
+    # Normalize the input into a trimesh.Scene so it can be exported to glb.
+    if isinstance(mesh, (str, Path)):
+        scene = trimesh.load(mesh, force='scene')
+    elif isinstance(mesh, trimesh.Scene):
+        scene = mesh
+    elif isinstance(mesh, (list, tuple)):
+        scene = trimesh.Scene(list(mesh))
+    else:
+        scene = trimesh.Scene(mesh)
+
+    with tempfile.TemporaryDirectory() as temp_dir:
+        temp_path = Path(temp_dir)
+        input_glb = temp_path / 'input.glb'
+        output_glb = temp_path / 'output.glb'
+        scene.export(str(input_glb))
+
+        script_path = temp_path / 'decimate_script.py'
+        _create_blender_decimate_script(
+            script_path, input_glb, output_glb, ratio)
+
+        cmd = [
+            blender_executable,
+            '--background',
+            '--python', str(script_path),
+        ]
+
+        if verbose:
+            print(f"Running Blender decimate with ratio={ratio}...")
+
+        result = subprocess.run(cmd, capture_output=not verbose, text=True)
+
+        if result.returncode != 0:
+            error_msg = (
+                "Blender decimation failed with return code "
+                f"{result.returncode}")
+            if not verbose and result.stderr:
+                error_msg += f"\nError: {result.stderr}"
+            raise RuntimeError(error_msg)
+
+        if not output_glb.exists():
+            raise RuntimeError(
+                f"Decimated output file not found: {output_glb}")
+
+        decimated = trimesh.load(str(output_glb), force='scene')
+
+    geometries = [g for g in decimated.dump()
+                  if isinstance(g, trimesh.Trimesh)]
+    if not geometries:
+        raise RuntimeError("Decimated mesh contains no Trimesh geometry")
+
+    if verbose:
+        total_faces = sum(len(g.faces) for g in geometries)
+        print(f"Decimation complete. Output faces: {total_faces}")
+
+    return geometries
+
+
+def _create_blender_decimate_script(script_path, input_path, output_path, ratio):
+    """Create a wrapper script that imports the core Blender decimate module.
+
+    Parameters
+    ----------
+    script_path : pathlib.Path
+        Path where the wrapper script will be written.
+    input_path : pathlib.Path
+        Path to the input glb file.
+    output_path : pathlib.Path
+        Path to the output glb file.
+    ratio : float
+        Collapse ratio for decimation.
+    """
+    from pathlib import Path
+
+    core_module_path = Path(__file__).parent / '_blender_decimate_core.py'
+
+    script_content = f"""import sys
+from pathlib import Path
+
+core_module_path = Path(r"{core_module_path}")
+sys.path.insert(0, str(core_module_path.parent))
+
+from _blender_decimate_core import decimate_glb_file
+
+input_path = Path(r"{input_path}")
+output_path = Path(r"{output_path}")
+ratio = {ratio}
+
+decimate_glb_file(input_path, output_path, ratio)
+"""
+
+    script_path.write_text(script_content)
+
+
 def _create_blender_wrapper_script(script_path, input_path, output_path, voxel_size, export_format):
     """Create a wrapper script that imports the core Blender remeshing module.
 
diff --git a/skrobot/utils/urdf.py b/skrobot/utils/urdf.py
@@ -53,6 +53,8 @@
                         'scale_factor': 1.0,
                         'blender_remesh': False,
                         'blender_voxel_size': 0.002,
+                        'blender_decimate': False,
+                        'blender_decimate_ratio': 0.1,
                         'blender_executable': None,
                         '_current_geometry_context': None,  # 'collision' or 'visual'
                         '_source_urdf_path': None,  # Original URDF file path for mesh resolution
@@ -85,6 +87,8 @@ def export_mesh_format(
         collision_mesh_format=None,
         blender_remesh=False,
         blender_voxel_size=0.002,
+        blender_decimate=False,
+        blender_decimate_ratio=0.1,
         blender_executable=None,
         remeshed_suffix='_remeshed',
         draco_compression=False):
@@ -99,6 +103,8 @@ def export_mesh_format(
     _CONFIGURABLE_VALUES["overwrite_mesh"] = overwrite_mesh
     _CONFIGURABLE_VALUES["blender_remesh"] = blender_remesh
     _CONFIGURABLE_VALUES["blender_voxel_size"] = blender_voxel_size
+    _CONFIGURABLE_VALUES["blender_decimate"] = blender_decimate
+    _CONFIGURABLE_VALUES["blender_decimate_ratio"] = blender_decimate_ratio
     _CONFIGURABLE_VALUES["blender_executable"] = blender_executable
     _CONFIGURABLE_VALUES["remeshed_suffix"] = remeshed_suffix
     _CONFIGURABLE_VALUES["draco_compression"] = draco_compression
@@ -113,6 +119,8 @@ def export_mesh_format(
     _CONFIGURABLE_VALUES["overwrite_mesh"] = False
     _CONFIGURABLE_VALUES["blender_remesh"] = False
     _CONFIGURABLE_VALUES["blender_voxel_size"] = 0.002
+    _CONFIGURABLE_VALUES["blender_decimate"] = False
+    _CONFIGURABLE_VALUES["blender_decimate_ratio"] = 0.1
     _CONFIGURABLE_VALUES["blender_executable"] = None
     _CONFIGURABLE_VALUES["remeshed_suffix"] = '_remeshed'
     _CONFIGURABLE_VALUES["draco_compression"] = False
@@ -1254,6 +1262,18 @@ def _to_xml(self, parent, path):
                     # Mark as processed
                     _REMESHED_FILES_CACHE[cache_key] = str(remeshed_path)
 
+        # Apply Blender decimation (collapse) if requested. This reduces the
+        # triangle count while preserving shape and colors, and works on
+        # Blender 5.x because the mesh is exchanged in glb (not Collada).
+        if _CONFIGURABLE_VALUES['blender_decimate'] and not blender_remesh_applied:
+            from skrobot.utils.blender_mesh import decimate_with_blender
+            meshes = decimate_with_blender(
+                meshes,
+                ratio=_CONFIGURABLE_VALUES['blender_decimate_ratio'],
+                blender_executable=_CONFIGURABLE_VALUES['blender_executable'],
+                verbose=True,
+            )
+
         if _CONFIGURABLE_VALUES["target_triangles"] is not None:
             from skrobot.utils.mesh import auto_simplify_quadric_decimation_with_texture_preservation
             meshes = auto_simplify_quadric_decimation_with_texture_preservation(
diff --git a/tests/skrobot_tests/test_console_scripts.py b/tests/skrobot_tests/test_console_scripts.py
@@ -76,12 +76,17 @@ def test_convert_urdf_mesh(self):
             if _find_blender_executable() is not None:
                 out_blender_dae = osp.join(urdf_dir, 'fetch_blender.urdf')
                 out_blender_stl = osp.join(urdf_dir, 'fetch_blender_stl.urdf')
+                out_blender_decimate = osp.join(
+                    urdf_dir, 'fetch_blender_decimate.urdf')
                 blender_cmds = [
                     'convert-urdf-mesh {} --blender-remesh --blender-voxel-size 0.005 --output {}'.format(
                         urdfpath, out_blender_dae),
                     ('convert-urdf-mesh {} --blender-remesh --blender-voxel-size 0.005 '
                      '--collision-mesh-format stl --output {}').format(
                         urdfpath, out_blender_stl),
+                    ('convert-urdf-mesh {} --blender-decimate '
+                     '--blender-decimate-ratio 0.5 --output {}').format(
+                        urdfpath, out_blender_decimate),
                 ]
                 cmds.extend(blender_cmds)
             failures = []
