Adjust unit conversion for mesh (#315)

* Adjust unit conversion

* Add transparency check to mesh loader and fix fully transparent PBRMaterials

- Implemented `get_transparency` function to retrieve the alpha value from mesh material's main_color.
- Updated `_load_meshes` function to check for transparency in meshes and set alpha to 255 for fully transparent PBRMaterials to ensure visibility.
- Imported `PBRMaterial` from `trimesh.visual.material` to support the transparency correction.

skrobot/utils/urdf.py

@@ -26,6 +26,7 @@
 import PIL
 import six
 import trimesh
+from trimesh.visual.material import PBRMaterial
 
 from skrobot.coordinates import normalize_vector
 from skrobot.coordinates import rpy_angle
@@ -59,6 +60,13 @@ def no_mesh_load_mode():
     _CONFIGURABLE_VALUES["no_mesh_load_mode"] = False
 
 
+def get_transparency(mesh):
+    if hasattr(mesh, 'visual') and hasattr(mesh.visual, 'material'):
+        material = mesh.visual.material
+        if hasattr(material, 'main_color'):
+            return material.main_color[3]
+
+
 def parse_origin(node):
     """Find the ``origin`` subelement of an XML node and convert it
 
@@ -249,6 +257,8 @@ def _load_meshes(filename):
             meshes = meshes.scaled(0.001)
         else:
             meshes = trimesh.load(filename)
+        if meshes.units is not None and meshes.units != 'meter':
+            meshes = meshes.convert_units('meter')
     except Exception as e:
         logger.error("Failed to load meshes from {}. Error: {}"
                      .format(filename, e))
@@ -271,6 +281,11 @@ def _load_meshes(filename):
     else:
         raise ValueError('Unable to load mesh from file')
 
+    for mesh in meshes:
+        transparency = get_transparency(mesh)
+        if transparency is not None and transparency == 0.0:
+            if isinstance(mesh.visual.material, PBRMaterial):
+                mesh.visual.material.baseColorFactor[3] = 255
     return meshes