[MISC] Remove fast_simplification submodule.

MANIFEST.in

@@ -3,5 +3,4 @@ include LICENSE
 recursive-include genesis/assets *
 recursive-include genesis/ext/pyrender/fonts *
 recursive-include genesis/ext/pyrender/shaders *
-include genesis/ext/VolumeSampling
-include genesis/ext/fast_simplification/*.h
+include genesis/ext/VolumeSampling

genesis/engine/mesh.py

@@ -1,6 +1,7 @@
 import os
 import pickle as pkl
 
+import fast_simplification
 import numpy as np
 import numpy.typing as npt
 import pyvista as pv
@@ -14,7 +15,6 @@
 import genesis.utils.gltf as gltf_utils
 import genesis.utils.usda as usda_utils
 import genesis.utils.particle as pu
-from genesis.ext import fast_simplification
 from genesis.repr_base import RBC
 
 
@@ -93,15 +93,23 @@ def decimate(self, decimate_face_num, decimate_aggressiveness, convexify):
         """
         if self._mesh.vertices.shape[0] > 3 and self._mesh.faces.shape[0] > decimate_face_num:
             self._mesh.process(validate=True)
-            self._mesh = trimesh.Trimesh(
-                *fast_simplification.simplify(
-                    self._mesh.vertices,
-                    self._mesh.faces,
-                    target_count=decimate_face_num,
-                    agg=decimate_aggressiveness,
-                    lossless=(decimate_aggressiveness == 0),
+            # TODO: lossless option support is pending on fast_simplification package.
+            # NOTE: https://github.com/pyvista/fast-simplification/pull/71
+            if decimate_aggressiveness == 0:
+                gs.logger.debug("Lossless simplification is not supported yet. Not applying simplification.")
+                self._mesh = trimesh.Trimesh(
+                    vertices=self._mesh.vertices,
+                    faces=self._mesh.faces,
+                )
+            else:
+                self._mesh = trimesh.Trimesh(
+                    *fast_simplification.simplify(
+                        self._mesh.vertices,
+                        self._mesh.faces,
+                        target_count=decimate_face_num,
+                        agg=decimate_aggressiveness,
+                    )
                 )
-            )
 
             # need to run convexify again after decimation, because sometimes decimating a convex-mesh can make it non-convex...
             if convexify: