fix(MeshHelper): get_triangles on meshes without indices

UnityPy/helpers/MeshHelper.py

@@ -635,9 +635,14 @@ def decompress_compressed_mesh(self):
             ]
 
     def get_triangles(self) -> List[List[Tuple[int, ...]]]:
-        assert self.m_IndexBuffer is not None
         assert self.src.m_SubMeshes is not None
 
+        if self.m_IndexBuffer is None:
+            # a mesh can have vertices without indices, e.g. an empty placeholder mesh
+            if any(m_SubMesh.indexCount for m_SubMesh in self.src.m_SubMeshes):
+                raise ValueError("Failed getting triangles. The submeshes have indices, but the mesh has none.")
+            return [[] for _ in self.src.m_SubMeshes]
+
         submeshes: List[List[Tuple[int, ...]]] = []
 
         for m_SubMesh in self.src.m_SubMeshes:

tests/test_mesh_helper.py

@@ -0,0 +1,47 @@
+import os
+
+import pytest
+
+import UnityPy
+from UnityPy.helpers.MeshHelper import MeshHandler
+
+SAMPLES = os.path.join(os.path.dirname(os.path.abspath(__file__)), "samples")
+
+
+def load_mesh():
+    env = UnityPy.load(os.path.join(SAMPLES, "xinzexi_2_n_tex"))
+    return next(obj.parse_as_object() for obj in env.objects if obj.type.name == "Mesh")
+
+
+def triangles(mesh):
+    handler = MeshHandler(mesh)
+    handler.process()
+    return handler.get_triangles()
+
+
+def test_mesh_without_submeshes_and_indices():
+    # vertices only, as in a placeholder mesh
+    mesh = load_mesh()
+    mesh.m_IndexBuffer = []
+    mesh.m_SubMeshes = []
+    assert triangles(mesh) == []
+    obj = mesh.export()
+    assert "\nv " in obj or obj.startswith("v ")
+    assert "\nf " not in obj
+
+
+def test_mesh_with_empty_submeshes_and_no_indices():
+    mesh = load_mesh()
+    mesh.m_IndexBuffer = []
+    for submesh in mesh.m_SubMeshes:
+        submesh.firstByte = 0
+        submesh.indexCount = 0
+    assert triangles(mesh) == [[] for _ in mesh.m_SubMeshes]
+
+
+def test_mesh_with_submesh_indices_but_no_index_buffer():
+    mesh = load_mesh()
+    assert any(submesh.indexCount for submesh in mesh.m_SubMeshes)
+    mesh.m_IndexBuffer = []
+    with pytest.raises(ValueError):
+        triangles(mesh)