fixes #2218

models/Mesh_PrimitiveMode_04.gltf

@@ -0,0 +1,71 @@
+{
+  "accessors": [
+    {
+      "bufferView": 0,
+      "componentType": 5126,
+      "count": 4,
+      "type": "VEC3",
+      "max": [
+        0.5,
+        0.5,
+        0.0
+      ],
+      "min": [
+        -0.5,
+        -0.5,
+        0.0
+      ],
+      "name": "Positions Accessor"
+    }
+  ],
+  "asset": {
+    "generator": "glTF Asset Generator",
+    "version": "2.0"
+  },
+  "buffers": [
+    {
+      "uri": "Mesh_PrimitiveMode_04.bin",
+      "byteLength": 48
+    }
+  ],
+  "bufferViews": [
+    {
+      "buffer": 0,
+      "byteLength": 48,
+      "name": "Positions"
+    }
+  ],
+  "materials": [
+    {
+      "pbrMetallicRoughness": {
+        "metallicFactor": 0.0
+      }
+    }
+  ],
+  "meshes": [
+    {
+      "primitives": [
+        {
+          "attributes": {
+            "POSITION": 0
+          },
+          "material": 0,
+          "mode": 5
+        }
+      ]
+    }
+  ],
+  "nodes": [
+    {
+      "mesh": 0
+    }
+  ],
+  "scene": 0,
+  "scenes": [
+    {
+      "nodes": [
+        0
+      ]
+    }
+  ]
+}

tests/test_gltf.py

@@ -1058,6 +1058,12 @@ def test_unitize_normals_null_values(self):
         # Check that the normals are still null
         assert g.np.allclose(reimported_mesh.vertex_normals[0], [0, 0, 0])
 
+    def test_no_indices(self):
+        # test mesh with no indices (faces should be generated correctly)
+        mesh = g.get_mesh("no_indices_3storybuilding.glb")
+        assert len(mesh.triangles) == 72
+        mesh = g.get_mesh("Mesh_PrimitiveMode_04.gltf")
+        assert len(mesh.triangles) == 1
 
 if __name__ == "__main__":
     g.trimesh.util.attach_to_log()

trimesh/exchange/gltf.py

@@ -1517,8 +1517,9 @@ def _read_buffers(
                     else:
                         # indices are apparently optional and we are supposed to
                         # do the same thing as webGL drawArrays?
+                        faces_len = int(3 * round(len(kwargs["vertices"]) / 3.))
                         kwargs["faces"] = np.arange(
-                            len(kwargs["vertices"]) * 3, dtype=np.int64
+                            faces_len, dtype=np.int64
                         ).reshape((-1, 3))
 
                     if "NORMAL" in attr: