Fix linestring to path (#2352)

This PR fixes an issues with converting `shapely.Linestring` to
`Path3D`.

tests/test_exchange.py

@@ -0,0 +1,38 @@
+from trimesh.path.exchange.misc import linestrings_to_path
+from trimesh.path.exchange.load import load_path
+
+from shapely.geometry import LineString, MultiLineString
+
+
+def test_linestrings_to_path():
+    line = LineString([(0, 0), (1, 1), (2, 0)])
+
+    result = linestrings_to_path(line)
+
+    assert len(result["entities"]) == 1
+    assert len(result["vertices"]) == 3
+
+
+def test_multilinestrings_to_path():
+    line = MultiLineString([
+        LineString([(0, 0), (1, 0), (2, 0)]),
+        LineString([(0, 1), (1, 1), (2, 1)])
+    ])
+
+    result = linestrings_to_path(line)
+
+    assert len(result["entities"]) == 2
+    assert len(result["vertices"]) == 6
+
+
+def test_load_path_with_multilinestrings():
+    line = MultiLineString([
+        LineString([(0, 0), (1, 0), (2, 0)]),
+        LineString([(0, 1), (1, 1), (2, 1)])
+    ])
+
+    result = load_path(line)
+
+    assert len(result.entities) == 2
+    assert len(result.vertices) == 6
+    assert result.length == 4

trimesh/path/exchange/misc.py

@@ -39,7 +39,7 @@ def dict_to_path(as_dict):
 
 def lines_to_path(lines: ArrayLike, index: Optional[NDArray[np.int64]] = None) -> Dict:
     """
-    Turn line segments into a Path2D or Path3D object.
+    Turn line segments into argument to be used for a Path2D or Path3D.
 
     Parameters
     ------------
@@ -50,7 +50,7 @@ def lines_to_path(lines: ArrayLike, index: Optional[NDArray[np.int64]] = None) -
 
     Returns
     -----------
-    kwargs : dict
+    kwargs : Dict
       kwargs for Path constructor
     """
     lines = np.asanyarray(lines, dtype=np.float64)
@@ -120,25 +120,29 @@ def polygon_to_path(polygon):
     return kwargs
 
 
-def linestrings_to_path(multi):
+def linestrings_to_path(multi) -> Dict:
     """
-    Load shapely LineString objects into a trimesh.path.Path2D object
+    Load shapely LineString objects into arguments to create a Path2D or Path3D.
 
     Parameters
     -------------
     multi : shapely.geometry.LineString or MultiLineString
-      Input 2D geometry
+      Input 2D or 3D geometry
 
     Returns
     -------------
-    kwargs : dict
-      Keyword arguments for Path2D constructor
+    kwargs : Dict
+      Keyword arguments for Path2D or Path3D constructor
     """
+    import shapely
+
     # append to result as we go
     entities = []
     vertices = []
 
-    if not util.is_sequence(multi):
+    if isinstance(multi, shapely.MultiLineString):
+        multi = list(multi.geoms)
+    else:
         multi = [multi]
 
     for line in multi: