Fix import of invalid triangles Issue #472

src/build123d/mesher.py

@@ -82,6 +82,7 @@
 # pylint: disable=no-name-in-module, import-error
 import copy
 import ctypes
+import itertools
 import math
 import os
 import sys
@@ -96,18 +97,21 @@
     BRepBuilderAPI_MakeSolid,
     BRepBuilderAPI_Sewing,
 )
+from OCP.BRepGProp import BRepGProp
 from OCP.BRepMesh import BRepMesh_IncrementalMesh
 from OCP.gp import gp_Pnt
 import OCP.TopAbs as ta
+from OCP.GProp import GProp_GProps
+from OCP.ShapeFix import ShapeFix_Shape
 from OCP.TopAbs import TopAbs_ShapeEnum
 from OCP.TopExp import TopExp_Explorer
 from OCP.TopoDS import TopoDS_Compound
 from OCP.TopLoc import TopLoc_Location
 
 from py_lib3mf import Lib3MF
 from build123d.build_enums import MeshType, Unit
-from build123d.geometry import Color, TOLERANCE
-from build123d.topology import Compound, Shape, Shell, Solid, downcast
+from build123d.geometry import Color, TOLERANCE, Vector
+from build123d.topology import Compound, Face, Shape, Shell, Solid, downcast
 
 
 class Mesher:
@@ -455,8 +459,11 @@ def _get_shape(self, mesh_3mf: Lib3MF.MeshObject) -> Shape:
             # Create the triangular face using the polygon
             polygon_builder = BRepBuilderAPI_MakePolygon(*ocp_vertices, Close=True)
             face_builder = BRepBuilderAPI_MakeFace(polygon_builder.Wire())
-            # Add new Face to Shell
-            shell_builder.Add(face_builder.Face())
+            facet = face_builder.Face()
+            facet_properties = GProp_GProps()
+            BRepGProp.SurfaceProperties_s(facet, facet_properties)
+            if facet_properties.Mass() != 0:  # Area==0 is an invalid facet
+                shell_builder.Add(facet)
 
         # Create the Shell(s) - if the object has voids there will be multiple
         shell_builder.Perform()

tests/test_mesher.py

@@ -190,5 +190,16 @@ def test_hollow_import(self):
         self.assertTrue(stl[0].is_valid())
 
 
+class TestImportDegenerateTriangles(unittest.TestCase):
+    def test_degenerate_import(self):
+        """Some STLs may contain 'triangles' where all three points are on a line"""
+        importer = Mesher()
+        stl = importer.read("cyl_w_rect_hole.stl")[0]
+        self.assertEqual(type(stl), Solid)
+        self.assertTrue(stl.is_manifold)
+        self.assertTrue(stl.is_valid())
+        self.assertEqual(sum(f.area == 0 for f in stl.faces()), 0)
+
+
 if __name__ == "__main__":
     unittest.main()