Modify STL export to enable non-relative tolerancing and speed up export. (#1432)

* Add arguments to allow non-relative and parallel mesh computation (default behavior is the same)

* Remove duplicate Perform() to approximately halve stl export time

* Paint it black (tm)

* Trust that `ascii` comes in as a bool

* Rename isParallel --> parallel, default True. Fix docstrings

* Rename `isToleranceRelative` to `relative`

Author: brhubbar

cadquery/occ_impl/shapes.py

@@ -417,6 +417,8 @@ def exportStl(
         tolerance: float = 1e-3,
         angularTolerance: float = 0.1,
         ascii: bool = False,
+        relative: bool = True,
+        parallel: bool = True,
     ) -> bool:
         """
         Exports a shape to a specified STL file.
@@ -428,17 +430,17 @@ def exportStl(
             Default is 1e-3, which is a good starting point for a range of cases.
         :param angularTolerance: Angular deflection setting which limits the angle between subsequent segments in a polyline. Default is 0.1.
         :param ascii: Export the file as ASCII (True) or binary (False) STL format.  Default is binary.
+        :param relative: If True, tolerance will be scaled by the size of the edge being meshed. Default is True.
+            Setting this value to True may cause large features to become faceted, or small features dense.
+        :param parallel: If True, OCCT will use parallel processing to mesh the shape. Default is True.
         """
-
-        mesh = BRepMesh_IncrementalMesh(self.wrapped, tolerance, True, angularTolerance)
-        mesh.Perform()
+        # The constructor used here automatically calls mesh.Perform(). https://dev.opencascade.org/doc/refman/html/class_b_rep_mesh___incremental_mesh.html#a3a383b3afe164161a3aa59a492180ac6
+        BRepMesh_IncrementalMesh(
+            self.wrapped, tolerance, relative, angularTolerance, parallel
+        )
 
         writer = StlAPI_Writer()
-
-        if ascii:
-            writer.ASCIIMode = True
-        else:
-            writer.ASCIIMode = False
+        writer.ASCIIMode = ascii
 
         return writer.Write(self.wrapped, fileName)