Limit DXF BSpline Degree

Most DXF renderers and processing tools can't handle BSplines with
degree >= 3 (order >= 4). For maximum compatibility, we should
approximate such BSplines using degree-3 splines. This change uses the
OpenCascade facilities to do so, though ezdxf.math also provides some
spline approximation facilities that could be used. Using the
OpenCascade approach allows us to match FreeCAD's parameters which are
presumably tuned on a diversity of real-world designs.

Fixes #1225

Author: sapphire-arches

cadquery/occ_impl/exporters/dxf.py

@@ -4,7 +4,8 @@
 from .utils import toCompound
 
 from OCP.gp import gp_Dir
-from OCP.GeomConvert import GeomConvert
+from OCP.GeomConvert import GeomConvert, GeomConvert_ApproxCurve
+from OCP.GeomAbs import GeomAbs_C0
 
 import ezdxf
 
@@ -72,12 +73,20 @@ def _dxf_spline(e: Edge, msp: ezdxf.layouts.Modelspace, plane: Plane):
     adaptor = e._geomAdaptor()
     curve = GeomConvert.CurveToBSplineCurve_s(adaptor.Curve().Curve())
 
-    spline = GeomConvert.SplitBSplineCurve_s(
+    full_spline = GeomConvert.SplitBSplineCurve_s(
         curve, adaptor.FirstParameter(), adaptor.LastParameter(), CURVE_TOLERANCE
     )
 
     # need to apply the transform on the geometry level
-    spline.Transform(plane.fG.wrapped.Trsf())
+    full_spline.Transform(plane.fG.wrapped.Trsf())
+
+    # approximate the transformed spline down to max degree 3 for
+    # compatibilitly with more DXF processing tools.
+    #
+    # parameters choosen to match FreeCAD 0.20
+    approx_spline = GeomConvert_ApproxCurve(full_spline, 0.001, GeomAbs_C0, 50, 3)
+
+    spline = approx_spline.Curve()
 
     order = spline.Degree() + 1
     knots = list(spline.KnotSequence())