Test and fix for exporting small arcs to SVG.

src/build123d/exporters.py

@@ -36,6 +36,7 @@ from copy import copy
 from enum import Enum, auto
 from os import PathLike, fsdecode
 from typing import Any, TypeAlias
+from warnings import warn
 
 from collections.abc import Callable, Iterable
 
@@ -1196,6 +1197,12 @@ class ExportSVG(Export2D):
 
     def _circle_segments(self, edge: Edge, reverse: bool) -> list[PathSegment]:
         # pylint: disable=too-many-locals
+        if edge.length < 1e-6:
+            warn(
+                "Skipping arc that is too small to export safely (length < 1e-6).",
+                stacklevel=7,
+            )
+            return []
         curve = edge.geom_adaptor()
         circle = curve.Circle()
         radius = circle.Radius()
@@ -1242,6 +1249,12 @@ class ExportSVG(Export2D):
 
     def _ellipse_segments(self, edge: Edge, reverse: bool) -> list[PathSegment]:
         # pylint: disable=too-many-locals
+        if edge.length < 1e-6:
+            warn(
+                "Skipping ellipse that is too small to export safely (length < 1e-6).",
+                stacklevel=7,
+            )
+            return []
         curve = edge.geom_adaptor()
         ellipse = curve.Ellipse()
         minor_radius = ellipse.MinorRadius()

tests/test_exporters.py

@@ -29,6 +29,7 @@ from build123d import (
     add,
     mirror,
     section,
+    ThreePointArc,
 )
 from build123d.exporters import ExportSVG, ExportDXF, Drawing, LineType
 
@@ -173,6 +174,24 @@ class ExportersTestCase(unittest.TestCase):
         svg.add_shape(sketch)
         svg.write("test-colors.svg")
 
+    def test_svg_small_arc(self):
+        pnts = ((0, 0), (0, 0.000001), (0.000001, 0))
+        small_arc = ThreePointArc(pnts).scale(0.01)
+        with self.assertWarns(UserWarning):
+            svg_exporter = ExportSVG()
+            segments = svg_exporter._circle_segments(small_arc.edges()[0], False)
+            self.assertEqual(len(segments), 0, "Small arc should produce no segments")
+
+    def test_svg_small_ellipse(self):
+        pnts = ((0, 0), (0, 0.000001), (0.000002, 0))
+        small_ellipse = ThreePointArc(pnts).scale(0.01)
+        with self.assertWarns(UserWarning):
+            svg_exporter = ExportSVG()
+            segments = svg_exporter._ellipse_segments(small_ellipse.edges()[0], False)
+            self.assertEqual(
+                len(segments), 0, "Small ellipse should produce no segments"
+            )
+
 
 @pytest.mark.parametrize(
     "format", (Path, fsencode, fsdecode), ids=["path", "bytes", "str"]