[Arc] return clearer errors when working with an invalid arclength

masque/shapes/arc.py

@@ -231,6 +231,8 @@ class Arc(PositionableImpl, Shape):
         if (num_vertices is None) and (max_arclen is None):
             raise PatternError('Max number of points and arclength left unspecified'
                                + ' (default was also overridden)')
+        if max_arclen is not None and (numpy.isnan(max_arclen) or max_arclen <= 0):
+            raise PatternError('Max arclength must be positive and not NaN')
 
         r0, r1 = self.radii
 
@@ -257,13 +259,19 @@ class Arc(PositionableImpl, Shape):
             return arc_lengths, tt
 
         wh = self.width / 2.0
+        arclen_limits: list[float] = []
+        if max_arclen is not None:
+            arclen_limits.append(max_arclen)
         if num_vertices is not None:
             n_pts = numpy.ceil(max(self.radii + wh) / min(self.radii) * num_vertices * 100).astype(int)
             perimeter_inner = get_arclens(n_pts, *a_ranges[0], dr=-wh)[0].sum()
             perimeter_outer = get_arclens(n_pts, *a_ranges[1], dr= wh)[0].sum()
             implied_arclen = (perimeter_outer + perimeter_inner + self.width * 2) / num_vertices
-            max_arclen = min(implied_arclen, max_arclen if max_arclen is not None else numpy.inf)
-        assert max_arclen is not None
+            if not (numpy.isnan(implied_arclen) or implied_arclen <= 0):
+                arclen_limits.append(implied_arclen)
+        if not arclen_limits:
+            raise PatternError('Arc polygonization could not determine a valid max_arclen')
+        max_arclen = min(arclen_limits)
 
         def get_thetas(inner: bool) -> NDArray[numpy.float64]:
             """ Figure out the parameter values at which we should place vertices to meet the arclength constraint"""

masque/test/test_shape_advanced.py

@@ -122,6 +122,12 @@ def test_curve_polygonizers_clamp_large_max_arclen() -> None:
         assert len(polys[0].vertices) >= 3
 
 
+def test_arc_polygonization_rejects_nan_implied_arclen() -> None:
+    arc = Arc(radii=(10, 20), angles=(0, numpy.nan), width=2)
+    with pytest.raises(PatternError, match='valid max_arclen'):
+        arc.to_polygons(num_vertices=24)
+
+
 def test_ellipse_integer_radii_scale_cleanly() -> None:
     ellipse = Ellipse(radii=(10, 20))
     ellipse.scale_by(0.5)