FIX: add extra condition on seam splitting

Author: greglucas

lib/cartopy/tests/mpl/baseline_images/mpl/test_examples/contour_label.png

@@ -605,9 +605,21 @@ cdef void _resample_recursive(
         # O(2^depth) calls for points outside the domain.
         return
 
-    # Check for a wrap-around: midpoint outside segment's [0,1] along-range
-    # means the geodesic crosses the domain boundary (e.g. antimeridian)
-    # without a state change.
+    # Check for a coordinate wrap-around at a projection seam (e.g., the
+    # antimeridian in PlateCarree). When p_mid falls outside the [0,1]
+    # along-range of the chord from p_start to p_end, one of two things has
+    # happened:
+    #
+    #   (1) Genuine seam: the projected path crosses a coordinate-space
+    #       discontinuity (longitude jumps from +180 to -180).
+    #       _find_valid_boundary will return p_in and p_out that are nearly
+    #       identical in parameter space (t) but far apart in coordinate space (p).
+    #
+    #   (2) High projection curvature: the path bends so sharply that the
+    #       midpoint of a straight chord misrepresents the actual arc, but
+    #       there is no coordinate discontinuity. _find_valid_boundary
+    #       converges to a tiny parameter interval, giving p_in and p_out
+    #       only ~threshold apart in coordinate space.
     seg_dx_w = p_end.x - p_start.x
     seg_dy_w = p_end.y - p_start.y
     seg_hq_w = seg_dx_w*seg_dx_w + seg_dy_w*seg_dy_w
@@ -616,24 +628,26 @@ cdef void _resample_recursive(
         mid_dy_w = p_mid.y - p_start.y
         along_w = (seg_dx_w*mid_dx_w + seg_dy_w*mid_dy_w) / seg_hq_w
         if along_w < 0.0 or along_w > 1.0:
-            # Wrap: find the boundary, split, and recurse each side.
             _find_valid_boundary(t_start, p_start, t_end, p_end,
                                  interpolator, domain, threshold,
                                  &t_in, &p_in, &t_out, &p_out,
                                  geom_fully_inside)
-            if t_in > t_start + 1e-6:
-                _resample_recursive(t_start, p_start, state_start,
-                                    t_in, p_in,
+            if (p_out.x - p_in.x)**2 + (p_out.y - p_in.y)**2 > (threshold * 50.0)**2:
+                # far apart coordinates indicating a seam/antimeridian, split the line here.
+                if t_in > t_start + 1e-6:
+                    _resample_recursive(t_start, p_start, state_start,
+                                        t_in, p_in,
+                                        interpolator, domain, threshold,
+                                        lines, depth - 1, geom_fully_inside)
+                else:
+                    lines.add_point_if_empty(p_start)
+                lines.new_line()
+                _resample_recursive(t_out, p_out, state_start,
+                                    t_end, p_end,
                                     interpolator, domain, threshold,
                                     lines, depth - 1, geom_fully_inside)
-            else:
-                lines.add_point_if_empty(p_start)
-            lines.new_line()
-            _resample_recursive(t_out, p_out, state_start,
-                                t_end, p_end,
-                                interpolator, domain, threshold,
-                                lines, depth - 1, geom_fully_inside)
-            return
+                return
+            # else: similar projected coordinates, no break needed
 
     # Normal curvature subdivision.
     _resample_recursive(t_start, p_start, state_start, t_mid, p_mid,