Backport PR #27045: Ensure valid path mangling for ContourLabeler

tacaswell · meeseeksmachine · commit cd46813c79b4 · 2023-10-25T20:43:16.000Z
diff --git a/lib/matplotlib/contour.py b/lib/matplotlib/contour.py
@@ -350,6 +350,11 @@ def _split_path_and_get_label_rotation(self, path, idx, screen_pos, lw, spacing=
         taken into account when breaking the path, but not when computing the angle.
         """
         if hasattr(self, "_old_style_split_collections"):
+            vis = False
+            for coll in self._old_style_split_collections:
+                vis |= coll.get_visible()
+                coll.remove()
+            self.set_visible(vis)
             del self._old_style_split_collections  # Invalidate them.
 
         xys = path.vertices
@@ -383,7 +388,7 @@ def _split_path_and_get_label_rotation(self, path, idx, screen_pos, lw, spacing=
         # If the path is closed, rotate it s.t. it starts at the label.
         is_closed_path = codes[stop - 1] == Path.CLOSEPOLY
         if is_closed_path:
-            cc_xys = np.concatenate([xys[idx:-1], xys[:idx+1]])
+            cc_xys = np.concatenate([cc_xys[idx:-1], cc_xys[:idx+1]])
             idx = 0
 
         # Like np.interp, but additionally vectorized over fp.
@@ -418,8 +423,13 @@ def interp_vec(x, xp, fp): return [np.interp(x, xp, col) for col in fp.T]
         new_code_blocks = []
         if is_closed_path:
             if i0 != -1 and i1 != -1:
-                new_xy_blocks.extend([[(x1, y1)], cc_xys[i1:i0+1], [(x0, y0)]])
-                new_code_blocks.extend([[Path.MOVETO], [Path.LINETO] * (i0 + 2 - i1)])
+                # This is probably wrong in the case that the entire contour would
+                # be discarded, but ensures that a valid path is returned and is
+                # consistent with behavior of mpl <3.8
+                points = cc_xys[i1:i0+1]
+                new_xy_blocks.extend([[(x1, y1)], points, [(x0, y0)]])
+                nlines = len(points) + 1
+                new_code_blocks.extend([[Path.MOVETO], [Path.LINETO] * nlines])
         else:
             if i0 != -1:
                 new_xy_blocks.extend([cc_xys[:i0 + 1], [(x0, y0)]])
diff --git a/lib/matplotlib/tests/baseline_images/test_contour/contour_disconnected_segments.png b/lib/matplotlib/tests/baseline_images/test_contour/contour_disconnected_segments.png
diff --git a/lib/matplotlib/tests/test_contour.py b/lib/matplotlib/tests/test_contour.py
@@ -116,11 +116,31 @@ def test_contour_manual_labels(split_collections):
 
     plt.figure(figsize=(6, 2), dpi=200)
     cs = plt.contour(x, y, z)
+
+    _maybe_split_collections(split_collections)
+
     pts = np.array([(1.0, 3.0), (1.0, 4.4), (1.0, 6.0)])
     plt.clabel(cs, manual=pts)
     pts = np.array([(2.0, 3.0), (2.0, 4.4), (2.0, 6.0)])
     plt.clabel(cs, manual=pts, fontsize='small', colors=('r', 'g'))
 
+
+@pytest.mark.parametrize("split_collections", [False, True])
+@image_comparison(['contour_disconnected_segments'],
+                  remove_text=True, style='mpl20', extensions=['png'])
+def test_contour_label_with_disconnected_segments(split_collections):
+    x, y = np.mgrid[-1:1:21j, -1:1:21j]
+    z = 1 / np.sqrt(0.01 + (x + 0.3) ** 2 + y ** 2)
+    z += 1 / np.sqrt(0.01 + (x - 0.3) ** 2 + y ** 2)
+
+    plt.figure()
+    cs = plt.contour(x, y, z, levels=[7])
+
+    # Adding labels should invalidate the old style
+    _maybe_split_collections(split_collections)
+
+    cs.clabel(manual=[(0.2, 0.1)])
+
     _maybe_split_collections(split_collections)
 
 
@@ -232,6 +252,9 @@ def test_labels(split_collections):
     disp_units = [(216, 177), (359, 290), (521, 406)]
     data_units = [(-2, .5), (0, -1.5), (2.8, 1)]
 
+    # Adding labels should invalidate the old style
+    _maybe_split_collections(split_collections)
+
     CS.clabel()
 
     for x, y in data_units:
@@ -338,6 +361,22 @@ def test_clabel_zorder(use_clabeltext, contour_zorder, clabel_zorder):
         assert clabel.get_zorder() == expected_clabel_zorder
 
 
+def test_clabel_with_large_spacing():
+    # When the inline spacing is large relative to the contour, it may cause the
+    # entire contour to be removed. In current implementation, one line segment is
+    # retained between the identified points.
+    # This behavior may be worth reconsidering, but check to be sure we do not produce
+    # an invalid path, which results in an error at clabel call time.
+    # see gh-27045 for more information
+    x = y = np.arange(-3.0, 3.01, 0.05)
+    X, Y = np.meshgrid(x, y)
+    Z = np.exp(-X**2 - Y**2)
+
+    fig, ax = plt.subplots()
+    contourset = ax.contour(X, Y, Z, levels=[0.01, 0.2, .5, .8])
+    ax.clabel(contourset, inline_spacing=100)
+
+
 # tol because ticks happen to fall on pixel boundaries so small
 # floating point changes in tick location flip which pixel gets
 # the tick.
