Don't clip clip paths to Figure bbox.

This kind of path clipping just cuts the path on the bounding box, which
produces an invalid clip path, since it is no longer closed or complete.

Fixes matplotlib#20127.

Author: QuLogic

lib/matplotlib/tests/test_artist.py

@@ -12,7 +12,7 @@
 import matplotlib.transforms as mtransforms
 import matplotlib.collections as mcollections
 import matplotlib.artist as martist
-from matplotlib.testing.decorators import image_comparison
+from matplotlib.testing.decorators import check_figures_equal, image_comparison
 
 
 def test_patch_transform_of_none():
@@ -121,6 +121,25 @@ def test_clipping():
     ax1.set_ylim([-3, 3])
 
 
+@check_figures_equal(extensions=['png'])
+def test_clipping_zoom(fig_test, fig_ref):
+    # This test places the Axes and sets its limits such that the clip path is
+    # outside the figure entirely. This should not break the clip path.
+    ax_test = fig_test.add_axes([0, 0, 1, 1])
+    l, = ax_test.plot([-3, 3], [-3, 3])
+    # Explicit Path instead of a Rectangle uses clip path processing, instead
+    # of a clip box optimization.
+    p = mpath.Path([[0, 0], [1, 0], [1, 1], [0, 1], [0, 0]])
+    p = mpatches.PathPatch(p, transform=ax_test.transData)
+    l.set_clip_path(p)
+
+    ax_ref = fig_ref.add_axes([0, 0, 1, 1])
+    ax_ref.plot([-3, 3], [-3, 3])
+
+    ax_ref.set(xlim=(0.5, 0.75), ylim=(0.5, 0.75))
+    ax_test.set(xlim=(0.5, 0.75), ylim=(0.5, 0.75))
+
+
 def test_cull_markers():
     x = np.random.random(20000)
     y = np.random.random(20000)

src/_backend_agg.cpp

@@ -134,8 +134,10 @@ bool RendererAgg::render_clippath(py::PathIterator &clippath,
 {
     typedef agg::conv_transform<py::PathIterator> transformed_path_t;
     typedef PathNanRemover<transformed_path_t> nan_removed_t;
-    typedef PathClipper<nan_removed_t> clipped_t;
-    typedef PathSnapper<clipped_t> snapped_t;
+    /* Unlike normal Paths, the clip path cannot be clipped to the Figure bbox,
+     * because it needs to remain a complete closed path, so there is no
+     * PathClipper<nan_removed_t> step. */
+    typedef PathSnapper<nan_removed_t> snapped_t;
     typedef PathSimplifier<snapped_t> simplify_t;
     typedef agg::conv_curve<simplify_t> curve_t;
 
@@ -151,8 +153,7 @@ bool RendererAgg::render_clippath(py::PathIterator &clippath,
         rendererBaseAlphaMask.clear(agg::gray8(0, 0));
         transformed_path_t transformed_clippath(clippath, trans);
         nan_removed_t nan_removed_clippath(transformed_clippath, true, clippath.has_curves());
-        clipped_t clipped_clippath(nan_removed_clippath, !clippath.has_curves(), width, height);
-        snapped_t snapped_clippath(clipped_clippath, snap_mode, clippath.total_vertices(), 0.0);
+        snapped_t snapped_clippath(nan_removed_clippath, snap_mode, clippath.total_vertices(), 0.0);
         simplify_t simplified_clippath(snapped_clippath,
                                        clippath.should_simplify() && !clippath.has_curves(),
                                        clippath.simplify_threshold());