matplotlib
diff --git a/‎lib/matplotlib/backends/backend_cairo.py‎
Lines changed: 18 additions & 3 deletions b/‎lib/matplotlib/backends/backend_cairo.py‎
Lines changed: 18 additions & 3 deletions
diff --git a/‎lib/matplotlib/tests/baseline_images/test_backends_rendering/blend_groups_agg.png‎
1.43 KB b/‎lib/matplotlib/tests/baseline_images/test_backends_rendering/blend_groups_agg.png‎
1.43 KB
diff --git a/‎lib/matplotlib/tests/baseline_images/test_backends_rendering/blend_groups_cairo.png‎
1.69 KB b/‎lib/matplotlib/tests/baseline_images/test_backends_rendering/blend_groups_cairo.png‎
1.69 KB
diff --git a/‎lib/matplotlib/tests/test_backends_rendering.py‎
Lines changed: 15 additions & 3 deletions b/‎lib/matplotlib/tests/test_backends_rendering.py‎
Lines changed: 15 additions & 3 deletions
@@ -378,10 +378,25 @@ def close_blend_group(self):
             ctx.save()
             self.gc.set_blend_mode(group_state.blend_mode)
             ctx.set_source(group)
-            if group_state.alpha != 1:
-                ctx.paint_with_alpha(group_state.alpha)
+
+            # For these two Porter-Duff compositing operators, we need to mask out fully
+            # transparent pixels, otherwise those pixels will override the backdrop
+            if group_state.blend_mode in {'knockout', 'clear'}:
+                # Extract the 8-bit alpha channel
+                mask_surface = ctx.get_target().create_similar_image(
+                    cairo.FORMAT_A8, self.width, self.height)
+                mask_ctx = cairo.Context(mask_surface)
+                mask_ctx.set_source(group)
+                mask_ctx.paint()
+                mask_surface.flush()
+
+                # Pixels that are not fully transparent are painted with the group alpha
+                mask_buf = np.asarray(mask_surface.get_data())
+                mask_buf[mask_buf > 0] = 255 * group_state.alpha + 0.5  # round to int
+                ctx.mask_surface(mask_surface)
             else:
-                ctx.paint()
+                ctx.paint_with_alpha(group_state.alpha)
+
             ctx.restore()
 
 
 
@@ -93,7 +93,7 @@ def draw(self, renderer):
         renderer.close_blend_group()
 
 
-def plot_blend_group_types(rasterize=False):
+def plot_blend_group_types(rasterize=False, porterduff=False):
     # Rows: top row is non-isolated, bottom row is isolated
     # Columns: left column is non-knockout, right column is knockout
     fig, axs = plt.subplots(2, 2, figsize=(3, 3), dpi=80, layout='constrained')
@@ -129,6 +129,18 @@ def plot_blend_group_types(rasterize=False):
             gray = Circle((0, 0), 0.1, fc='gray', zorder=0)
             axs[i, j].add_artist(gray)
 
+    # For backends that support the Porter-Duff compositing operators, additionally
+    # test 'knockout' and 'clear' as the blend mode for an isolated group
+    if porterduff:
+        for x, group_blend_mode, group_alpha in [(-0.6, 'knockout', 1),
+                                                 (-0.2, 'knockout', 0.6),
+                                                 (0.2, 'clear', 0.6),
+                                                 (0.6, 'clear', 1)]:
+            circle = Circle((x, -0.8), 0.15, fc='k', alpha=0.75)
+            group = ArtistGroup([circle], group_blend_mode=group_blend_mode,
+                                group_alpha=group_alpha)
+            axs[1, 0].add_artist(group)
+
 
 @image_comparison(['blend_modes_agg.png'], style='mpl20')
 def test_blend_modes_agg():
@@ -184,7 +196,7 @@ def test_blend_groups_agg():
     # The top-right panel (knockout but not isolated) is not supported, so will be
     # rendered like the top-left panel (neither knockout nor isolated)
 
-    plot_blend_group_types()
+    plot_blend_group_types(porterduff=True)
 
 
 @pytest.mark.backend('cairo')
@@ -193,7 +205,7 @@ def test_blend_groups_cairo():
     # The top-right panel (knockout but not isolated) is not supported, so will be
     # rendered like the top-left panel (neither knockout nor isolated)
 
-    plot_blend_group_types()
+    plot_blend_group_types(porterduff=True)
 
 
 @image_comparison(['blend_groups_svg.svg'], style='mpl20')