matplotlib · ayshih · Aug 28, 2026 · Aug 28, 2026
diff --git a/lib/matplotlib/backend_bases.py b/lib/matplotlib/backend_bases.py
@@ -158,6 +158,9 @@ def __init__(self):
         self._raster_depth = 0
         self._rasterizing = False
 
+        self._use_blend_group_for_contourf = False
+        self._use_blend_group_for_pcolor = False
+
     def open_group(self, s, gid=None):
         """
         Open a grouping element with label *s* and *gid* (if set) as id.

diff --git a/lib/matplotlib/backends/backend_agg.py b/lib/matplotlib/backends/backend_agg.py
@@ -85,6 +85,9 @@ def __init__(self, width, height, dpi):
 
         self._override_blend_mode_to_knockout = False
 
+        self._use_blend_group_for_contourf = True
+        self._use_blend_group_for_pcolor = True
+
         self._update_methods()
         self.mathtext_parser = MathTextParser('path')
 

diff --git a/lib/matplotlib/backends/backend_cairo.py b/lib/matplotlib/backends/backend_cairo.py
@@ -104,6 +104,9 @@ def __init__(self, dpi):
 
         self._override_blend_mode_to_knockout = False
 
+        self._use_blend_group_for_contourf = True
+        self._use_blend_group_for_pcolor = True
+
     def set_context(self, ctx):
         surface = ctx.get_target()
         if hasattr(surface, "get_width") and hasattr(surface, "get_height"):

diff --git a/lib/matplotlib/collections.py b/lib/matplotlib/collections.py
@@ -2546,6 +2546,7 @@ def draw(self, renderer):
         if not self.get_visible():
             return
         renderer.open_group(self.__class__.__name__, self.get_gid())
+
         transform = self.get_transform()
         offset_trf = self.get_offset_transform()
         offsets = self.get_offsets()
@@ -2580,14 +2581,24 @@ def draw(self, renderer):
             renderer.draw_gouraud_triangles(
                 gc, triangles, colors, transform.frozen())
         else:
+            isolate = (renderer._use_blend_group_for_pcolor and self._antialiased
+                       and (self._edgecolors.size == 0 or self._edgecolors[0][3] == 0))
+            if isolate:
+                gc.set_blend_mode("plus")
+                renderer.open_blend_group(self.get_blend_mode())
+
             renderer.draw_quad_mesh(
                 gc, transform.frozen(),
                 coordinates.shape[1] - 1, coordinates.shape[0] - 1,
                 coordinates, offsets, offset_trf,
                 # Backends expect flattened rgba arrays (n*m, 4) for fc and ec
                 self.get_facecolor().reshape((-1, 4)),
                 self._antialiased, self.get_edgecolors().reshape((-1, 4)))
+
+            if isolate:
+                renderer.close_blend_group()
         gc.restore()
+
         renderer.close_group(self.__class__.__name__)
         self.stale = False
 
@@ -2688,6 +2699,21 @@ def get_edgecolor(self):
             return ec
         return ec[unmasked_polys, :]
 
+    @artist.allow_rasterization
+    def draw(self, renderer):
+        isolate = (renderer._use_blend_group_for_pcolor and np.all(self._antialiaseds)
+                   and (self._edgecolors.size == 0 or self._edgecolors[0][3] == 0))
+        if isolate:
+            blend_mode = self.get_blend_mode()
+            self.set_blend_mode("plus")
+            renderer.open_blend_group(blend_mode)
+
+        super().draw(renderer)
+
+        if isolate:
+            renderer.close_blend_group()
+            self.set_blend_mode(blend_mode)
+
     def get_facecolor(self):
         # docstring inherited
         # We only want to return the facecolors of the polygons

diff --git a/lib/matplotlib/contour.py b/lib/matplotlib/contour.py
@@ -1285,11 +1285,25 @@ def find_nearest_contour(self, x, y, indices=None, pixel=True):
     def draw(self, renderer):
         paths = self._paths
         n_paths = len(paths)
+        edgecolors = self.get_edgecolors()
         if not self.filled or all(hatch is None for hatch in self.hatches):
+            isolate = (renderer._use_blend_group_for_contourf and
+                       self.filled and self.get_antialiased() and
+                       (edgecolors.size == 0 or edgecolors[0][3] == 0))
+
+            if isolate:
+                blend_mode = self.get_blend_mode()
+                self.set_blend_mode("plus")
+                renderer.open_blend_group(blend_mode)
+
             super().draw(renderer)
+
+            if isolate:
+                renderer.close_blend_group()
+                self.set_blend_mode(blend_mode)
+
             return
         # In presence of hatching, draw contours one at a time.
-        edgecolors = self.get_edgecolors()
         if edgecolors.size == 0:
             edgecolors = ("none",)
         for idx in range(n_paths):

diff --git a/lib/matplotlib/tests/baseline_images/test_axes/pcolormesh_antialiasing.png b/lib/matplotlib/tests/baseline_images/test_axes/pcolormesh_antialiasing.png
diff --git a/lib/matplotlib/tests/baseline_images/test_contour/contourf_antialiasing.png b/lib/matplotlib/tests/baseline_images/test_contour/contourf_antialiasing.png
diff --git a/lib/matplotlib/tests/test_axes.py b/lib/matplotlib/tests/test_axes.py
@@ -1507,6 +1507,32 @@ def test_pcolormesh_alpha():
     ax4.pcolormesh(Qx, Qy, Z, cmap=cmap, shading='gouraud', zorder=1)
 
 
+@image_comparison(['pcolormesh_antialiasing.png'], style='mpl20')
+def test_pcolormesh_antialiasing():
+    N = 5
+    data = np.arange(N**2, dtype=float).reshape((N, N))
+    data[2, 2] = np.nan
+
+    x, y = np.meshgrid(np.arange(N + 1), np.arange(N + 1))
+
+    rotation = mtransforms.Affine2D().rotate(1)
+
+    fig, axs = plt.subplots(2, 3, figsize=(5, 3.5), layout="constrained")
+
+    for i, antialiased in enumerate([None, False, True]):
+        kwargs = {'cmap': 'jet', 'alpha': 0.5}
+        if antialiased is not None:
+            kwargs['antialiased'] = antialiased
+
+        axs[0, i].pcolormesh(x, y, data, transform=rotation + axs[0, i].transData,
+                             **kwargs)
+        axs[1, i].pcolor(x, y, data, transform=rotation + axs[1, i].transData, **kwargs)
+
+        for j in range(2):
+            axs[j, i].set_aspect("equal")
+            axs[j, i].set_axis_off()
+
+
 @pytest.mark.parametrize("dims,alpha", [(3, 1), (4, 0.5)])
 @check_figures_equal()
 def test_pcolormesh_rgba(fig_test, fig_ref, dims, alpha):

diff --git a/lib/matplotlib/tests/test_contour.py b/lib/matplotlib/tests/test_contour.py
@@ -896,3 +896,23 @@ def test_clabel_manual_subset():
     cs = ax.contour([[1, 2], [3, 4]], levels=[1.5, 2.5, 3.5])
     # Attempt to label only one specific level manually
     ax.clabel(cs, levels=[2.5], manual=[(0.5, 0.5)])
+
+
+@image_comparison(['contourf_antialiasing.png'], style='mpl20')
+def test_contourf_antialiasing():
+    x = np.arange(1, 6)
+    y = x.reshape(-1, 1)
+    data = (x * y).astype(float)
+    data[2, 2] = np.nan
+
+    fig, axs = plt.subplots(1, 3, figsize=(5, 2), layout="constrained")
+
+    for i, antialiased in enumerate([None, False, True]):
+        kwargs = {'cmap': 'jet', 'alpha': 0.5}
+        if antialiased is not None:
+            kwargs['antialiased'] = antialiased
+
+        axs[i].contourf(data, levels=np.arange(1, 25, 1), extend="both", **kwargs)
+
+        axs[i].set_aspect("equal")
+        axs[i].set_axis_off()