matplotlib · jklymak · Mar 25, 2019 · Mar 18, 2019 · Mar 19, 2019
diff --git a/lib/matplotlib/backends/backend_mixed.py b/lib/matplotlib/backends/backend_mixed.py
@@ -62,27 +62,16 @@ def __init__(self, figure, width, height, dpi, vector_renderer,
 
         self._bbox_inches_restore = bbox_inches_restore
 
-        self._set_current_renderer(vector_renderer)
-
-    _methods = """
-        close_group draw_image draw_markers draw_path
-        draw_path_collection draw_quad_mesh draw_tex draw_text
-        finalize flipy get_canvas_width_height get_image_magnification
-        get_texmanager get_text_width_height_descent new_gc open_group
-        option_image_nocomposite points_to_pixels strip_math
-        start_filter stop_filter draw_gouraud_triangle
-        draw_gouraud_triangles option_scale_image
-        _text2path _get_text_path_transform height width
-        """.split()
-
-    def _set_current_renderer(self, renderer):
-        self._renderer = renderer
-
-        for method in self._methods:
-            if hasattr(renderer, method):
-                setattr(self, method, getattr(renderer, method))
-        renderer.start_rasterizing = self.start_rasterizing
-        renderer.stop_rasterizing = self.stop_rasterizing
+        self._renderer = vector_renderer
+
+    def __getattr__(self, attr):
+        # Proxy everything that hasn't been overridden to the base
+        # renderer. Things that *are* overridden can call methods
+        # on self._renderer directly, but must not cache/store
+        # methods (because things like RendererAgg change their
+        # methods on the fly in order to optimise proxying down
+        # to the underlying C implementation).
+        return getattr(self._renderer, attr)
 
     def start_rasterizing(self):
         """
@@ -105,7 +94,7 @@ def start_rasterizing(self):
         if self._rasterizing == 0:
             self._raster_renderer = self._raster_renderer_class(
                 self._width*self.dpi, self._height*self.dpi, self.dpi)
-            self._set_current_renderer(self._raster_renderer)
+            self._renderer = self._raster_renderer
         self._rasterizing += 1
 
     def stop_rasterizing(self):
@@ -119,7 +108,7 @@ def stop_rasterizing(self):
         """
         self._rasterizing -= 1
         if self._rasterizing == 0:
-            self._set_current_renderer(self._vector_renderer)
+            self._renderer = self._vector_renderer
 
             height = self._height * self.dpi
             buffer, bounds = self._raster_renderer.tostring_rgba_minimized()

diff --git a/lib/matplotlib/tests/baseline_images/test_agg_filter/agg_filter_alpha.pdf b/lib/matplotlib/tests/baseline_images/test_agg_filter/agg_filter_alpha.pdf
diff --git a/lib/matplotlib/tests/baseline_images/test_agg_filter/agg_filter_alpha.png b/lib/matplotlib/tests/baseline_images/test_agg_filter/agg_filter_alpha.png
diff --git a/lib/matplotlib/tests/test_agg_filter.py b/lib/matplotlib/tests/test_agg_filter.py
@@ -0,0 +1,30 @@
+import numpy as np
+
+import matplotlib.pyplot as plt
+from matplotlib.testing.decorators import image_comparison
+
+
+@image_comparison(baseline_images=['agg_filter_alpha'],
+                  extensions=['png', 'pdf'])
+def test_agg_filter_alpha():
+    ax = plt.axes()
+    x, y = np.mgrid[0:7, 0:8]
+    data = x**2 - y**2
+    mesh = ax.pcolormesh(data, cmap='Reds', zorder=5)
+
+    def manual_alpha(im, dpi):
+        im[:, :, 3] *= 0.6
+        print('CALLED')
+        return im, 0, 0
+
+    # Note: Doing alpha like this is not the same as setting alpha on
+    # the mesh itself. Currently meshes are drawn as independent patches,
+    # and we see fine borders around the blocks of color. See the SO
+    # question for an example: https://stackoverflow.com/questions/20678817
+    mesh.set_agg_filter(manual_alpha)
+
+    # Currently we must enable rasterization for this to have an effect in
+    # the PDF backend.
+    mesh.set_rasterized(True)
+
+    ax.plot([0, 4, 7], [1, 3, 8])