Cairo backend: Fix alpha render of collections

joh · joh · commit 325e698384b4 · 2018-11-08T14:59:17.000+01:00
Fixes #12773
diff --git a/lib/matplotlib/backends/backend_cairo.py b/lib/matplotlib/backends/backend_cairo.py
@@ -303,14 +303,14 @@ def _draw_paths():
             gc_vars, rgb_fc = reuse_key
             gc = copy.copy(gc0)
             # We actually need to call the setters to reset the internal state.
-            vars(gc).update(gc_vars)
             for k, v in gc_vars.items():
                 if k == "_linestyle":  # Deprecated, no effect.
                     continue
                 try:
                     getattr(gc, "set" + k)(v)
                 except (AttributeError, TypeError) as e:
                     pass
+            vars(gc).update(gc_vars)
             gc.ctx.new_path()
             paths, transforms = zip(*grouped_draw)
             grouped_draw.clear()
@@ -327,7 +327,11 @@ def _draw_paths():
                          .translate(xo, yo - self.height).scale(1, -1))
             # rgb_fc could be a ndarray, for which equality is elementwise.
             new_key = vars(gc0), tuple(rgb_fc) if rgb_fc is not None else None
-            if new_key == reuse_key:
+            # don't do grouped draw when doing alpha blending
+            alpha_blending = gc0.get_forced_alpha() and gc0.get_alpha() < 1.0
+            if not alpha_blending and rgb_fc is not None:
+                alpha_blending = len(rgb_fc) == 4 and rgb_fc[-1] < 1.0
+            if new_key == reuse_key and not alpha_blending:
                 grouped_draw.append((path, transform))
             else:
                 _draw_paths()
