Use the mask from the stored array

Author: efiring

lib/matplotlib/collections.py

@@ -2098,8 +2098,10 @@ def _convert_mesh_to_triangles(self, coordinates):
         ], axis=2).reshape((-1, 3, 2))
 
         c = self.get_facecolor().reshape((*coordinates.shape[:2], 4))
-        mask = c[..., 3] == 0
-        c[mask, 3] = np.nan
+        z = self.get_array()
+        mask = z.mask if np.ma.is_masked(z) else None
+        if mask is not None:
+            c[mask, 3] = np.nan
         c_a = c[:-1, :-1]
         c_b = c[:-1, 1:]
         c_c = c[1:, 1:]
@@ -2111,8 +2113,8 @@ def _convert_mesh_to_triangles(self, coordinates):
             c_c, c_d, c_center,
             c_d, c_a, c_center,
         ], axis=2).reshape((-1, 3, 4))
-        mask = np.isnan(colors[..., 2, 3])
-        return triangles[~mask], colors[~mask]
+        tmask = np.isnan(colors[..., 2, 3])
+        return triangles[~tmask], colors[~tmask]
 
     @artist.allow_rasterization
     def draw(self, renderer):