Fixed the webagg backend to handle figures with transparency.

Author: pelson

lib/matplotlib/backends/backend_nbagg.py

@@ -1,4 +1,8 @@
 """Interactive figures in the IPython notebook"""
+# Note: There is a notebook in
+# lib/matplotlib/backends/web_backend/nbagg_uat.ipynb to help verify
+# that changes made maintain expected behaviour.
+
 from base64 import b64encode
 from contextlib import contextmanager
 import json
@@ -33,8 +37,8 @@ def __call__(self, block=None):
         for manager in managers:
             manager.show()
 
-        if not is_interactive() and manager in Gcf._activeQue:
-            Gcf._activeQue.remove(manager)
+            if not is_interactive() and manager in Gcf._activeQue:
+                Gcf._activeQue.remove(manager)
 
 
 show = Show()
@@ -189,19 +193,6 @@ def start_event_loop(self, timeout):
     def stop_event_loop(self):
         FigureCanvasBase.stop_event_loop_default(self)
 
-    def draw(self):
-        renderer = self.get_renderer()
-
-        self._png_is_old = True
-
-        backend_agg.RendererAgg.lock.acquire()
-        try:
-            self.figure.draw(renderer)
-        finally:
-            backend_agg.RendererAgg.lock.release()
-            # Swap the frames
-            self.manager.refresh_all()
-
 
 def new_figure_manager(num, *args, **kwargs):
     """

lib/matplotlib/backends/backend_webagg_core.py

@@ -71,7 +71,7 @@ def show(self):
         show()
 
     def draw(self):
-        renderer = self.get_renderer()
+        renderer = self.get_renderer(cleared=True)
 
         self._png_is_old = True
 
@@ -91,21 +91,25 @@ def get_diff_image(self):
             # The buffer is created as type uint32 so that entire
             # pixels can be compared in one numpy call, rather than
             # needing to compare each plane separately.
-            buff = np.frombuffer(
-                self.get_renderer().buffer_rgba(), dtype=np.uint32)
-            buff.shape = (
-                self._renderer.height, self._renderer.width)
+            renderer = self.get_renderer()
+            buff = np.frombuffer(renderer.buffer_rgba(), dtype=np.uint32)
 
-            if not self._force_full:
-                last_buffer = np.frombuffer(
-                    self._last_renderer.buffer_rgba(), dtype=np.uint32)
-                last_buffer.shape = (
-                    self._renderer.height, self._renderer.width)
+            buff.shape = (renderer.height, renderer.width)
+
+            # If any pixels have transparency, we need to force a full draw
+            # as we cannot overlay new on top of old. 
+            pixels = buff.view(dtype=np.uint8).reshape(buff.shape + (4,))
+            some_transparency = np.any(pixels[:, :, 3] != 255)
+
+            output = buff
+
+            if not self._force_full and not some_transparency:
+                last_buffer = np.frombuffer(self._last_renderer.buffer_rgba(),
+                                            dtype=np.uint32)
+                last_buffer.shape = (renderer.height, renderer.width)
 
                 diff = buff != last_buffer
                 output = np.where(diff, buff, 0)
-            else:
-                output = buff
 
             # Clear out the PNG data buffer rather than recreating it
             # each time.  This reduces the number of memory
@@ -122,7 +126,7 @@ def get_diff_image(self):
 
             # Swap the renderer frames
             self._renderer, self._last_renderer = (
-                self._last_renderer, self._renderer)
+                self._last_renderer, renderer)
             self._force_full = False
             self._png_is_old = False
         return self._png_buffer.getvalue()
@@ -147,6 +151,9 @@ def get_renderer(self, cleared=None):
                 w, h, self.figure.dpi)
             self._lastKey = key
 
+        elif cleared:
+            self._renderer.clear()
+
         return self._renderer
 
     def handle_event(self, event):

lib/matplotlib/backends/web_backend/mpl.js

@@ -60,6 +60,7 @@ mpl.figure = function(figure_id, websocket, ondownload, parent_element) {
         }
 
     this.imageObj.onload = function() {
+            fig.context.clearRect(0, 0, fig.canvas.width, fig.canvas.height);
             fig.context.drawImage(fig.imageObj, 0, 0);
             fig.waiting = false;
         };
@@ -322,6 +323,7 @@ mpl.figure.prototype._make_on_message_function = function(fig) {
                 (window.URL || window.webkitURL).revokeObjectURL(
                     fig.imageObj.src);
             }
+
             fig.imageObj.src = (window.URL || window.webkitURL).createObjectURL(
                 evt.data);
             fig.updated_canvas_event();

lib/matplotlib/backends/web_backend/nbagg_uat.ipynb

@@ -1,7 +1,7 @@
 {
  "metadata": {
   "name": "",
-  "signature": "sha256:34c3a5a9d82d5a3c433f0c03f0717d43e1d14eaf78f88a64f094d104fa636ddc"
+  "signature": "sha256:1d491e506b54b126f6971897d3249a9e6f96f9d50bf4e4ba8d179c6b7b1aefa8"
  },
  "nbformat": 3,
  "nbformat_minor": 0,
@@ -24,7 +24,6 @@
       "import matplotlib\n",
       "reload(matplotlib)\n",
       "\n",
-      "\n",
       "matplotlib.use('nbagg')\n",
       "\n",
       "import matplotlib.backends.backend_nbagg\n",
@@ -47,6 +46,9 @@
      "cell_type": "code",
      "collapsed": false,
      "input": [
+      "import matplotlib.backends.backend_webagg_core\n",
+      "reload(matplotlib.backends.backend_webagg_core)\n",
+      "\n",
       "import matplotlib.pyplot as plt\n",
       "plt.interactive(False)\n",
       "\n",
@@ -382,14 +384,6 @@
      "language": "python",
      "metadata": {},
      "outputs": []
-    },
-    {
-     "cell_type": "code",
-     "collapsed": false,
-     "input": [],
-     "language": "python",
-     "metadata": {},
-     "outputs": []
     }
    ],
    "metadata": {}