Merge consecutive rasterizations

samtygier · samtygier · commit 3ae9d441a401 · 2020-04-16T21:09:14.000+01:00
In vector output it is possible to flag artists to be rasterized. In many cases with multiple rasterized objects there can be significant file size savings by combining the rendered bitmaps into a single bitmap. This achieved by splitting some of the work of the renderers stop_rasterizing() out into a flush_rasterizing() function. This gets called when for a non-rasterized artist, and does the flushing only if needed. It must also be called after every element in the Figure has been drawn to finalize any remaining rasterized elements. This fixes #17149
diff --git a/lib/matplotlib/artist.py b/lib/matplotlib/artist.py
@@ -35,6 +35,8 @@ def draw_wrapper(artist, renderer, *args, **kwargs):
         try:
             if artist.get_rasterized():
                 renderer.start_rasterizing()
+            else:
+                renderer.flush_rasterizing()
             if artist.get_agg_filter() is not None:
                 renderer.start_filter()
 
diff --git a/lib/matplotlib/backend_bases.py b/lib/matplotlib/backend_bases.py
@@ -685,6 +685,9 @@ def stop_rasterizing(self):
         Used by `.MixedModeRenderer`.
         """
 
+    def flush_rasterizing(self):
+        pass
+
     def start_filter(self):
         """
         Switch to a temporary renderer for image filtering effects.
diff --git a/lib/matplotlib/backends/backend_mixed.py b/lib/matplotlib/backends/backend_mixed.py
@@ -53,6 +53,7 @@ def __init__(self, figure, width, height, dpi, vector_renderer,
 
         self._raster_renderer = None
         self._rasterizing = 0
+        self._rasterizing_unflushed = False
 
         # A reference to the figure is needed as we need to change
         # the figure dpi before and after the rasterization. Although
@@ -84,11 +85,12 @@ def start_rasterizing(self):
             r = process_figure_for_rasterizing(self.figure,
                                                self._bbox_inches_restore)
             self._bbox_inches_restore = r
-        if self._rasterizing == 0:
+        if self._rasterizing == 0 and not self._rasterizing_unflushed:
             self._raster_renderer = self._raster_renderer_class(
                 self._width*self.dpi, self._height*self.dpi, self.dpi)
             self._renderer = self._raster_renderer
         self._rasterizing += 1
+        self._rasterizing_unflushed = True
 
     def stop_rasterizing(self):
         """
@@ -101,7 +103,15 @@ def stop_rasterizing(self):
         "raster" mode is exited.
         """
         self._rasterizing -= 1
-        if self._rasterizing == 0:
+
+        if self._bbox_inches_restore:  # when tight bbox is used
+            r = process_figure_for_rasterizing(self.figure,
+                                               self._bbox_inches_restore,
+                                               self._figdpi)
+            self._bbox_inches_restore = r
+
+    def flush_rasterizing(self):
+        if self._rasterizing == 0 and self._rasterizing_unflushed:
             self._renderer = self._vector_renderer
 
             height = self._height * self.dpi
@@ -122,12 +132,7 @@ def stop_rasterizing(self):
                     image)
             self._raster_renderer = None
             self._rasterizing = False
+            self._rasterizing_unflushed = False
 
             # restore the figure dpi.
             self.figure.set_dpi(self._figdpi)
-
-        if self._bbox_inches_restore:  # when tight bbox is used
-            r = process_figure_for_rasterizing(self.figure,
-                                               self._bbox_inches_restore,
-                                               self._figdpi)
-            self._bbox_inches_restore = r
diff --git a/lib/matplotlib/figure.py b/lib/matplotlib/figure.py
@@ -1730,7 +1730,7 @@ def draw(self, renderer):
             self.patch.draw(renderer)
             mimage._draw_list_compositing_images(
                 renderer, self, artists, self.suppressComposite)
-
+            renderer.flush_rasterizing()
             renderer.close_group('figure')
         finally:
             self.stale = False
