DOC: add a blitting tutorial

doc/api/animation_api.rst

@@ -55,7 +55,7 @@ performance), to be non-blocking, not repeatedly start/stop the GUI
 event loop, handle repeats, multiple animated axes, and easily save
 the animation to a movie file.
 
-'Blitting' is a `old technique
+'Blitting' is a `standard technique
 <https://en.wikipedia.org/wiki/Bit_blit>`__ in computer graphics.  The
 general gist is to take an existing bit map (in our case a mostly
 rasterized figure) and then 'blit' one more artist on top.  Thus, by

lib/matplotlib/backend_bases.py

@@ -1671,6 +1671,7 @@ class FigureCanvasBase:
 
     @cbook._classproperty
     def supports_blit(cls):
+        """If this Canvas sub-class supports blitting."""
         return (hasattr(cls, "copy_from_bbox")
                 and hasattr(cls, "restore_region"))
 

tutorials/advanced/blitting.py

@@ -0,0 +1,224 @@
+"""
+=================
+Blitting tutorial
+=================
+
+'Blitting' is a `standard technique
+<https://en.wikipedia.org/wiki/Bit_blit>`__ in raster graphics that,
+in the context of Matplotlib, can be used to (drastically) improve
+performance of interactive figures. For example, the
+:mod:`~.animation` and :mod:`~.widgets` modules use blitting
+internally. Here, we demonstrate how to implement your own blitting, outside
+of these classes.
+
+The source of the performance gains is simply not re-doing work we do
+not have to.  If the limits of an Axes have not changed, then there is
+no need to re-draw all of the ticks and tick-labels (particularly
+because text is one of the more expensive things to render).
+
+The procedure to save our work is roughly:
+
+- draw the figure, but exclude any artists marked as 'animated'
+- save a copy of the RBGA buffer
+
+In the future, to update the 'animated' artists we
+
+- restore our copy of the RGBA buffer
+- redraw only the animated artists
+- show the resulting image on the screen
+
+thus saving us from having to re-draw everything which is _not_
+animated.  One consequence of this procedure is that your animated
+artists are always drawn at a higher z-order than the static artists.
+
+Not all backends support blitting.  You can check if a given canvas does via
+the `.FigureCanvasBase.supports_blit` property.
+
+.. warning::
+
+   This code does not work with the OSX backend (but does work with other
+   GUI backends on mac).
+
+Minimal example
+---------------
+
+We can use the `.FigureCanvasAgg` methods
+`~.FigureCanvasAgg.copy_from_bbox` and
+`~.FigureCanvasAgg.restore_region` in conjunction with setting
+``animated=True`` on our artist to implement a minimal example that
+uses blitting to accelerate rendering
+
+"""
+
+import matplotlib.pyplot as plt
+import numpy as np
+
+x = np.linspace(0, 2 * np.pi, 100)
+
+fig, ax = plt.subplots()
+
+# animated=True tells matplotlib to only draw the artist when we
+# explicitly request it
+(ln,) = ax.plot(x, np.sin(x), animated=True)
+
+# make sure the window is raised, but the script keeps going
+plt.show(block=False)
+
+# stop to admire our empty window axes and ensure it is rendered at
+# least once.
+#
+# We need to fully draw the figure at its final size on the screen
+# before we continue on so that :
+#  a) we have the correctly sized and drawn background to grab
+#  b) we have a cached renderer so that ``ax.draw_artist`` works
+# so we spin the event loop to let the backend process any pending operations
+plt.pause(0.1)
+
+# get copy of entire figure (everything inside fig.bbox) sans animated artist
+bg = fig.canvas.copy_from_bbox(fig.bbox)
+# draw the animated artist, this uses a cached renderer
+ax.draw_artist(ln)
+# show the result to the screen, this pushes the updated RGBA buffer from the
+# renderer to the GUI framework so you can see it
+fig.canvas.blit(fig.bbox)
+
+for j in range(100):
+    # reset the background back in the canvas state, screen unchanged
+    fig.canvas.restore_region(bg)
+    # update the artist, neither the canvas state nor the screen have changed
+    ln.set_ydata(np.sin(x + (j / 100) * np.pi))
+    # re-render the artist, updating the canvas state, but not the screen
+    ax.draw_artist(ln)
+    # copy the image to the GUI state, but screen might not changed yet
+    fig.canvas.blit(fig.bbox)
+    # flush any pending GUI events, re-painting the screen if needed
+    fig.canvas.flush_events()
+    # you can put a pause in if you want to slow things down
+    # plt.pause(.1)
+
+###############################################################################
+# This example works and shows a simple animation, however because we
+# are only grabbing the background once, if the size of the figure in
+# pixels changes (due to either the size or dpi of the figure
+# changing) , the background will be invalid and result in incorrect
+# (but sometimes cool looking!) images.  There is also a global
+# variable and a fair amount of boiler plate which suggests we should
+# wrap this in a class.
+#
+# Class-based example
+# -------------------
+#
+# We can use a class to encapsulate the boilerplate logic and state of
+# restoring the background, drawing the artists, and then blitting the
+# result to the screen.  Additionally, we can use the ``'draw_event'``
+# callback to capture a new background whenever a full re-draw
+# happens to handle resizes correctly.
+
+
+class BlitManager:
+    def __init__(self, canvas, animated_artists=()):
+        """
+        Parameters
+        ----------
+        canvas : FigureCanvasAgg
+            The canvas to work with, this only works for sub-classes of the Agg
+            canvas which have the `~FigureCanvasAgg.copy_from_bbox` and
+            `~FigureCanvasAgg.restore_region` methods.
+
+        animated_artists : Iterable[Artist]
+            List of the artists to manage
+        """
+        self.canvas = canvas
+        self._bg = None
+        self._artists = []
+
+        for a in animated_artists:
+            self.add_artist(a)
+        # grab the background on every draw
+        self.cid = canvas.mpl_connect("draw_event", self.on_draw)
+
+    def on_draw(self, event):
+        """Callback to register with 'draw_event'."""
+        cv = self.canvas
+        if event is not None:
+            if event.canvas != cv:
+                raise RuntimeError
+        self._bg = cv.copy_from_bbox(cv.figure.bbox)
+        self._draw_animated()
+
+    def add_artist(self, art):
+        """
+        Add an artist to be managed.
+
+        Parameters
+        ----------
+        art : Artist
+
+            The artist to be added.  Will be set to 'animated' (just
+            to be safe).  *art* must be in the figure associated with
+            the canvas this class is managing.
+
+        """
+        if art.figure != self.canvas.figure:
+            raise RuntimeError
+        art.set_animated(True)
+        self._artists.append(art)
+
+    def _draw_animated(self):
+        """Draw all of the animated artists."""
+        fig = self.canvas.figure
+        for a in self._artists:
+            fig.draw_artist(a)
+
+    def update(self):
+        """Update the screen with animated artists."""
+        cv = self.canvas
+        fig = cv.figure
+        # paranoia in case we missed the draw event,
+        if self._bg is None:
+            self.on_draw(None)
+        else:
+            # restore the background
+            cv.restore_region(self._bg)
+            # draw all of the animated artists
+            self._draw_animated()
+            # update the GUI state
+            cv.blit(fig.bbox)
+        # let the GUI event loop process anything it has to do
+        cv.flush_events()
+
+
+###############################################################################
+# Here is how we would use our class.  This is a slightly more complicated
+# example than the first case as we add a text frame counter as well.
+
+# make a new figure
+fig, ax = plt.subplots()
+# add a line
+(ln,) = ax.plot(x, np.sin(x), animated=True)
+# add a frame number
+fr_number = ax.annotate(
+    "0",
+    (0, 1),
+    xycoords="axes fraction",
+    xytext=(10, -10),
+    textcoords="offset points",
+    ha="left",
+    va="top",
+    animated=True,
+)
+bm = BlitManager(fig.canvas, [ln, fr_number])
+# make sure our window is on the screen and drawn
+plt.show(block=False)
+plt.pause(.1)
+
+for j in range(100):
+    # update the artists
+    ln.set_ydata(np.sin(x + (j / 100) * np.pi))
+    fr_number.set_text("frame: {j}".format(j=j))
+    # tell the blitting manager to do it's thing
+    bm.update()
+
+###############################################################################
+# This class does not depend on `.pyplot` and is suitable to embed
+# into larger GUI application.