DOC: copy edit tutorial

tacaswell · QuLogic · brunobeltran · tacaswell · commit e243ecc8666f · 2020-06-15T16:02:42.000-04:00
Co-authored-by: Elliott Sales de Andrade &lt;quantum.analyst@gmail.com&gt;
Co-authored-by: Bruno Beltran &lt;brunobeltran0@gmail.com&gt;
Co-authored-by: Tim Hoffmann&lt;2836374+timhoffm@users.noreply.github.com&gt;
diff --git a/lib/matplotlib/backend_bases.py b/lib/matplotlib/backend_bases.py
@@ -1604,6 +1604,7 @@ 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"))
 
diff --git a/tutorials/intermediate/blitting.py b/tutorials/intermediate/blitting.py
@@ -1,24 +1,25 @@
 """
 =================
-Blitting Tutorial
+Blitting tutorial
 =================
 
 'Blitting' is a `standard technique
-<https://en.wikipedia.org/wiki/Bit_blit>`__ in computer graphics that
-in the context of matplotlib can be used to (drastically) improve
-performance of interactive figures.  It is used internally by the
-:mod:`~.animation` and :mod:`~.widgets` modules for this reason.
+<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.  For example, if the limits of an Axes have not changed,
-then there is no reason we should re-draw all of the ticks and
-tick-labels (particularly because text is one of the more expensive
-things to render).
+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 an artists marked as 'animated'
-- save a copy of the Agg RBGA buffer
+- 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
 
@@ -27,50 +28,76 @@
 - show the resulting image on the screen
 
 thus saving us from having to re-draw everything which is _not_
-animated.
+animated.  One consequence of this procedure is that your animated
+artists are always drawn at a higher z-order than the static artists.
 
-Simple Example
---------------
+Not all backends support blitting.  You can check if a given canvas does via
+the `.FigureCanvasBase.supports_blit` property.
+
+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
 
-We can implement this via methods on `.CanvasAgg` and setting
-``animated=True`` on our artist.
 """
 
 import matplotlib.pyplot as plt
 import numpy as np
 
-x = np.linspace(0, 2*np.pi, 100)
+x = np.linspace(0, 2 * np.pi, 100)
 
 fig, ax = plt.subplots()
-# animated=True makes the artist be excluded from normal draw tree
-ln, = ax.plot(x, np.sin(x), animated=True)
 
-# stop to admire our empty window axes and ensure it is drawn
-plt.pause(.1)
+# 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)
 
-# save a copy of the image sans animated artist
+# 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
+# draw the animated artist, this uses a cached renderer
 ax.draw_artist(ln)
-# show the result to the screen
+# 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):
-    # put the un-changed background back
+    # reset the background back in the canvas state, screen unchanged
     fig.canvas.restore_region(bg)
-    # update the artist.
+    # 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
+    # re-render the artist, updating the canvas state, but not the screen
     ax.draw_artist(ln)
-    # copy the result to the screen
+    # 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 dpi of the figure change, the
-# background will be invalid and result in incorrect images.  There is also a
-# global variable and a fair amount of boiler plate which suggests we should
+# 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
@@ -80,21 +107,20 @@
 # 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.
+# happens to handle resizes correctly.
 
 
 class BlitManager:
-
-    def __init__(self, canvas, animated_artists):
+    def __init__(self, canvas, animated_artists=()):
         """
         Parameters
         ----------
-        canvas : CanvasAgg
+        canvas : FigureCanvasAgg
             The canvas to work with, this only works for sub-classes of the Agg
-            canvas which have the `~CanvasAgg.copy_from_bbox` and
-            `~CanvasAgg.restore_region` methods.
+            canvas which have the `~FigureCanvasAgg.copy_from_bbox` and
+            `~FigureCanvasAgg.restore_region` methods.
 
-        animated_artists : Optional[List[Artist]]
+        animated_artists : Iterable[Artist]
             List of the artists to manage
         """
         self.canvas = canvas
@@ -104,11 +130,10 @@ def __init__(self, canvas, animated_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)
+        self.cid = canvas.mpl_connect("draw_event", self.on_draw)
 
     def on_draw(self, event):
-        """Callback to register with 'draw_event'
-        """
+        """Callback to register with 'draw_event'."""
         cv = self.canvas
         if event is not None:
             if event.canvas != cv:
@@ -117,67 +142,78 @@ def on_draw(self, event):
         self._draw_animated()
 
     def add_artist(self, art):
-        """Add a artist to be managed
+        """
+        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.
+
+            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
-        """
+        """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
-        """
+        """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 old background
+            # restore the background
             cv.restore_region(self._bg)
             # draw all of the animated artists
             self._draw_animated()
-            # update the screen
+            # update the GUI state
             cv.blit(fig.bbox)
         # let the GUI event loop process anything it has to do
         cv.flush_events()
 
 
 ###############################################################################
-# And now use our class.  This is a slightly more complicated example of the
-# first case as we add a text frame counter as well.
+# 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)
+(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)
+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))
+    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.
