Small improvements to canvasagg example.

anntzer · anntzer · commit 3124bdaf714a · 2021-06-07T07:19:49.000+02:00
In particular, show how Pillow can be used to save images in formats not
directly supported by Matplotlib.  Also directly convert the buffer to
an array (with only brief, in-passing mention of memoryviews), as
memoryviews are rather technical.
diff --git a/examples/user_interfaces/canvasagg.py b/examples/user_interfaces/canvasagg.py
@@ -14,9 +14,12 @@
     the backend to "Agg" would be sufficient.
 
 In this example, we show how to save the contents of the agg canvas to a file,
-and how to extract them to a string, which can in turn be passed off to PIL or
-put in a numpy array.  The latter functionality allows e.g. to use Matplotlib
-inside a cgi-script *without* needing to write a figure to disk.
+and how to extract them to a numpy array, which can in turn be passed off
+to Pillow_.  The latter functionality allows e.g. to use Matplotlib inside a
+cgi-script *without* needing to write a figure to disk, and to write images in
+any format supported by Pillow.
+
+.. _Pillow: https://pillow.readthedocs.io/
 """
 
 from matplotlib.backends.backend_agg import FigureCanvasAgg
@@ -39,13 +42,14 @@
 # etc.).
 fig.savefig("test.png")
 
-# Option 2: Retrieve a view on the renderer buffer...
+# Option 2: Retrieve a memoryview on the renderer buffer, and convert it to a
+# numpy array.
 canvas.draw()
-buf = canvas.buffer_rgba()
-# ... convert to a NumPy array ...
-X = np.asarray(buf)
+rgba = np.asarray(canvas.buffer_rgba())
 # ... and pass it to PIL.
-im = Image.fromarray(X)
+im = Image.fromarray(rgba)
+# This image can then be saved to any format supported by Pillow, e.g.:
+im.save("test.bmp")
 
 # Uncomment this line to display the image using ImageMagick's `display` tool.
 # im.show()
