Composite against white, not the savefig.facecolor rc, in print_jpeg.

anntzer · anntzer · commit f1677a71ff1c · 2019-09-09T22:35:06.000+02:00
jpeg doesn't support alpha transparency, so we need to paste the image
against some solid background before saving it.  Pasting against the
savefig.facecolor rc, as the code did before, is just wrong: it makes
e.g.
```
plot([1, 2])
rcParams["savefig.facecolor"] = "blue"
savefig("/tmp/foo.jpg", facecolor=(1, 0, 0, .5))
```
result in a purple background (blue + 0.5 red), instead of a red one (as
for png -- the facecolor kwarg should just override the rcParam).
Likewise the test_jpeg_alpha test was just wrong (indeed, trying to run
the same code but saving as png shows that transparent=True should
override the savefig.facecolor rcParam).

Instead, paste the image against a white background, which fixes the
issues (and is tested by the fixed test_jpeg_alpha).
diff --git a/lib/matplotlib/backends/backend_agg.py b/lib/matplotlib/backends/backend_agg.py
@@ -584,12 +584,10 @@ def print_jpg(self, filename_or_obj, *args, dryrun=False,
             FigureCanvasAgg.draw(self)
             if dryrun:
                 return
-            # The image is "pasted" onto a white background image to safely
-            # handle any transparency
+            # The image is pasted onto a white background image to handle
+            # transparency.
             image = Image.fromarray(np.asarray(self.buffer_rgba()))
-            rgba = mcolors.to_rgba(rcParams['savefig.facecolor'])
-            color = tuple([int(x * 255) for x in rgba[:3]])
-            background = Image.new('RGB', image.size, color)
+            background = Image.new('RGB', image.size, "white")
             background.paste(image, image)
             if pil_kwargs is None:
                 pil_kwargs = {}
diff --git a/lib/matplotlib/tests/test_image.py b/lib/matplotlib/tests/test_image.py
@@ -674,8 +674,7 @@ def test_jpeg_alpha():
     plt.figimage(im)
 
     buff = io.BytesIO()
-    with rc_context({'savefig.facecolor': 'red'}):
-        plt.savefig(buff, transparent=True, format='jpg', dpi=300)
+    plt.savefig(buff, facecolor="red", format='jpg', dpi=300)
 
     buff.seek(0)
     image = Image.open(buff)
