Fix #5302: Proper alpha-blending for jpeg

mdboom · mdboom · commit bdea63d1c852 · 2015-10-26T14:12:47.000-04:00
diff --git a/lib/matplotlib/backends/backend_agg.py b/lib/matplotlib/backends/backend_agg.py
@@ -551,7 +551,6 @@ def print_to_buffer(self):
         return result
 
     if _has_pil:
-
         # add JPEG support
         def print_jpg(self, filename_or_obj, *args, **kwargs):
             """
@@ -573,14 +572,18 @@ def print_jpg(self, filename_or_obj, *args, **kwargs):
             buf, size = self.print_to_buffer()
             if kwargs.pop("dryrun", False):
                 return
+            # The image is "pasted" onto a white background image to safely
+            # handle any transparency
             image = Image.frombuffer('RGBA', size, buf, 'raw', 'RGBA', 0, 1)
+            background = Image.new('RGB', size, (255, 255, 255))
+            background.paste(image, image)
             options = restrict_dict(kwargs, ['quality', 'optimize',
                                              'progressive'])
 
             if 'quality' not in options:
                 options['quality'] = rcParams['savefig.jpeg_quality']
 
-            return image.save(filename_or_obj, format='jpeg', **options)
+            return background.save(filename_or_obj, format='jpeg', **options)
         print_jpeg = print_jpg
 
         # add TIFF support
diff --git a/lib/matplotlib/tests/test_image.py b/lib/matplotlib/tests/test_image.py
@@ -453,6 +453,28 @@ def test_nonuniformimage_setnorm():
     im = NonUniformImage(ax)
     im.set_norm(plt.Normalize())
 
+@knownfailureif(not HAS_PIL)
+@cleanup
+def test_jpeg_alpha():
+    plt.figure(figsize=(1, 1), dpi=300)
+    # Create an image that is all black, with a gradient from 0-1 in
+    # the alpha channel from left to right.
+    im = np.zeros((300, 300, 4), dtype=np.float)
+    im[..., 3] = np.linspace(0.0, 1.0, 300)
+
+    plt.figimage(im)
+
+    buff = io.BytesIO()
+    plt.savefig(buff, transparent=True, format='jpg', dpi=300)
+
+    buff.seek(0)
+    image = Image.open(buff)
+
+    # If this fails, there will be only one color (all black). If this
+    # is working, we should have all 256 shades of grey represented.
+    assert len(image.getcolors(256)) == 256
+
+
 if __name__=='__main__':
     import nose
     nose.runmodule(argv=['-s','--with-doctest'], exit=False)
