Improve handling of alpha when saving to jpeg.

anntzer · anntzer · commit 757fe20df5d9 · 2019-10-19T19:46:39.000+02:00
We can compose the figure facecolor against a white background before
rendering the image, rather than after.  This saves an image composition
step.
diff --git a/lib/matplotlib/backends/backend_agg.py b/lib/matplotlib/backends/backend_agg.py
@@ -563,23 +563,25 @@ def print_jpg(self, filename_or_obj, *args, dryrun=False, pil_kwargs=None,
             `PIL.Image.save` when saving the figure.  These take precedence
             over *quality*, *optimize* and *progressive*.
         """
-        FigureCanvasAgg.draw(self)
+        # Remove transparency by alpha-blending on an assumed white background.
+        r, g, b, a = mcolors.to_rgba(self.figure.get_facecolor())
+        try:
+            self.figure.set_facecolor(a * np.array([r, g, b]) + 1 - a)
+            FigureCanvasAgg.draw(self)
+        finally:
+            self.figure.set_facecolor((r, g, b, a))
         if dryrun:
             return
-        # The image is "pasted" onto a white background image to safely
-        # handle any transparency
-        image = Image.fromarray(np.asarray(self.buffer_rgba()))
-        background = Image.new("RGB", image.size, "white")
-        background.paste(image, image)
         if pil_kwargs is None:
             pil_kwargs = {}
         for k in ["quality", "optimize", "progressive"]:
             if k in kwargs:
                 pil_kwargs.setdefault(k, kwargs[k])
         pil_kwargs.setdefault("quality", rcParams["savefig.jpeg_quality"])
         pil_kwargs.setdefault("dpi", (self.figure.dpi, self.figure.dpi))
-        return background.save(
-            filename_or_obj, format='jpeg', **pil_kwargs)
+        # Drop alpha channel now.
+        return (Image.fromarray(np.asarray(self.buffer_rgba())[..., :3])
+                .save(filename_or_obj, format='jpeg', **pil_kwargs))
 
     print_jpeg = print_jpg
 
