Drop alpha channel when saving comparison failure diff image.

anntzer · anntzer · commit 377dba445e24 · 2018-04-18T09:10:19.000-07:00
We drop (for the better or the worse) the alpha channel when *comparing*
the images (in `compare_images`), but not when saving; thus, a
comparison failure where additionally the reference has no alpha but
the generated image has an alpha channel would so far raise a spurious
"image sizes do not match" when trying to save the diff image, rather
than actually saving the diff image as expected.

Noted while running the Matplotlib test suite with mplcairo.
diff --git a/lib/matplotlib/testing/compare.py b/lib/matplotlib/testing/compare.py
@@ -422,8 +422,9 @@ def compare_images(expected, actual, tol, in_decorator=False):
 
 
 def save_diff_image(expected, actual, output):
-    expectedImage = _png.read_png(expected)
-    actualImage = _png.read_png(actual)
+    # Drop alpha channels, similarly to compare_images.
+    expectedImage = _png.read_png(expected)[..., :3]
+    actualImage = _png.read_png(actual)[..., :3]
     actualImage, expectedImage = crop_to_same(
         actual, actualImage, expected, expectedImage)
     expectedImage = np.array(expectedImage).astype(float)
