Test for alpha preservation in ImageEnhance, #899

Tests/test_imageenhance.py

@@ -22,6 +22,34 @@ class TestImageEnhance(PillowTestCase):
         ImageEnhance.Sharpness(im).enhance(0.5)
 
 
+    def _half_transparent_image(self):
+        # returns an image, half transparent, half solid
+        im = hopper('RGB')
+        
+        transparent = Image.new('L', im.size, 0)
+        solid = Image.new('L', (im.size[0]//2, im.size[1]), 255)
+        transparent.paste(solid, (0,0))
+        im.putalpha(transparent)
+
+        return im
+
+    def _check_alpha(self,im, original, op, amount):
+        self.assertEqual(im.getbands(), original.getbands())
+        self.assert_image_equal(im.split()[-1], original.split()[-1],
+                                "Diff on %s: %s" % (op, amount))
+    
+    def test_alpha(self):
+        # Issue https://github.com/python-pillow/Pillow/issues/899
+        # Is alpha preserved through image enhancement?
+
+        original = self._half_transparent_image()
+
+        for op in ['Color', 'Brightness', 'Contrast', 'Sharpness']:
+            for amount in [0,0.5,1.0]:
+                self._check_alpha(getattr(ImageEnhance,op)(original).enhance(amount),
+                                  original, op, amount)
+                
+
 if __name__ == '__main__':
     unittest.main()