Merge pull request #909 from wiredfool/imageenhance

Retain alpha in ImageEnhance operations

PIL/ImageEnhance.py

@@ -47,8 +47,11 @@ class Color(_Enhance):
     """
     def __init__(self, image):
         self.image = image
-        self.degenerate = image.convert("L").convert(image.mode)
+        self.intermediate_mode = 'L'
+        if 'A' in image.getbands():
+            self.intermediate_mode = 'LA'
 
+        self.degenerate = image.convert(self.intermediate_mode).convert(image.mode)
 
 class Contrast(_Enhance):
     """Adjust image contrast.
@@ -62,6 +65,9 @@ class Contrast(_Enhance):
         mean = int(ImageStat.Stat(image.convert("L")).mean[0] + 0.5)
         self.degenerate = Image.new("L", image.size, mean).convert(image.mode)
 
+        if 'A' in image.getbands():
+            self.degenerate.putalpha(image.split()[-1])
+
 
 class Brightness(_Enhance):
     """Adjust image brightness.
@@ -74,6 +80,9 @@ class Brightness(_Enhance):
         self.image = image
         self.degenerate = Image.new(image.mode, image.size, 0)
 
+        if 'A' in image.getbands():
+            self.degenerate.putalpha(image.split()[-1])
+
 
 class Sharpness(_Enhance):
     """Adjust image sharpness.
@@ -85,3 +94,6 @@ class Sharpness(_Enhance):
     def __init__(self, image):
         self.image = image
         self.degenerate = image.filter(ImageFilter.SMOOTH)
+
+        if 'A' in image.getbands():
+            self.degenerate.putalpha(image.split()[-1])

Tests/test_imageenhance.py

@@ -1,4 +1,4 @@
-from helper import unittest, PillowTestCase, lena
+from helper import unittest, PillowTestCase, hopper
 
 from PIL import Image
 from PIL import ImageEnhance
@@ -10,10 +10,10 @@ class TestImageEnhance(PillowTestCase):
 
         # FIXME: assert_image
         # Implicit asserts no exception:
-        ImageEnhance.Color(lena()).enhance(0.5)
-        ImageEnhance.Contrast(lena()).enhance(0.5)
-        ImageEnhance.Brightness(lena()).enhance(0.5)
-        ImageEnhance.Sharpness(lena()).enhance(0.5)
+        ImageEnhance.Color(hopper()).enhance(0.5)
+        ImageEnhance.Contrast(hopper()).enhance(0.5)
+        ImageEnhance.Brightness(hopper()).enhance(0.5)
+        ImageEnhance.Sharpness(hopper()).enhance(0.5)
 
     def test_crash(self):
 
@@ -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()