Fix for alpha preservation in ImageEnhance, #899

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 @@ def __init__(self, image):
         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 @@ def __init__(self, image):
         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])