Added Image average_difference

Tests/helper.py

@@ -33,19 +33,6 @@ else:
         pass
 
 
-def convert_to_comparable(a, b):
-    new_a, new_b = a, b
-    if a.mode == 'P':
-        new_a = Image.new('L', a.size)
-        new_b = Image.new('L', b.size)
-        new_a.putdata(a.getdata())
-        new_b.putdata(b.getdata())
-    elif a.mode == 'I;16':
-        new_a = a.convert('I')
-        new_b = b.convert('I')
-    return new_a, new_b
-
-
 class PillowTestCase(unittest.TestCase):
 
     def __init__(self, *args, **kwargs):
@@ -130,14 +117,7 @@ class PillowTestCase(unittest.TestCase):
             a.size, b.size,
             msg or "got size %r, expected %r" % (a.size, b.size))
 
-        a, b = convert_to_comparable(a, b)
-
-        diff = 0
-        for ach, bch in zip(a.split(), b.split()):
-            chdiff = ImageMath.eval("abs(a - b)", a=ach, b=bch).convert('L')
-            diff += sum(i * num for i, num in enumerate(chdiff.histogram()))
-
-        ave_diff = float(diff)/(a.size[0]*a.size[1])
+        ave_diff = a.average_difference(b)
         try:
             self.assertGreaterEqual(
                 epsilon, ave_diff,

Tests/test_image.py

@@ -1,6 +1,6 @@
 from helper import unittest, PillowTestCase, hopper
 
-from PIL import Image
+from PIL import Image, ImageFilter
 from PIL._util import py3
 import os
 
@@ -492,6 +492,27 @@ class TestImage(PillowTestCase):
 
         self.assertEqual(ext_individual, ext_multiple)
 
+    def test_average_difference(self):
+        for mode_a, mode_b in [
+            ("RGB", "RGBA"),
+            ("P", "RGB"),
+            ("I;16", "RGB")
+        ]:
+            im_a = hopper(mode_a)
+            im_b = hopper(mode_b)
+            self.assertEqual(im_a.average_difference(im_b), 0)
+
+        im = hopper()
+        im_resized = im.resize((im.width * 2, im.height * 2))
+        diff = im.average_difference(im_resized)
+        self.assertEqual(diff, 0)
+        self.assertEqual(im_resized.average_difference(im), diff)
+
+        im_blur = im.filter(ImageFilter.BLUR)
+        diff = im.average_difference(im_blur)
+        self.assertNotEqual(diff, 0)
+        self.assertEqual(im_blur.average_difference(im), diff)
+
     def test_remap_palette(self):
         # Test illegal image mode
         im = hopper()

src/PIL/Image.py

@@ -851,6 +851,41 @@ class Image(object):
         """
         pass
 
+    def average_difference(self, im):
+        """
+        Returns the average pixel value difference between this and another
+        image
+
+        :param im: Comparison image
+        """
+        from . import ImageMath
+
+        a, b = self, im
+        if a.mode == 'I;16':
+            a = a.convert('I')
+        if b.mode != a.mode:
+            b = b.convert(a.mode)
+
+        if a.mode == 'P':
+            a_data = a.getdata()
+            b_data = b.getdata()
+            a = new('L', a.size)
+            b = new('L', b.size)
+            a.putdata(a_data)
+            b.putdata(b_data)
+
+        if (a.width * a.height) < (b.width * b.height):
+            b = b.resize(a.size)
+        elif (b.width * b.height) < (a.width * a.height):
+            a = a.resize(b.size)
+
+        diff = 0
+        for ach, bch in zip(a.split(), b.split()):
+            chdiff = ImageMath.eval("abs(a - b)", a=ach, b=bch).convert('L')
+            diff += sum(i * num for i, num in enumerate(chdiff.histogram()))
+
+        return float(diff) / (a.size[0] * a.size[1])
+
     def convert(self, mode=None, matrix=None, dither=None,
                 palette=WEB, colors=256):
         """