Add extended box blur.

PIL/ImageOps.py

@@ -20,6 +20,7 @@
 from PIL import Image
 from PIL._util import isStringType
 import operator
+import math
 from functools import reduce
 
 
@@ -458,3 +459,18 @@ def box_blur(image, radius):
     image.load()
 
     return image._new(image.im.box_blur(radius))
+
+
+def extended_box_blur(image, radius, n=3):
+    sigma2 = float(radius) * radius / n
+    # http://www.mia.uni-saarland.de/Publications/gwosdek-ssvm11.pdf
+    # [7] Box length.
+    L = math.sqrt(12.0 * sigma2 + 1.0)
+    # [11] Integer part of box radius.
+    l = math.floor((L - 1.0) / 2.0)
+    # [14], [Fig. 2] Fractional part of box radius.
+    a = (2 * l + 1) * (l * (l + 1) - 3 * sigma2)
+    a /= 6 * (sigma2 - (l + 1) * (l + 1))
+
+    image.load()
+    return image._new(image.im.box_blur(l + a, n))

Tests/test_box_blur.py

@@ -13,14 +13,43 @@ sample.putdata(sum([
 ], []))
 
 
+class ImageMock(object):
+    def __init__(self):
+        self.im = self
+
+    def load(self):
+        pass
+
+    def _new(self, im):
+        return im
+
+    def box_blur(self, radius, n):
+        return radius, n
+
+
 class TestBoxBlurApi(PillowTestCase):
 
-    def test_imageops(self):
+    def test_imageops_box_blur(self):
         i = ImageOps.box_blur(sample, 1)
         self.assertEqual(i.mode, sample.mode)
         self.assertEqual(i.size, sample.size)
         self.assertIsInstance(i, Image.Image)
 
+    def test_imageops_extended_box_blur(self):
+        i = ImageOps.extended_box_blur(sample, 1)
+        self.assertEqual(i.mode, sample.mode)
+        self.assertEqual(i.size, sample.size)
+        self.assertIsInstance(i, Image.Image)
+
+    def test_extended_box_blur_radius(self):
+        mock = ImageMock()
+        self.assertEqual((0.25, 3), ImageOps.extended_box_blur(mock, 1))
+        self.assertEqual((0.25, 3), ImageOps.extended_box_blur(mock, 1, 3))
+        self.assertAlmostEqual(ImageOps.extended_box_blur(mock, .5, 3)[0],
+                               0.0455, delta=0.0001)
+        self.assertAlmostEqual(ImageOps.extended_box_blur(mock, 35, 3)[0],
+                               34.49, delta=0.01)
+
 
 class TestBoxBlur(PillowTestCase):