serious test for kernel filters

Tests/test_image_filter.py

@@ -94,6 +94,28 @@ class TestImageFilter(PillowTestCase):
         self.assertEqual(rankfilter.size, 1)
         self.assertEqual(rankfilter.rank, 2)
 
+    def test_consistency_3x3(self):
+        im = Image.open("Tests/images/hopper.bmp")
+        emboss = im.filter(ImageFilter.Kernel((3, 3),
+            (-1, -1,  0,
+             -1,  0,  1,
+              0,  1,  1), .3))
+
+        self.assert_image_equal(
+            emboss, Image.open("Tests/images/hopper_emboss.bmp"))
+
+    def test_consistency_5x5(self):
+        im = Image.open("Tests/images/hopper.bmp")
+        emboss = im.filter(ImageFilter.Kernel((5, 5),
+            (-1, -1, -1, -1,  0,
+             -1, -1, -1,  0,  1,
+             -1, -1,  0,  1,  1,
+             -1,  0,  1,  1,  1,
+              0,  1,  1,  1,  1), 0.3))
+
+        self.assert_image_equal(
+            emboss, Image.open("Tests/images/hopper_emboss_more.bmp"))
+
 
 if __name__ == '__main__':
     unittest.main()

libImaging/Filter.c

@@ -92,6 +92,9 @@ ImagingFilter(Imaging im, int xsize, int ysize, const FLOAT32* kernel,
     if (!imOut)
         return NULL;
 
+    // Add one time for rounding
+    offset += 0.5;
+
     /* brute force kernel implementations */
 #define KERNEL3x3(image, kernel, d) ( \
     (int) image[y+1][x-d] * kernel[0] + \