Added threshold parameter to ImageDraw.floodfill (#2599)

* added thresh option and test

* fixed up, test works and passes

* Update test_imagedraw.py

* Update test_imagedraw.py

* Update ImageDraw.py

* removed pypy skip decorator from thresh test

* Update ImageDraw.py

PIL/ImageDraw.py

@@ -315,7 +315,7 @@ def getdraw(im=None, hints=None):
     return im, handler
 
 
-def floodfill(image, xy, value, border=None):
+def floodfill(image, xy, value, border=None, thresh=0):
     """
     (experimental) Fills a bounded region with a given color.
 
@@ -326,13 +326,17 @@ def floodfill(image, xy, value, border=None):
         pixels with a color different from the border color.  If not given,
         the region consists of pixels having the same color as the seed
         pixel.
+    :param thresh: Optional threshold value which specifies a maximum 
+        tolerable difference of a pixel value from the 'background' in 
+        order for it to be replaced. Useful for filling regions of non-
+        homogeneous, but similar, colors.
     """
     # based on an implementation by Eric S. Raymond
     pixel = image.load()
     x, y = xy
     try:
         background = pixel[x, y]
-        if background == value:
+        if _color_diff(value, background) <= thresh:
             return  # seed point already has fill color
         pixel[x, y] = value
     except (ValueError, IndexError):
@@ -348,7 +352,7 @@ def floodfill(image, xy, value, border=None):
                     except IndexError:
                         pass
                     else:
-                        if p == background:
+                        if _color_diff(p, background) <= thresh:
                             pixel[s, t] = value
                             newedge.append((s, t))
             edge = newedge
@@ -366,3 +370,10 @@ def floodfill(image, xy, value, border=None):
                             pixel[s, t] = value
                             newedge.append((s, t))
             edge = newedge
+
+  
+def _color_diff(rgb1, rgb2):
+    """
+    Uses 1-norm distance to calculate difference between two rgb values.
+    """
+    return abs(rgb1[0]-rgb2[0]) +  abs(rgb1[1]-rgb2[1]) +  abs(rgb1[2]-rgb2[2])

Tests/test_imagedraw.py

@@ -400,6 +400,26 @@ class TestImageDraw(PillowTestCase):
         self.assert_image_equal(
             im, Image.open("Tests/images/imagedraw_floodfill2.png"))
 
+
+    def test_floodfill_thresh(self):
+        # floodfill() is experimental
+
+        # Arrange
+        im = Image.new("RGB", (W, H))
+        draw = ImageDraw.Draw(im)
+        draw.rectangle(BBOX2, outline="darkgreen", fill="green")
+        centre_point = (int(W/2), int(H/2))
+
+        # Act
+        ImageDraw.floodfill(
+            im, centre_point, ImageColor.getrgb("red"),
+            thresh=30)
+        del draw
+        
+        # Assert
+        self.assert_image_equal(
+            im, Image.open("Tests/images/imagedraw_floodfill2.png"))
+
     def create_base_image_draw(self, size,
                                mode=DEFAULT_MODE,
                                background1=WHITE,