BUG Fix Sobel filtering on newer numpy versions

Some slight change in recent numpy versions triggered dodgy code in
sobel(). Curiously enough, Matlab seems to also not behave consistently
across different versions and output very different results with minor
differences in the input.

ChangeLog

@@ -4,6 +4,7 @@ Version 1.2.1+
 	* Allow floating point inputs to ``cwatershed()``
 	* Correctly check for float16 & float128 inputs
 	* Make sobel into a pure function (i.e., do not normalize its input)
+	* Fix sobel filtering
 
 Version 1.2.1 2014-07-21 by luispedro
 	* Explicitly set numpy.include_dirs() in setup.py [patch by Andrew

mahotas/edge.py

@@ -1,11 +1,12 @@
 # -*- coding: utf-8 -*-
-# Copyright (C) 2008-2013, Luis Pedro Coelho <luis@luispedro.org>
+# Copyright (C) 2008-2014, Luis Pedro Coelho <luis@luispedro.org>
 # vim: set ts=4 sts=4 sw=4 expandtab smartindent:
 # 
 # License: MIT (see COPYING file)
 
 from __future__ import division
 import numpy as np
+import mahotas as mh
 from . import convolve
 
 _hsobel_filter = np.array([
@@ -63,12 +64,7 @@ def sobel(img, just_filter=False):
     if just_filter:
         return filtered
     thresh = 2*np.sqrt(filtered.mean())
-    filtered *= (np.sqrt(filtered) > thresh)
-
-    r,c = filtered.shape
-    x = (filtered > np.hstack((np.zeros((r,1)),filtered[:,:-1]))) & (filtered > np.hstack((filtered[:,1:], np.zeros((r,1)))))
-    y = (filtered > np.vstack((np.zeros(c),filtered[:-1,:]))) & (filtered > np.vstack((filtered[1:,:], np.zeros(c))))
-    return x | y
+    return mh.regmax(filtered) * (np.sqrt(filtered) > thresh)
 
 
 

mahotas/tests/test_edge.py

@@ -47,13 +47,12 @@ def test_sobel_pure():
 
 def test_find_edge():
     f = np.zeros((32,48))
-    f[:,16:] = 255
+    f[:,16:] = 1
     f = mh.gaussian_filter(f,4)
     fs = mh.sobel(f)
     assert np.all(fs[:,15] > 0)
 
 @raises(ValueError)
 def test_3d_error():
-    import mahotas as mh
     f = np.zeros((32,16,3))
     sobel(f)