Fix corner_subpix function for corners near border

skimage/feature/corner.py

@@ -2,7 +2,7 @@ import numpy as np
 from scipy import ndimage
 from scipy import stats
 from skimage.color import rgb2grey
-from skimage.util import img_as_float
+from skimage.util import img_as_float, pad
 from skimage.feature import peak_local_max
 
 
@@ -356,6 +356,11 @@ def corner_subpix(image, corners, window_size=11, alpha=0.99):
     # window extent in one direction
     wext = (window_size - 1) / 2
 
+    image = pad(image, pad_width=wext, mode='constant', constant_values=0)
+
+    # add pad width, make sure to not modify the input values in-place
+    corners = corners + wext
+
     # normal equation arrays
     N_dot = np.zeros((2, 2), dtype=np.double)
     N_edge = np.zeros((2, 2), dtype=np.double)
@@ -449,6 +454,9 @@ def corner_subpix(image, corners, window_size=11, alpha=0.99):
         elif corner_class == 1:
             corners_subpix[i, :] = y0 + est_edge[0], x0 + est_edge[1]
 
+    # subtract pad width
+    corners_subpix -= wext
+
     return corners_subpix
 
 

skimage/feature/tests/test_corner.py

@@ -1,5 +1,5 @@
 import numpy as np
-from numpy.testing import assert_array_equal
+from numpy.testing import assert_array_equal, assert_almost_equal
 
 from skimage import data
 from skimage import img_as_float
@@ -101,6 +101,22 @@ def test_subpix():
     assert_array_equal(subpix[0], (24.5, 24.5))
 
 
+def test_subpix_border():
+    img = np.zeros((50, 50))
+    img[1:25,1:25] = 255
+    img[25:-1,25:-1] = 255
+    corner = corner_peaks(corner_harris(img), min_distance=1)
+    subpix = corner_subpix(img, corner, window_size=11)
+    ref = np.array([[ 0.52040816,  0.52040816],
+                    [ 0.52040816, 24.47959184],
+                    [24.47959184,  0.52040816],
+                    [24.5       , 24.5       ],
+                    [24.52040816, 48.47959184],
+                    [48.47959184, 24.52040816],
+                    [48.47959184, 48.47959184]])
+    assert_almost_equal(subpix, ref)
+
+
 def test_num_peaks():
     """For a bunch of different values of num_peaks, check that
     peak_local_max returns exactly the right amount of peaks. Test