Fix division by zero bug in corner detectors

skimage/feature/corner.py

@@ -91,8 +91,13 @@ def corner_kitchen_rosenfeld(image):
     imxx, imxy = _compute_derivatives(imx)
     imyx, imyy = _compute_derivatives(imy)
 
-    response = (imxx * imy**2 + imyy * imx**2 - 2 * imxy * imx * imy) \
-               / (imx**2 + imy**2)
+    numerator = (imxx * imy**2 + imyy * imx**2 - 2 * imxy * imx * imy)
+    denominator = (imx**2 + imy**2)
+
+    response = np.zeros_like(image, dtype=np.double)
+
+    mask = denominator != 0
+    response[mask] = numerator[mask] / denominator[mask]
 
     return response
 
@@ -311,8 +316,13 @@ def corner_foerstner(image, sigma=1):
     # trace
     traceA = Axx + Ayy
 
-    w = detA / traceA
-    q = 4 * detA / traceA**2
+    w = np.zeros_like(image, dtype=np.double)
+    q = np.zeros_like(image, dtype=np.double)
+
+    mask = traceA != 0
+
+    w[mask] = detA[mask] / traceA[mask]
+    q[mask] = 4 * detA[mask] / traceA[mask]**2
 
     return w, q
 

skimage/feature/tests/test_corner.py

@@ -5,7 +5,8 @@ from skimage import data
 from skimage import img_as_float
 
 from skimage.feature import (corner_moravec, corner_harris, corner_shi_tomasi,
-                             corner_subpix, peak_local_max, corner_peaks)
+                             corner_subpix, peak_local_max, corner_peaks,
+                             corner_kitchen_rosenfeld, corner_foerstner)
 
 
 def test_square_image():
@@ -124,6 +125,21 @@ def test_corner_peaks():
     assert len(corners) == 4
 
 
+def test_blank_image_nans():
+    """Some of the corner detectors had a weakness in terms of returning
+    NaN when presented with regions of constant intensity. This should
+    be fixed by now. We test whether each detector returns something
+    finite in the case of constant input"""
+
+    detectors = [corner_moravec, corner_harris, corner_shi_tomasi,
+                 corner_kitchen_rosenfeld, corner_foerstner]
+    constant_image = np.zeros((20, 20))
+
+    for det in detectors:
+        response = det(constant_image)
+        assert np.all(np.isfinite(response))
+
+
 if __name__ == '__main__':
     from numpy import testing
     testing.run_module_suite()