Merge pull request #1660 from jwiggins/issue-1586

Raise an error for negative sigma values in guassian_filter.

skimage/filters/_gaussian.py

@@ -93,6 +93,8 @@ def gaussian_filter(image, sigma, output=None, mode='nearest', cval=0,
                "3D image with last dimension of length 3.")
         warnings.warn(RuntimeWarning(msg))
         multichannel = True
+    if np.any(np.asarray(sigma) < 0.0):
+        raise ValueError("Sigma values less than zero are not valid")
     if multichannel:
         # do not filter across channels
         if not isinstance(sigma, coll.Iterable):

skimage/filters/tests/test_gaussian.py

@@ -1,8 +1,18 @@
 import numpy as np
+from numpy.testing import assert_raises
 from skimage.filters._gaussian import gaussian_filter
 from skimage._shared._warnings import expected_warnings
 
 
+def test_negative_sigma():
+    a = np.zeros((3, 3))
+    a[1, 1] = 1.
+    assert_raises(ValueError, gaussian_filter, a, sigma=-1.0)
+    assert_raises(ValueError, gaussian_filter, a, sigma=[-1.0, 1.0])
+    assert_raises(ValueError, gaussian_filter, a,
+                  sigma=np.asarray([-1.0, 1.0]))
+
+
 def test_null_sigma():
     a = np.zeros((3, 3))
     a[1, 1] = 1.