Added default value to gaussian filter (sigma parameter)

skimage/filters/_gaussian.py

@@ -9,7 +9,7 @@ from .._shared.utils import warn
 __all__ = ['gaussian']
 
 
-def gaussian(image, sigma, output=None, mode='nearest', cval=0,
+def gaussian(image, sigma=1, output=None, mode='nearest', cval=0,
              multichannel=None):
     """Multi-dimensional Gaussian filter
 
@@ -17,7 +17,7 @@ def gaussian(image, sigma, output=None, mode='nearest', cval=0,
     ----------
     image : array-like
         input image (grayscale or color) to filter.
-    sigma : scalar or sequence of scalars
+    sigma : scalar or sequence of scalars, optional
         standard deviation for Gaussian kernel. The standard
         deviations of the Gaussian filter are given for each axis as a
         sequence, or as a single number, in which case it is equal for

skimage/filters/tests/test_gaussian.py

@@ -19,6 +19,12 @@ def test_null_sigma():
     assert np.all(gaussian(a, 0) == a)
 
 
+def test_default_sigma():
+    a = np.zeros((3, 3))
+    a[1, 1] = 1.
+    assert np.all(gaussian(a) == gaussian(a, sigma=1))
+
+
 def test_energy_decrease():
     a = np.zeros((3, 3))
     a[1, 1] = 1.