Require median filter radius >= 2

The octagon structuring element used by _ctmf.pyx does not scale correctly for smaller radii.

skimage/filter/ctmf.py

@@ -47,6 +47,9 @@ def median_filter(image, mask=None, radius=2, percent=50):
     if image.ndim != 2:
         raise TypeError("The input 'image' must be a two dimensional array.")
 
+    if radius < 2:
+        raise ValueError("The input 'radius' must be >= 2.")
+
     if mask is None:
         mask = np.ones(image.shape, dtype=np.bool)
     mask = np.ascontiguousarray(mask, dtype=np.bool)

skimage/filter/tests/test_ctmf.py

@@ -1,4 +1,5 @@
 import numpy as np
+from nose.tools import raises
 
 from skimage.filter import median_filter
 
@@ -110,5 +111,11 @@ def test_default_values():
     np.testing.assert_array_equal(result1, result2)
 
 
+@raises(ValueError)
+def test_insufficient_size():
+    img = (np.random.random((20, 20)) * 255).astype(np.uint8)
+    median_filter(img, radius=1)
+
+
 if __name__ == "__main__":
     np.testing.run_module_suite()