Raise warning for sigma parameter and add to TODO for next release

TODO.txt

@@ -1,15 +1,15 @@
 Version 0.10
 ------------
-* Remove deprecated functions:
-    - ``skimage.filter.rank.*``
-* Remove deprecated parameter ``epsilon`` of ``skimage.viewer.LineProfile``
-* Remove backwards-compatability of ``skimage.measure.regionprops``
+* Remove deprecated functions in `skimage.filter.rank.*`
+* Remove deprecated parameter `epsilon` of `skimage.viewer.LineProfile`
+* Remove backwards-compatability of `skimage.measure.regionprops`
+* Remove {`ratio`, `sigma`} deprecation warnings of `skimage.segmentation.slic`
 
 Version 0.9
 -----------
 * Remove deprecated functions
-    - ``skimage.filter.denoise_tv_chambolle``
-    - ``skimage.morphology.is_local_maximum``
-    - ``skimage.transform.hough``
-    - ``skimage.transform.probabilistic_hough``
-    - ``skimage.transform.hough_peaks``
+    - `skimage.filter.denoise_tv_chambolle`
+    - `skimage.morphology.is_local_maximum`
+    - `skimage.transform.hough`
+    - `skimage.transform.probabilistic_hough`
+    - `skimage.transform.hough_peaks`

skimage/segmentation/slic_superpixels.py

@@ -10,7 +10,7 @@ from skimage.segmentation._slic import _slic_cython
 from skimage.color import rgb2lab
 
 
-def slic(image, n_segments=100, compactness=10., max_iter=10, sigma=0,
+def slic(image, n_segments=100, compactness=10., max_iter=10, sigma=None,
          multichannel=True, convert2lab=True, ratio=None):
     """Segments image using k-means clustering in Color-(x,y,z) space.
 
@@ -80,9 +80,13 @@ def slic(image, n_segments=100, compactness=10., max_iter=10, sigma=0,
     >>> segments = slic(img, n_segments=100, ratio=20)
     """
 
+    if sigma is None:
+        warnings.warn('Default value of keyword `sigma` changed from ``1`` '
+                      'to ``0``.')
+        sigma = 0
     if ratio is not None:
-        msg = 'Keyword `ratio` is deprecated. Use `compactness` instead.'
-        warnings.warn(msg)
+        warnings.warn('Keyword `ratio` is deprecated. Use `compactness` '
+                      'instead.')
         compactness = ratio
 
     image = img_as_float(image)