Update treatment of convert2lab in slic

It still defaults to `True` but only when the last dimension of the
input array could be construed as RGB.

Also, update ValueError description in docstring.

skimage/segmentation/slic_superpixels.py

@@ -12,7 +12,7 @@
 
 
 def slic(image, n_segments=100, compactness=10., max_iter=10, sigma=0,
-         spacing=None, multichannel=True, convert2lab=True,
+         spacing=None, multichannel=True, convert2lab=None,
          enforce_connectivity=False, min_size_factor=0.5, max_size_factor=3,
          slic_zero=False):
     """Segments image using k-means clustering in Color-(x,y,z) space.
@@ -47,8 +47,9 @@ def slic(image, n_segments=100, compactness=10., max_iter=10, sigma=0,
         channels or another spatial dimension.
     convert2lab : bool, optional
         Whether the input should be converted to Lab colorspace prior to
-        segmentation. For this purpose, the input is assumed to be RGB. Highly
-        recommended.
+        segmentation. The input image *must* be RGB. Highly recommended.
+        This option defaults to ``True`` when ``multichannel=True`` *and*
+        ``image.shape[-1] == 3``.
     enforce_connectivity: bool, optional (default False)
         Whether the generated segments are connected or not
     min_size_factor: float, optional
@@ -68,9 +69,8 @@ def slic(image, n_segments=100, compactness=10., max_iter=10, sigma=0,
     Raises
     ------
     ValueError
-        If:
-            - the image dimension is not 2 or 3 and `multichannel == False`, OR
-            - the image dimension is not 3 or 4 and `multichannel == True`
+        If ``convert2lab`` is set to ``True`` but the last array
+        dimension is not of length 3.
 
     Notes
     -----
@@ -141,10 +141,11 @@ def slic(image, n_segments=100, compactness=10., max_iter=10, sigma=0,
         sigma = list(sigma) + [0]
         image = ndimage.gaussian_filter(image, sigma)
 
-    if convert2lab and multichannel:
-        if image.shape[3] != 3:
+    if multichannel and (convert2lab or convert2lab is None):
+        if image.shape[-1] != 3 and convert2lab:
             raise ValueError("Lab colorspace conversion requires a RGB image.")
-        image = rgb2lab(image)
+        elif image.shape[-1] == 3:
+            image = rgb2lab(image)
 
     depth, height, width = image.shape[:3]
 

skimage/segmentation/tests/test_slic.py

@@ -1,5 +1,4 @@
 import itertools as it
-import warnings
 import numpy as np
 from numpy.testing import assert_equal, assert_raises
 from skimage.segmentation import slic
@@ -14,9 +13,27 @@ def test_color_2d():
     img += 0.01 * rnd.normal(size=img.shape)
     img[img > 1] = 1
     img[img < 0] = 0
-    with warnings.catch_warnings():
-        warnings.simplefilter("ignore")
-        seg = slic(img, n_segments=4, sigma=0)
+    seg = slic(img, n_segments=4, sigma=0, enforce_connectivity=False)
+
+    # we expect 4 segments
+    assert_equal(len(np.unique(seg)), 4)
+    assert_equal(seg.shape, img.shape[:-1])
+    assert_equal(seg[:10, :10], 0)
+    assert_equal(seg[10:, :10], 2)
+    assert_equal(seg[:10, 10:], 1)
+    assert_equal(seg[10:, 10:], 3)
+
+
+def test_multichannel_2d():
+    rnd = np.random.RandomState(0)
+    img = np.zeros((20, 20, 8))
+    img[:10, :10, 0:2] = 1
+    img[:10, 10:, 2:4] = 1
+    img[10:, :10, 4:6] = 1
+    img[10:, 10:, 6:8] = 1
+    img += 0.01 * rnd.normal(size=img.shape)
+    img = np.clip(img, 0, 1, out=img)
+    seg = slic(img, n_segments=4, enforce_connectivity=False)
 
     # we expect 4 segments
     assert_equal(len(np.unique(seg)), 4)
@@ -158,9 +175,7 @@ def test_slic_zero():
     img += 0.01 * rnd.normal(size=img.shape)
     img[img > 1] = 1
     img[img < 0] = 0
-    with warnings.catch_warnings():
-        warnings.simplefilter("ignore")
-        seg = slic(img, n_segments=4, sigma=0, slic_zero=True)
+    seg = slic(img, n_segments=4, sigma=0, slic_zero=True)
 
     # we expect 4 segments
     assert_equal(len(np.unique(seg)), 4)