Fix image dimension sanitizing at function start

`np.atleast_3d` will add a singleton dimension at the end of an array
if needed. This is not the correct thing to do if `multichannel=False`
based on the subsequent lines. If the input image was 2D with shape
`(40, 50)` and `multichannel=False`, then `np.atleast_3d` gives it
shape `(40, 50, 1)`, and then, because `multichannel=False`, the rest
of the code gives it shape `(40, 50, 1, 1)`. This results in the final
returned array having shape `(40, 50, 1)` instead of the desired
`(40, 50)`.

This commit fixes that and updates the test to detect this failure.

skimage/segmentation/slic_superpixels.py

@@ -51,14 +51,12 @@ def slic(image, n_segments=100, compactness=10., max_iter=20, sigma=0,
     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`, OR
-            - `multichannel == True` and the length of the last dimension of
-            the image is not 3, OR
+            - the image dimension is not 3 or 4 and `multichannel == True`
 
     Notes
     -----
-    If `sigma > 0` as is default, the image is smoothed using a Gaussian kernel
-    prior to segmentation.
+    If `sigma > 0`, the image is smoothed using a Gaussian kernel prior to
+    segmentation.
 
     The image is rescaled to be in [0, 1] prior to processing.
 
@@ -88,15 +86,18 @@ def slic(image, n_segments=100, compactness=10., max_iter=20, sigma=0,
         compactness = ratio
 
     image = img_as_float(image)
-    image = np.atleast_3d(image)
-
-    if image.ndim == 3:
-        if multichannel:
-            # Make 2D image 3D with depth = 1
-            image = image[np.newaxis, ...]
-        else:
-            # Add channel as single last dimension
-            image = image[..., np.newaxis]
+    is2d = False
+    if image.ndim == 2:
+        # 2D grayscale image
+        image = image[np.newaxis, ..., np.newaxis]
+        is2d = True
+    elif image.ndim == 3 and multichannel:
+        # Make 2D multichannel image 3D with depth = 1
+        image = image[np.newaxis, ...]
+        is2d = True
+    elif image.ndim == 3 and not multichannel:
+        # Add channel as single last dimension
+        image = image[..., np.newaxis]
 
     if not isinstance(sigma, coll.Iterable):
         sigma = np.array([sigma, sigma, sigma])
@@ -135,7 +136,7 @@ def slic(image, n_segments=100, compactness=10., max_iter=20, sigma=0,
 
     labels = _slic_cython(image, segments, max_iter)
 
-    if labels.shape[0] == 1:
+    if is2d:
         labels = labels[0]
 
     return labels

skimage/segmentation/tests/test_slic.py

@@ -20,6 +20,7 @@ def test_color_2d():
 
     # we expect 4 segments
     assert_equal(len(np.unique(seg)), 4)
+    assert_equal(seg.shape, img.shape[:-1])
     assert_array_equal(seg[:10, :10], 0)
     assert_array_equal(seg[10:, :10], 2)
     assert_array_equal(seg[:10, 10:], 1)
@@ -39,6 +40,7 @@ def test_gray_2d():
                multichannel=False, convert2lab=False)
 
     assert_equal(len(np.unique(seg)), 4)
+    assert_equal(seg.shape, img.shape)
     assert_array_equal(seg[:10, :10], 0)
     assert_array_equal(seg[10:, :10], 2)
     assert_array_equal(seg[:10, 10:], 1)