unwrap: Warn about singleton dimensions in 3D arrays.

skimage/exposure/tests/test_unwrap.py

@@ -70,7 +70,10 @@ def check_wrap_around(ndim, axis):
     index_first = tuple([0] * ndim)
     index_last = tuple([-1 if n == axis else 0 for n in range(ndim)])
     # unwrap the image without wrap around
-    image_unwrap_no_wrap_around = unwrap_phase(image_wrapped)
+    with warnings.catch_warnings():
+        # We do not want warnings about length 1 dimensions
+        warnings.simplefilter("ignore")
+        image_unwrap_no_wrap_around = unwrap_phase(image_wrapped)
     print('endpoints without wrap_around:',
           image_unwrap_no_wrap_around[index_first],
           image_unwrap_no_wrap_around[index_last])
@@ -79,7 +82,10 @@ def check_wrap_around(ndim, axis):
                - image_unwrap_no_wrap_around[index_last]) > np.pi
     # unwrap the image with wrap around
     wrap_around = [n == axis for n in range(ndim)]
-    image_unwrap_wrap_around = unwrap_phase(image_wrapped, wrap_around)
+    with warnings.catch_warnings():
+        # We do not want warnings about length 1 dimensions
+        warnings.simplefilter("ignore")
+        image_unwrap_wrap_around = unwrap_phase(image_wrapped, wrap_around)
     print('endpoints with wrap_around:',
           image_unwrap_wrap_around[index_first],
           image_unwrap_wrap_around[index_last])

skimage/exposure/unwrap.py

@@ -1,4 +1,5 @@
 import numpy as np
+import warnings
 
 from ._unwrap_2d import unwrap_2d
 from ._unwrap_3d import unwrap_3d
@@ -53,6 +54,9 @@ def unwrap_phase(image, wrap_around=False):
     else:
         raise ValueError('wrap_around must be a bool or a sequence with '
                          'length equal to the dimensionality of image')
+    if image.ndim == 3 and 1 in image.shape:
+        warnings.warn('image is 3D and has a length 1 dimension; consider '
+                      'using a 2D array to use the 2D unwrapping algorithm')
 
     if np.ma.isMaskedArray(image):
         mask = np.require(image.mask, np.uint8, ['C'])