Do not allow view_as_* to return copies (closes #866)

doc/source/api_changes.txt

@@ -1,6 +1,9 @@
 Version 0.10
 ------------
 - Removed ``skimage.io.video`` functionality due to broken gstreamer bindings
+- The functions `skimage.util.view_as_blocks` and
+  `skimage.util.view_as_windows` now raise exceptions if the input data is not
+  contiguous.
 
 Version 0.9
 -----------

skimage/util/shape.py

@@ -11,16 +11,16 @@ def view_as_blocks(arr_in, block_shape):
 
     Parameters
     ----------
-    arr_in: ndarray
-        The n-dimensional input array.
+    arr_in : ndarray
+        Contiguous N-d input array.
 
-    block_shape: tuple
+    block_shape : tuple
         The shape of the block. Each dimension must divide evenly into the
         corresponding dimensions of `arr_in`.
 
     Returns
     -------
-    arr_out: ndarray
+    arr_out : ndarray
         Block view of the input array.
 
     Examples
@@ -70,8 +70,6 @@ def view_as_blocks(arr_in, block_shape):
            [[[76, 77],
              [82, 83]]]])
     """
-
-    # -- basic checks on arguments
     if not isinstance(block_shape, tuple):
         raise TypeError('block needs to be a tuple')
 
@@ -88,6 +86,9 @@ def view_as_blocks(arr_in, block_shape):
         raise ValueError("'block_shape' is not compatible with 'arr_in'")
 
     # -- restride the array to build the block view
+    if not arr_in.flags.contiguous:
+        raise ValueError("Cannot provide views on a non-contiguous input array")
+
     arr_in = np.ascontiguousarray(arr_in)
 
     new_shape = tuple(arr_shape / block_shape) + tuple(block_shape)
@@ -107,7 +108,7 @@ def view_as_windows(arr_in, window_shape, step=1):
     Parameters
     ----------
     arr_in: ndarray
-        The n-dimensional input array.
+        Contiguous N-d input array.
     window_shape: tuple
         Defines the shape of the elementary n-dimensional orthotope
         (better know as hyperrectangle [1]_) of the rolling window view.
@@ -228,6 +229,9 @@ def view_as_windows(arr_in, window_shape, step=1):
         raise ValueError("`window_shape` is too small")
 
     # -- build rolling window view
+    if not arr_in.flags.contiguous:
+        raise ValueError("Cannot provide views on a non-contiguous input array")
+
     arr_in = np.ascontiguousarray(arr_in)
 
     new_shape = tuple((arr_shape - window_shape) // step + 1) + \

skimage/util/tests/test_shape.py

@@ -1,6 +1,6 @@
 import numpy as np
 from nose.tools import raises
-from numpy.testing import assert_equal
+from numpy.testing import assert_equal, assert_raises
 from skimage.util.shape import view_as_blocks, view_as_windows
 
 
@@ -147,5 +147,13 @@ def test_view_as_windows_With_skip():
     assert_equal(C.shape, (1, 1, 2, 2))
 
 
+def test_views_non_contiguous():
+    A = np.arange(16).reshape((4, 4))
+    A = A[::2, :]
+
+    assert_raises(ValueError, view_as_blocks, A, (2, 2))
+    assert_raises(ValueError, view_as_windows, A, (2, 2))
+
+
 if __name__ == '__main__':
     np.testing.run_module_suite()