From b0d3b92e74cdef187830513c360cc16b7306fec5 Mon Sep 17 00:00:00 2001
From: Stefan van der Walt <stefan@sun.ac.za>
Date: Sun, 1 Sep 2013 23:44:23 +0200
Subject: [PATCH] Add skip parameter to view_as_windows.

---
 skimage/util/shape.py            | 18 ++++++++++++++----
 skimage/util/tests/test_shape.py | 16 ++++++++++++++++
 2 files changed, 30 insertions(+), 4 deletions(-)

diff --git a/skimage/util/shape.py b/skimage/util/shape.py
index 0126d2e3..1d606b42 100644
--- a/skimage/util/shape.py
+++ b/skimage/util/shape.py
@@ -98,7 +98,7 @@ def view_as_blocks(arr_in, block_shape):
     return arr_out
 
 
-def view_as_windows(arr_in, window_shape):
+def view_as_windows(arr_in, window_shape, step=1):
     """Rolling window view of the input n-dimensional array.
 
     Windows are overlapping views of the input array, with adjacent windows
@@ -108,10 +108,12 @@ def view_as_windows(arr_in, window_shape):
     ----------
     arr_in: ndarray
         The n-dimensional input array.
-
     window_shape: tuple
         Defines the shape of the elementary n-dimensional orthotope
         (better know as hyperrectangle [1]_) of the rolling window view.
+    step : int
+        Number of elements to skip when moving the window forward (by
+        default, move forward by one).
 
     Returns
     -------
@@ -212,6 +214,9 @@ def view_as_windows(arr_in, window_shape):
     if not (len(window_shape) == arr_in.ndim):
         raise ValueError("'window_shape' is incompatible with 'arr_in.shape'")
 
+    if step < 1:
+        raise ValueError("`step` must be >= 1")
+
     arr_shape = np.array(arr_in.shape)
     window_shape = np.array(window_shape, dtype=arr_shape.dtype)
 
@@ -224,8 +229,13 @@ def view_as_windows(arr_in, window_shape):
     # -- build rolling window view
     arr_in = np.ascontiguousarray(arr_in)
 
-    new_shape = tuple(arr_shape - window_shape + 1) + tuple(window_shape)
-    new_strides = arr_in.strides + arr_in.strides
+    new_shape = tuple((arr_shape - window_shape) // step + 1) + \
+                tuple(window_shape)
+
+    arr_strides = np.array(arr_in.strides)
+    new_strides = np.concatenate(
+                  (arr_strides * step, arr_strides)
+                  )
 
     arr_out = as_strided(arr_in, shape=new_shape, strides=new_strides)
 
diff --git a/skimage/util/tests/test_shape.py b/skimage/util/tests/test_shape.py
index 8c62a191..b6975d0f 100644
--- a/skimage/util/tests/test_shape.py
+++ b/skimage/util/tests/test_shape.py
@@ -141,5 +141,21 @@ def test_view_as_windows_2D():
                                 [17, 18, 19]]]]))
 
 
+def test_view_as_windows_With_skip():
+    A = np.arange(20).reshape((5, 4))
+    B = view_as_windows(A, (2, 2), step=2)
+    assert_equal(B, [[[[0, 1],
+                       [4, 5]],
+                      [[2, 3],
+                       [6, 7]]],
+                     [[[8, 9],
+                       [12, 13]],
+                      [[10, 11],
+                       [14, 15]]]])
+
+    C = view_as_windows(A, (2, 2), step=4)
+    assert_equal(C.shape, (1, 1, 2, 2))
+
+
 if __name__ == '__main__':
     np.testing.run_module_suite()