Add test for new fast correlation

skan/test/test_vendored_correlate.py

@@ -1,6 +1,9 @@
 from time import time
+from functools import reduce
 import numpy as np
 from skan.vendored import thresholding as th
+from skimage.transform import integral_image
+from scipy import ndimage as ndi
 
 
 class Timer:
@@ -25,3 +28,16 @@ def test_fast_sauvola():
     with Timer() as t1:
         th.threshold_sauvola(image, window_size=w1)
     assert t1.interval < 2 * t0.interval
+
+
+def test_reference_correlation():
+    ndim = 4
+    shape = np.random.randint(0, 20, size=ndim)
+    x = np.random.random(shape)
+    kern = reduce(np.outer, [[-1, 0, 0, 1]] * ndim).reshape((4,) * ndim)
+    px = np.pad(x, (2, 1), mode='reflect')
+    pxi = integral_image(px)
+    mean_fast = th.correlate_nonzeros(pxi, kern / 3 ** ndim)
+    mean_ref = ndi.correlate(x, np.ones((3,) * ndim) / 3 ** ndim,
+                             mode='mirror')
+    np.testing.assert_allclose(mean_fast, mean_ref)