Add nearest mode for positions outside image

skimage/transform/_geometric.py

@@ -832,11 +832,8 @@ def warp(image, inverse_map=None, map_args={}, output_shape=None, order=1,
     ishape = np.array(image.shape)
     bands = ishape[2]
 
-    # use fast Cython version for specific parameters
-    fast_modes = ('constant', 'reflect', 'wrap')
-    fast_orders = (0, 1, 3)
-
-    if order in fast_orders and mode in fast_modes and not map_args:
+    # use fast Cython version for specific interpolation orders
+    if order in (0, 1, 3) and not map_args:
         matrix = None
         if isinstance(inverse_map, HOMOGRAPHY_TRANSFORMS):
             matrix = inverse_map._matrix

skimage/transform/_warps_cy.pyx

@@ -86,9 +86,9 @@ def _warp_fast(np.ndarray image, np.ndarray H, output_shape=None, int order=1,
     cdef np.ndarray[dtype=np.double_t, ndim=2, mode="c"] M = \
          np.ascontiguousarray(H)
 
-    if mode not in ('constant', 'wrap', 'reflect'):
+    if mode not in ('constant', 'wrap', 'reflect', 'nearest'):
         raise ValueError("Invalid mode specified.  Please use "
-                         "`constant`, `wrap` or `reflect`.")
+                         "`constant`, `nearest`, `wrap` or `reflect`.")
     cdef char mode_c = ord(mode[0].upper())
 
     cdef int out_r, out_c

skimage/transform/tests/test_warps.py

@@ -58,7 +58,7 @@ def test_fast_homography():
     coords = warp_coords(tform.inverse, (img.shape[0], img.shape[1]))
 
     for order in range(4):
-        for mode in ('constant', 'reflect', 'wrap'):
+        for mode in ('constant', 'reflect', 'wrap', 'nearest'):
             p0 = map_coordinates(img, coords, mode=mode, order=order)
             p1 = warp(img, tform, mode=mode, order=order)