Add nearest mode for positions outside image

skimage/_shared/interpolation.pyx

@@ -19,8 +19,8 @@ cdef inline double nearest_neighbour_interpolation(double* image, int rows,
         Shape of image.
     r, c : int
         Position at which to interpolate.
-    mode : {'C', 'W', 'R'}
-        Wrapping mode. Constant, Wrap or Reflect.
+    mode : {'C', 'W', 'R', 'N'}
+        Wrapping mode. Constant, Wrap, Reflect or Nearest.
     cval : double
         Constant value to use for constant mode.
 
@@ -48,8 +48,8 @@ cdef inline double bilinear_interpolation(double* image, int rows, int cols,
         Shape of image.
     r, c : int
         Position at which to interpolate.
-    mode : {'C', 'W', 'R'}
-        Wrapping mode. Constant, Wrap or Reflect.
+    mode : {'C', 'W', 'R', 'N'}
+        Wrapping mode. Constant, Wrap, Reflect or Nearest.
     cval : double
         Constant value to use for constant mode.
 
@@ -111,8 +111,8 @@ cdef inline double bicubic_interpolation(double* image, int rows, int cols,
         Shape of image.
     r, c : int
         Position at which to interpolate.
-    mode : {'C', 'W', 'R'}
-        Wrapping mode. Constant, Wrap or Reflect.
+    mode : {'C', 'W', 'R', 'N'}
+        Wrapping mode. Constant, Wrap, Reflect or Nearest.
     cval : double
         Constant value to use for constant mode.
 
@@ -160,8 +160,8 @@ cdef inline double get_pixel(double* image, int rows, int cols, int r, int c,
         Shape of image.
     r, c : int
         Position at which to get the pixel.
-    mode : {'C', 'W', 'R'}
-        Wrapping mode. Constant, Wrap or Reflect.
+    mode : {'C', 'W', 'R', 'N'}
+        Wrapping mode. Constant, Wrap, Reflect or Nearest.
     cval : double
         Constant value to use for constant mode.
 
@@ -190,28 +190,33 @@ cdef inline int coord_map(int dim, int coord, char mode):
         Maximum coordinate.
     coord : int
         Coord provided by user.  May be < 0 or > dim.
-    mode : {'W', 'R'}
+    mode : {'W', 'R', 'N'}
         Whether to wrap or reflect the coordinate if it
         falls outside [0, dim).
 
     """
     dim = dim - 1
     if mode == 'R': # reflect
-        if (coord < 0):
+        if coord < 0:
             # How many times times does the coordinate wrap?
-            if (<int>(-coord / dim) % 2 != 0):
+            if <int>(-coord / dim) % 2 != 0:
                 return dim - <int>(-coord % dim)
             else:
                 return <int>(-coord % dim)
-        elif (coord > dim):
-            if (<int>(coord / dim) % 2 != 0):
+        elif coord > dim:
+            if <int>(coord / dim) % 2 != 0:
                 return <int>(dim - (coord % dim))
             else:
                 return <int>(coord % dim)
     elif mode == 'W': # wrap
-        if (coord < 0):
+        if coord < 0:
             return <int>(dim - (-coord % dim))
-        elif (coord > dim):
+        elif coord > dim:
             return <int>(coord % dim)
+    elif mode == 'N': # nearest
+        if coord < 0:
+            return 0
+        elif coord > dim:
+            return dim
 
     return coord

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)