DOC: add visual example of edge modes

doc/examples/plot_edge_modes.py

@@ -0,0 +1,36 @@
+"""
+=========================
+Interpolation: Edge Modes
+=========================
+
+This example illustrates the different edge modes available during
+interpolation in routines such as ``skimage.transform.rescale`` and
+``skimage.transform.resize``.
+"""
+from skimage._shared._interpolation_test import extend_image
+import skimage.data
+import matplotlib.pyplot as plt
+import numpy as np
+
+img = np.zeros((9, 9))
+img[:8, :8] += 1
+img[:4, :4] += 1
+img[:2, :2] += 1
+img[:1, :1] += 2
+
+modes = ['constant', 'nearest', 'wrap', 'reflect']
+fig, axes = plt.subplots(1, 4, figsize=(15, 5))
+for n, mode in enumerate(modes):
+    img_extended = extend_image(img, pad=img.shape[0], mode=mode)
+    axes[n].imshow(img_extended, cmap=plt.cm.gray, interpolation='nearest')
+    axes[n].plot([8.5, 8.5], [8.5, 17.5], 'y--', linewidth=0.5)
+    axes[n].plot([17.5, 17.5], [8.5, 17.5], 'y--', linewidth=0.5)
+    axes[n].plot([8.5, 17.5], [8.5, 8.5], 'y--', linewidth=0.5)
+    axes[n].plot([8.5, 17.5], [17.5, 17.5], 'y--', linewidth=0.5)
+    axes[n].set_axis_off()
+    axes[n].set_aspect('equal')
+    axes[n].set_title(mode)
+
+plt.tight_layout()
+
+plt.show()

skimage/_shared/_interpolation_test.pyx

@@ -1,5 +1,31 @@
 from interpolation cimport coord_map as _coord_map
+from interpolation cimport get_pixel2d
+import numpy as np
+cimport numpy as cnp
+
 
 def coord_map(Py_ssize_t dim, long coord, mode):
+    """ interpolation.coord_map python wrapper """
     cdef char mode_c = ord(mode[0].upper())
-    return _coord_map(dim, coord, mode_c)
+    return _coord_map(dim, coord, mode_c)
+
+
+def extend_image(image, pad=10, mode='C', cval=0):
+    """ can be used to verify proper get_pixel2d behavior. """
+    cdef:
+        Py_ssize_t rows = image.shape[0]
+        Py_ssize_t cols = image.shape[1]
+        long ro, co
+        char mode_c = ord(mode[0].upper())
+
+    image = np.ascontiguousarray(image.astype(np.float64))
+    output_shape = np.asarray(image.shape) + 2*pad
+    image_out = np.zeros(output_shape, dtype=image.dtype)
+    for r in range(-pad, rows+pad):
+        for c in range(-pad, cols+pad):
+            ro = r + pad
+            co = c + pad
+            image_out[ro, co] = get_pixel2d(<double*> cnp.PyArray_DATA(image),
+                                          rows, cols, <long> r, <long> c,
+                                          mode_c, <double> cval)
+    return image_out

skimage/_shared/tests/test_interpolation.py

@@ -1,6 +1,7 @@
 from skimage._shared._interpolation_test import coord_map
 from numpy.testing import assert_array_equal
 
+
 def test_coord_map():
 
     reflect = [coord_map(4, n, 'R') for n in range(-6, 6)]
@@ -16,4 +17,4 @@ def test_coord_map():
     assert_array_equal(nearest, expected_neareset)
 
     other = [coord_map(4, n, 'undefined') for n in range(-6, 6)]
-    assert_array_equal(other, list(range(-6, 6)))
+    assert_array_equal(other, list(range(-6, 6)))