Add support for negative indexes/slices.

skimage/novice/_novice.py

@@ -38,26 +38,25 @@ def _verify_picture_index(index):
         if isinstance(dim_slice, int):
             index[i] = dim_slice = slice(dim_slice, dim_slice + 1)
 
-        if dim_slice.start is not None and dim_slice.start < 0:
-            raise IndexError("Negative slicing not supported")
-
-        if dim_slice.stop is not None and dim_slice.stop < 0:
-            raise IndexError("Negative slicing not supported")
-
         if dim_slice.step is not None and dim_slice.step != 1:
             raise IndexError("Only a step size of 1 is supported")
 
     return tuple(index)
 
 
+def rgb_transpose(array):
+    """Return RGB array with first 2 axes transposed."""
+    return np.transpose(array, (1, 0, 2))
+
+
 def array_to_xy_origin(image):
     """Return view of image transformed from array to Cartesian origin."""
-    return np.transpose(image[::-1], (1, 0, 2))
+    return rgb_transpose(image[::-1])
 
 
 def xy_to_array_origin(image):
     """Return view of image transformed from Cartesian to array origin."""
-    return np.transpose(image[:, ::-1], (1, 0, 2))
+    return rgb_transpose(image[:, ::-1])
 
 
 class Pixel(object):
@@ -151,6 +150,10 @@ class Pixel(object):
         self._picture.xy_array[self._x, self._y] = self.rgb
         self._picture._array_modified()
 
+    def __eq__(self, other):
+        if isinstance(other, Pixel):
+            return self.rgb == other.rgb
+
     def __repr__(self):
         args = self.red, self.green, self.blue
         return "Pixel(red={0}, green={1}, blue={2})".format(*args)
@@ -396,8 +399,6 @@ class Picture(object):
         """Return `Picture`s for slices and `Pixel`s for indexes."""
         xy_index = _verify_picture_index(xy_index)
         if all(isinstance(index, int) for index in xy_index):
-            if any(index < 0 for index in xy_index):
-                raise IndexError("Negative indices not supported")
             return self._makepixel(*xy_index)
         else:
             return Picture(xy_array=self.xy_array[xy_index])

skimage/novice/tests/test_novice.py

@@ -4,7 +4,8 @@ import tempfile
 import numpy as np
 from numpy.testing import assert_equal, raises, assert_allclose
 from skimage import novice
-from skimage.novice._novice import array_to_xy_origin, xy_to_array_origin
+from skimage.novice._novice import (array_to_xy_origin, xy_to_array_origin,
+                                    rgb_transpose)
 from skimage import data_dir
 
 
@@ -164,7 +165,7 @@ def test_picture_slice():
     assert_allclose(subpic.array, array[x_slice, :])
 
 
-def test_slicing():
+def test_move_slice():
     h, w = 3, 12
     array = _array_2d_to_RGB(np.linspace(0, 255, h * w).reshape(h, w))
     array = array.astype(np.uint8)
@@ -184,6 +185,26 @@ def test_slicing():
     assert pic[:rest, :] == pic_orig[cut:, :]
 
 
+def test_negative_index():
+    n = 10
+    array = _array_2d_to_RGB(np.arange(0, n)[np.newaxis, :])
+    # Test both x and y indices.
+    pic = novice.Picture(array=array)
+    assert pic[-1, 0] == pic[n - 1, 0]
+    pic = novice.Picture(array=rgb_transpose(array))
+    assert pic[0, -1] == pic[0, n - 1]
+
+
+def test_negative_slice():
+    n = 10
+    array = _array_2d_to_RGB(np.arange(0, n)[np.newaxis, :])
+    # Test both x and y slices.
+    pic = novice.Picture(array=array)
+    assert pic[-3:, 0] == pic[n - 3:, 0]
+    pic = novice.Picture(array=rgb_transpose(array))
+    assert pic[0, -3:] == pic[0, n - 3:]
+
+
 @raises(IndexError)
 def test_1d_getitem_raises():
     pic = novice.Picture.from_size((1, 1))
@@ -214,18 +235,6 @@ def test_getitem_with_step_raises():
     pic[::2, ::2]
 
 
-@raises(IndexError)
-def test_negative_index_raises():
-    pic = novice.Picture.from_size((1, 1))
-    pic[-1, -1]
-
-
-@raises(IndexError)
-def test_negative_slice_raises():
-    pic = novice.Picture.from_size((1, 1))
-    pic[-1:, -1:]
-
-
 @raises(IndexError)
 def test_out_of_bounds_indexing():
     pic = novice.open(SMALL_IMAGE_PATH)