Change rank.pyx to _rank.py

- Cython was unnecessary in the main rank module
- Don't shadow package name in the module
- Add support for int8 images.

skimage/filter/rank/__init__.py

@@ -1,6 +1,6 @@
-from .rank import (autolevel, bottomhat, equalize, gradient, maximum, mean,
-                   meansubtraction, median, minimum, modal, morph_contr_enh,
-                   pop, threshold, tophat, noise_filter, entropy, otsu)
+from ._rank import (autolevel, bottomhat, equalize, gradient, maximum, mean,
+                    meansubtraction, median, minimum, modal, morph_contr_enh,
+                    pop, threshold, tophat, noise_filter, entropy, otsu)
 from .percentile_rank import (percentile_autolevel, percentile_gradient,
                               percentile_mean, percentile_mean_subtraction,
                               percentile_morph_contr_enh, percentile,

skimage/filter/rank/_rank.py

@@ -40,13 +40,10 @@ def _apply(func8, func16, image, selem, out, mask, shift_x, shift_y):
     if image is out:
         raise NotImplementedError("Cannot perform rank operation in place.")
 
-    if image.dtype == np.uint8:
-        if func8 is None:
-            raise TypeError("Not implemented for uint8 image.")
-        if out is None:
-            out = np.zeros(image.shape, dtype=np.uint8)
-        func8(image, selem, shift_x=shift_x, shift_y=shift_y,
-              mask=mask, out=out)
+    is_8bit = image.dtype in (np.uint8, np.int8)
+
+    if func8 is not None and (is_8bit or func16 is None):
+        out = _apply8(func8, image, selem, out, mask, shift_x, shift_y)
     elif image.dtype == np.uint16:
         if func16 is None:
             raise TypeError("Not implemented for uint16 image.")
@@ -63,6 +60,15 @@ def _apply(func8, func16, image, selem, out, mask, shift_x, shift_y):
     return out
 
 
+def _apply8(func8, image, selem, out, mask, shift_x, shift_y):
+    if out is None:
+        out = np.zeros(image.shape, dtype=np.uint8)
+    image = img_as_ubyte(image)
+    func8(image, selem, shift_x=shift_x, shift_y=shift_y,
+          mask=mask, out=out)
+    return out
+
+
 def autolevel(image, selem, out=None, mask=None, shift_x=False, shift_y=False):
     """Autolevel image using local histogram.
 

skimage/filter/rank/tests/test_rank.py

@@ -1,6 +1,7 @@
 import numpy as np
 from numpy.testing import run_module_suite, assert_array_equal, assert_raises
 
+from skimage import img_as_ubyte
 from skimage import data, util
 from skimage.morphology import cmorph, disk
 from skimage.filter import rank
@@ -186,6 +187,29 @@ def test_compare_autolevels_16bit():
     assert_array_equal(loc_autolevel, loc_perc_autolevel)
 
 
+def test_compare_8bit_unsigned_vs_signed():
+    # filters applied on 8-bit image ore 16-bit image (having only real 8-bit of
+    # dynamic) should be identical
+
+    # Create signed int8 image that and convert it to uint8
+    image = img_as_ubyte(data.camera())
+    image[image > 127] = 0
+    image_s = image.astype(np.int8)
+    image_u = img_as_ubyte(image_s)
+
+    assert_array_equal(image_u, img_as_ubyte(image_s))
+
+    methods = ['autolevel', 'bottomhat', 'equalize', 'gradient', 'maximum',
+               'mean', 'meansubtraction', 'median', 'minimum', 'modal',
+               'morph_contr_enh', 'pop', 'threshold', 'tophat']
+
+    for method in methods:
+        func = getattr(rank, method)
+        out_u = func(image_u, disk(3))
+        out_s = func(image_s, disk(3))
+        assert_array_equal(out_u, out_s)
+
+
 def test_compare_8bit_vs_16bit():
     # filters applied on 8-bit image ore 16-bit image (having only real 8-bit of
     # dynamic) should be identical