Add parameter to flip structuring element about its origin

This parameter only affects structuring elements (selem) with even-numbered sides, since an odd-numbered selem is centered about its origin. Note the selem isn't actually flipped, but shifted (i.e. I assume a selem that is symmetric in *shape*)

scikits/image/morphology/cmorph.pyx

@@ -8,6 +8,7 @@ import numpy as np
 
 cimport numpy as np
 cimport cython
+from cpython cimport bool
 
 STREL_DTYPE = np.uint8
 ctypedef np.uint8_t STREL_DTYPE_t
@@ -21,9 +22,14 @@ cdef inline int int_min(int a, int b): return a if a <= b else b
 @cython.boundscheck(False)
 def dilate(np.ndarray[IMAGE_DTYPE_t, ndim=2] image not None,
            np.ndarray[IMAGE_DTYPE_t, ndim=2] selem not None,
-           np.ndarray[IMAGE_DTYPE_t, ndim=2] out):
+           np.ndarray[IMAGE_DTYPE_t, ndim=2] out,
+           bool flip):
     cdef int hw = selem.shape[0] // 2
     cdef int hh = selem.shape[1] // 2
+    if flip:
+        hw -= 1
+        hh -= 1
+
     cdef int width = image.shape[0], height = image.shape[1]
     if out is None:
         out = np.zeros([width, height], dtype=IMAGE_DTYPE)
@@ -64,9 +70,14 @@ def dilate(np.ndarray[IMAGE_DTYPE_t, ndim=2] image not None,
 @cython.boundscheck(False)
 def erode(np.ndarray[IMAGE_DTYPE_t, ndim=2] image not None,
           np.ndarray[IMAGE_DTYPE_t, ndim=2] selem not None,
-          np.ndarray[IMAGE_DTYPE_t, ndim=2] out):
+          np.ndarray[IMAGE_DTYPE_t, ndim=2] out,
+          bool flip):
     cdef int hw = selem.shape[0] // 2
     cdef int hh = selem.shape[1] // 2
+    if flip:
+        hw -= 1
+        hh -= 1
+
     cdef int width = image.shape[0], height = image.shape[1]
     if out is None:
         out = np.zeros([width, height], dtype=IMAGE_DTYPE)

scikits/image/morphology/grey.py

@@ -9,7 +9,7 @@ import numpy as np
 
 eps = np.finfo(float).eps
 
-def greyscale_erode(image, selem, out=None):
+def greyscale_erode(image, selem, out=None, flip=False):
     """
     Performs a greyscale morphological erosion on an image given a flat
     structuring element. The eroded pixel at (i,j) is the minimum over all
@@ -27,6 +27,10 @@ def greyscale_erode(image, selem, out=None):
        The array to store the result of the morphology. If None is
        passed, a new array will be allocated.
 
+    flip : bool
+        Flip structuring element about center point. This only affects
+        eccentric structuring elements (i.e. selem with even numbered sides).
+
     Returns
     -------
     eroded : ndarray
@@ -36,12 +40,12 @@ def greyscale_erode(image, selem, out=None):
         raise NotImplementedError("In-place erosion not supported!")
     try:
         import scikits.image.morphology.cmorph as cmorph
-        out = cmorph.erode(image, selem, out=out)
+        out = cmorph.erode(image, selem, out=out, flip=flip)
         return out;
     except ImportError:
         raise ImportError("cmorph extension not available.")
 
-def greyscale_dilate(image, selem, out=None):
+def greyscale_dilate(image, selem, out=None, flip=False):
     """
     Performs a greyscale morphological dilation on an image given a flat
     structuring element. The dilated pixel at (i,j) is the maximum over all
@@ -60,6 +64,10 @@ def greyscale_dilate(image, selem, out=None):
        The array to store the result of the morphology. If None, is
        passed, a new array will be allocated.
 
+    flip : bool
+        Flip structuring element about center point. This only affects
+        eccentric structuring elements (i.e. selem with even numbered sides).
+
     Returns
     -------
     dilated : ndarray
@@ -69,7 +77,7 @@ def greyscale_dilate(image, selem, out=None):
         raise NotImplementedError("In-place dilation not supported!")
     try:
         from . import cmorph
-        out = cmorph.dilate(image, selem, out=out)
+        out = cmorph.dilate(image, selem, out=out, flip=flip)
         return out;
     except ImportError:
         raise ImportError("cmorph extension not available.")
@@ -79,6 +87,7 @@ def greyscale_open(image, selem, out=None):
     Performs a greyscale morphological opening on an image given a flat
     structuring element defined as a erosion followed by a dilation.
 
+
     Parameters
     ----------
     image : ndarray