Pull request: morphology fix

[Tony Yu]

I just submitted a pull request for a somewhat-subtle bug with some
morphology operations:

https://github.com/scikits-image/scikits.image/pull/62

Overly detailed explanation:

Using a structuring element with one-or-both sides being even-numbered gives
buggy output for morphological open/close and white/black tophat operations.
These issues arise because even-numbered structuring elements (selems)
aren't centered on a pixel. The dilate and erode operations are (correctly)
defined so that they are symmetric (dilation on a white feature is exactly
the opposite of erosion on an equivalent black feature). When chaining them
(e.g. open() = dilate(erode())), however, this symmetry ends up shifting
features. To fix this, one of the operations should be shifted. For example,
if erosion is done with the selem shifted slightly to the lower-right, the
dilation operation should shift the selem to the upper-left. This change
prevents features from being shifted.

The open/close operations aren't that bad: they just shift features. But,
when calling white/black tophat operations, the shifting can lead to
over/underflow of pixels. I've attached examples of the lena image after
calling white tophat, both before and after the fix. The underflow should be
apparent in the first image.

Best,
-Tony


Example code:

import os
import numpy as np
import matplotlib.pyplot as plt

from scikits.image import data_dir
from scikits.image.morphology import *

lena = np.load(os.path.join(data_dir, 'lena_GRAY_U8.npy'))

plt.imshow(greyscale_white_top_hat(lena, square(4)))
plt.show()
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