mirror of
https://github.com/wassname/scikit-image.git
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Christian Sachs
316 lines
10 KiB
Python
316 lines
10 KiB
Python
"""
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Algorithms for computing the skeleton of a binary image
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"""
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import numpy as np
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from scipy import ndimage as ndi
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from ._skeletonize_cy import _fast_skeletonize, _skeletonize_loop, _table_lookup_index
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# --------- Skeletonization by morphological thinning ---------
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def skeletonize(image):
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"""Return the skeleton of a binary image.
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Thinning is used to reduce each connected component in a binary image
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to a single-pixel wide skeleton.
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Parameters
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----------
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image : numpy.ndarray
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A binary image containing the objects to be skeletonized. '1'
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represents foreground, and '0' represents background. It
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also accepts arrays of boolean values where True is foreground.
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Returns
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-------
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skeleton : ndarray
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A matrix containing the thinned image.
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See also
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--------
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medial_axis
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Notes
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-----
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The algorithm [1]_ works by making successive passes of the image,
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removing pixels on object borders. This continues until no
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more pixels can be removed. The image is correlated with a
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mask that assigns each pixel a number in the range [0...255]
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corresponding to each possible pattern of its 8 neighbouring
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pixels. A look up table is then used to assign the pixels a
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value of 0, 1, 2 or 3, which are selectively removed during
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the iterations.
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Note that this algorithm will give different results than a
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medial axis transform, which is also often referred to as
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"skeletonization".
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References
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----------
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.. [1] A fast parallel algorithm for thinning digital patterns,
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T. Y. Zhang and C. Y. Suen, Communications of the ACM,
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March 1984, Volume 27, Number 3.
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Examples
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--------
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>>> X, Y = np.ogrid[0:9, 0:9]
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>>> ellipse = (1./3 * (X - 4)**2 + (Y - 4)**2 < 3**2).astype(np.uint8)
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>>> ellipse
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array([[0, 0, 0, 1, 1, 1, 0, 0, 0],
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[0, 0, 1, 1, 1, 1, 1, 0, 0],
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[0, 0, 1, 1, 1, 1, 1, 0, 0],
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[0, 0, 1, 1, 1, 1, 1, 0, 0],
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[0, 0, 1, 1, 1, 1, 1, 0, 0],
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[0, 0, 1, 1, 1, 1, 1, 0, 0],
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[0, 0, 1, 1, 1, 1, 1, 0, 0],
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[0, 0, 1, 1, 1, 1, 1, 0, 0],
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[0, 0, 0, 1, 1, 1, 0, 0, 0]], dtype=uint8)
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>>> skel = skeletonize(ellipse)
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>>> skel.astype(np.uint8)
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array([[0, 0, 0, 0, 0, 0, 0, 0, 0],
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[0, 0, 0, 0, 0, 0, 0, 0, 0],
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[0, 0, 0, 0, 0, 0, 0, 0, 0],
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[0, 0, 0, 0, 1, 0, 0, 0, 0],
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[0, 0, 0, 0, 1, 0, 0, 0, 0],
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[0, 0, 0, 0, 1, 0, 0, 0, 0],
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[0, 0, 0, 0, 1, 0, 0, 0, 0],
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[0, 0, 0, 0, 0, 0, 0, 0, 0],
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[0, 0, 0, 0, 0, 0, 0, 0, 0]], dtype=uint8)
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"""
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# convert to unsigned int (this should work for boolean values)
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image = image.astype(np.uint8)
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# check some properties of the input image:
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# - 2D
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# - binary image with only 0's and 1's
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if image.ndim != 2:
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raise ValueError('Skeletonize requires a 2D array')
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if not np.all(np.in1d(image.flat, (0, 1))):
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raise ValueError('Image contains values other than 0 and 1')
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return _fast_skeletonize(image)
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# --------- Skeletonization by medial axis transform --------
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_eight_connect = ndi.generate_binary_structure(2, 2)
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def medial_axis(image, mask=None, return_distance=False):
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"""
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Compute the medial axis transform of a binary image
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Parameters
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----------
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image : binary ndarray, shape (M, N)
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The image of the shape to be skeletonized.
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mask : binary ndarray, shape (M, N), optional
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If a mask is given, only those elements in `image` with a true
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value in `mask` are used for computing the medial axis.
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return_distance : bool, optional
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If true, the distance transform is returned as well as the skeleton.
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Returns
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-------
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out : ndarray of bools
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Medial axis transform of the image
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dist : ndarray of ints, optional
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Distance transform of the image (only returned if `return_distance`
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is True)
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See also
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--------
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skeletonize
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Notes
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-----
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This algorithm computes the medial axis transform of an image
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as the ridges of its distance transform.
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The different steps of the algorithm are as follows
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* A lookup table is used, that assigns 0 or 1 to each configuration of
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the 3x3 binary square, whether the central pixel should be removed
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or kept. We want a point to be removed if it has more than one neighbor
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and if removing it does not change the number of connected components.
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* The distance transform to the background is computed, as well as
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the cornerness of the pixel.
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* The foreground (value of 1) points are ordered by
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the distance transform, then the cornerness.
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* A cython function is called to reduce the image to its skeleton. It
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processes pixels in the order determined at the previous step, and
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removes or maintains a pixel according to the lookup table. Because
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of the ordering, it is possible to process all pixels in only one
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pass.
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Examples
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--------
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>>> square = np.zeros((7, 7), dtype=np.uint8)
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>>> square[1:-1, 2:-2] = 1
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>>> square
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array([[0, 0, 0, 0, 0, 0, 0],
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[0, 0, 1, 1, 1, 0, 0],
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[0, 0, 1, 1, 1, 0, 0],
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[0, 0, 1, 1, 1, 0, 0],
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[0, 0, 1, 1, 1, 0, 0],
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[0, 0, 1, 1, 1, 0, 0],
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[0, 0, 0, 0, 0, 0, 0]], dtype=uint8)
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>>> medial_axis(square).astype(np.uint8)
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array([[0, 0, 0, 0, 0, 0, 0],
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[0, 0, 1, 0, 1, 0, 0],
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[0, 0, 0, 1, 0, 0, 0],
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[0, 0, 0, 1, 0, 0, 0],
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[0, 0, 0, 1, 0, 0, 0],
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[0, 0, 1, 0, 1, 0, 0],
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[0, 0, 0, 0, 0, 0, 0]], dtype=uint8)
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"""
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global _eight_connect
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if mask is None:
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masked_image = image.astype(np.bool)
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else:
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masked_image = image.astype(bool).copy()
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masked_image[~mask] = False
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#
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# Build lookup table - three conditions
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# 1. Keep only positive pixels (center_is_foreground array).
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# AND
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# 2. Keep if removing the pixel results in a different connectivity
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# (if the number of connected components is different with and
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# without the central pixel)
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# OR
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# 3. Keep if # pixels in neighbourhood is 2 or less
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# Note that table is independent of image
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center_is_foreground = (np.arange(512) & 2**4).astype(bool)
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table = (center_is_foreground # condition 1.
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&
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(np.array([ndi.label(_pattern_of(index), _eight_connect)[1] !=
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ndi.label(_pattern_of(index & ~ 2**4),
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_eight_connect)[1]
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for index in range(512)]) # condition 2
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np.array([np.sum(_pattern_of(index)) < 3 for index in range(512)]))
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# condition 3
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)
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# Build distance transform
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distance = ndi.distance_transform_edt(masked_image)
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if return_distance:
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store_distance = distance.copy()
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# Corners
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# The processing order along the edge is critical to the shape of the
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# resulting skeleton: if you process a corner first, that corner will
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# be eroded and the skeleton will miss the arm from that corner. Pixels
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# with fewer neighbors are more "cornery" and should be processed last.
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# We use a cornerness_table lookup table where the score of a
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# configuration is the number of background (0-value) pixels in the
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# 3x3 neighbourhood
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cornerness_table = np.array([9 - np.sum(_pattern_of(index))
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for index in range(512)])
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corner_score = _table_lookup(masked_image, cornerness_table)
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# Define arrays for inner loop
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i, j = np.mgrid[0:image.shape[0], 0:image.shape[1]]
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result = masked_image.copy()
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distance = distance[result]
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i = np.ascontiguousarray(i[result], dtype=np.intp)
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j = np.ascontiguousarray(j[result], dtype=np.intp)
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result = np.ascontiguousarray(result, np.uint8)
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# Determine the order in which pixels are processed.
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# We use a random # for tiebreaking. Assign each pixel in the image a
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# predictable, random # so that masking doesn't affect arbitrary choices
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# of skeletons
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#
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generator = np.random.RandomState(0)
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tiebreaker = generator.permutation(np.arange(masked_image.sum()))
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order = np.lexsort((tiebreaker,
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corner_score[masked_image],
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distance))
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order = np.ascontiguousarray(order, dtype=np.int32)
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table = np.ascontiguousarray(table, dtype=np.uint8)
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# Remove pixels not belonging to the medial axis
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_skeletonize_loop(result, i, j, order, table)
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result = result.astype(bool)
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if not mask is None:
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result[~mask] = image[~mask]
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if return_distance:
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return result, store_distance
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else:
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return result
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def _pattern_of(index):
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"""
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Return the pattern represented by an index value
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Byte decomposition of index
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"""
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return np.array([[index & 2**0, index & 2**1, index & 2**2],
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[index & 2**3, index & 2**4, index & 2**5],
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[index & 2**6, index & 2**7, index & 2**8]], bool)
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def _table_lookup(image, table):
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"""
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Perform a morphological transform on an image, directed by its
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neighbors
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Parameters
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----------
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image : ndarray
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A binary image
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table : ndarray
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A 512-element table giving the transform of each pixel given
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the values of that pixel and its 8-connected neighbors.
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border_value : bool
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The value of pixels beyond the border of the image.
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Returns
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-------
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result : ndarray of same shape as `image`
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Transformed image
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Notes
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-----
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The pixels are numbered like this::
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0 1 2
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3 4 5
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6 7 8
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The index at a pixel is the sum of 2**<pixel-number> for pixels
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that evaluate to true.
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"""
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#
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# We accumulate into the indexer to get the index into the table
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# at each point in the image
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#
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if image.shape[0] < 3 or image.shape[1] < 3:
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image = image.astype(bool)
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indexer = np.zeros(image.shape, int)
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indexer[1:, 1:] += image[:-1, :-1] * 2**0
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indexer[1:, :] += image[:-1, :] * 2**1
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indexer[1:, :-1] += image[:-1, 1:] * 2**2
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indexer[:, 1:] += image[:, :-1] * 2**3
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indexer[:, :] += image[:, :] * 2**4
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indexer[:, :-1] += image[:, 1:] * 2**5
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indexer[:-1, 1:] += image[1:, :-1] * 2**6
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indexer[:-1, :] += image[1:, :] * 2**7
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indexer[:-1, :-1] += image[1:, 1:] * 2**8
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else:
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indexer = _table_lookup_index(np.ascontiguousarray(image, np.uint8))
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image = table[indexer]
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return image
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