mirror of
https://github.com/wassname/scikit-image.git
synced 2026-06-29 09:06:25 +08:00
Johannes Schönberger
264 lines
8.4 KiB
Python
264 lines
8.4 KiB
Python
__all__ = ['hough', 'hough_peaks', 'probabilistic_hough']
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from itertools import izip as zip
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import numpy as np
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from scipy import ndimage
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from ._hough_transform import _probabilistic_hough
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from skimage import measure, morphology
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def _hough(img, theta=None):
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if img.ndim != 2:
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raise ValueError('The input image must be 2-D')
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if theta is None:
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theta = np.linspace(-np.pi / 2, np.pi / 2, 180)
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# compute the vertical bins (the distances)
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d = np.ceil(np.hypot(*img.shape))
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nr_bins = 2 * d
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bins = np.linspace(-d, d, nr_bins)
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# allocate the output image
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out = np.zeros((nr_bins, len(theta)), dtype=np.uint64)
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# precompute the sin and cos of the angles
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cos_theta = np.cos(theta)
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sin_theta = np.sin(theta)
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# find the indices of the non-zero values in
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# the input image
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y, x = np.nonzero(img)
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# x and y can be large, so we can't just broadcast to 2D
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# arrays as we may run out of memory. Instead we process
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# one vertical slice at a time.
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for i, (cT, sT) in enumerate(zip(cos_theta, sin_theta)):
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# compute the base distances
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distances = x * cT + y * sT
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# round the distances to the nearest integer
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# and shift them to a nonzero bin
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shifted = np.round(distances) - bins[0]
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# cast the shifted values to ints to use as indices
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indices = shifted.astype(np.int)
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# use bin count to accumulate the coefficients
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bincount = np.bincount(indices)
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# finally assign the proper values to the out array
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out[:len(bincount), i] = bincount
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return out, theta, bins
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_py_hough = _hough
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# try to import and use the faster Cython version if it exists
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try:
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from ._hough_transform import _hough
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except ImportError:
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pass
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def probabilistic_hough(img, threshold=10, line_length=50, line_gap=10,
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theta=None):
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"""Return lines from a progressive probabilistic line Hough transform.
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Parameters
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----------
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img : (M, N) ndarray
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Input image with nonzero values representing edges.
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threshold : int
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Threshold
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line_length : int, optional (default 50)
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Minimum accepted length of detected lines.
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Increase the parameter to extract longer lines.
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line_gap : int, optional, (default 10)
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Maximum gap between pixels to still form a line.
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Increase the parameter to merge broken lines more aggresively.
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theta : 1D ndarray, dtype=double, optional, default (-pi/2 .. pi/2)
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Angles at which to compute the transform, in radians.
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Returns
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-------
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lines : list
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List of lines identified, lines in format ((x0, y0), (x1, y0)), indicating
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line start and end.
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References
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----------
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.. [1] C. Galamhos, J. Matas and J. Kittler, "Progressive probabilistic
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Hough transform for line detection", in IEEE Computer Society
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Conference on Computer Vision and Pattern Recognition, 1999.
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"""
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return _probabilistic_hough(img, threshold, line_length, line_gap, theta)
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def hough(img, theta=None):
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"""Perform a straight line Hough transform.
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Parameters
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----------
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img : (M, N) ndarray
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Input image with nonzero values representing edges.
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theta : 1D ndarray of double
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Angles at which to compute the transform, in radians.
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Defaults to -pi/2 .. pi/2
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Returns
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-------
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H : 2-D ndarray of uint64
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Hough transform accumulator.
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theta : ndarray
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Angles at which the transform was computed.
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distances : ndarray
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Distance values.
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Examples
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--------
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Generate a test image:
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>>> img = np.zeros((100, 150), dtype=bool)
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>>> img[30, :] = 1
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>>> img[:, 65] = 1
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>>> img[35:45, 35:50] = 1
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>>> for i in range(90):
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... img[i, i] = 1
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>>> img += np.random.random(img.shape) > 0.95
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Apply the Hough transform:
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>>> out, angles, d = hough(img)
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.. plot:: hough_tf.py
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"""
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return _hough(img, theta)
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def hough_peaks(hspace, angles, dists, min_distance=10, min_angle=10,
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threshold=None, num_peaks=np.inf):
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"""Return peaks in hough transform.
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Identifies most prominent lines separated by a certain angle and distance in
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a hough transform. Non-maximum suppression with different sizes is applied
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separately in the first (distances) and second (angles) dimension of the
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hough space to identify peaks.
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Parameters
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----------
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hspace : (N, M) array
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Hough space returned by the `hough` function.
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angles : (M,) array
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Angles returned by the `hough` function. Assumed to be continuous
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(`angles[-1] - angles[0] == PI`).
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dists : (N, ) array
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Distances returned by the `hough` function.
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min_distance : int
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Minimum distance separating lines (maximum filter size for first
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dimension of hough space).
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min_angle : int
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Minimum angle separating lines (maximum filter size for second
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dimension of hough space).
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threshold : float
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Minimum intensity of peaks. Default is `0.5 * max(hspace)`.
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num_peaks : int
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Maximum number of peaks. When the number of peaks exceeds `num_peaks`,
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return `num_peaks` coordinates based on peak intensity.
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Returns
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-------
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hspace, angles, dists : tuple of array
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Peak values in hough space, angles and distances.
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Examples
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--------
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>>> import numpy as np
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>>> from skimage.transform import hough, hough_peaks
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>>> from skimage.draw import line
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>>> img = np.zeros((15, 15), dtype=np.bool_)
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>>> rr, cc = line(0, 0, 14, 14)
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>>> img[rr, cc] = 1
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>>> rr, cc = line(0, 14, 14, 0)
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>>> img[cc, rr] = 1
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>>> hspace, angles, dists = hough(img)
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>>> hspace, angles, dists = hough_peaks(hspace, angles, dists)
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>>> angles
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array([ 0.74590887, -0.79856126])
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>>> dists
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array([ 10.74418605, 0.51162791])
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"""
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hspace = hspace.copy()
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rows, cols = hspace.shape
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if threshold is None:
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threshold = 0.5 * np.max(hspace)
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distance_size = 2 * min_distance + 1
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angle_size = 2 * min_angle + 1
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hspace_max = ndimage.maximum_filter1d(hspace, size=distance_size, axis=0,
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mode='constant', cval=0)
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hspace_max = ndimage.maximum_filter1d(hspace_max, size=angle_size, axis=1,
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mode='constant', cval=0)
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mask = (hspace == hspace_max)
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hspace *= mask
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hspace_t = hspace > threshold
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label_hspace = morphology.label(hspace_t)
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props = measure.regionprops(label_hspace, ['Centroid'])
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coords = np.array([np.round(p['Centroid']) for p in props], dtype=int)
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hspace_peaks = []
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dist_peaks = []
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angle_peaks = []
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# relative coordinate grid for local neighbourhood suppression
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dist_ext, angle_ext = np.mgrid[-min_distance:min_distance + 1,
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-min_angle:min_angle + 1]
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for dist_idx, angle_idx in coords:
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accum = hspace[dist_idx, angle_idx]
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if accum > threshold:
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# absolute coordinate grid for local neighbourhood suppression
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dist_nh = dist_idx + dist_ext
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angle_nh = angle_idx + angle_ext
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# no reflection for distance neighbourhood
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dist_in = np.logical_and(dist_nh > 0, dist_nh < rows)
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dist_nh = dist_nh[dist_in]
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angle_nh = angle_nh[dist_in]
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# reflect angles and assume angles are continuous, e.g.
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# (..., 88, 89, -90, -89, ..., 89, -90, -89, ...)
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angle_low = angle_nh < 0
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dist_nh[angle_low] = rows - dist_nh[angle_low]
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angle_nh[angle_low] += cols
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angle_high = angle_nh >= cols
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dist_nh[angle_high] = rows - dist_nh[angle_high]
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angle_nh[angle_high] -= cols
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# suppress neighbourhood
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hspace[dist_nh, angle_nh] = 0
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# add current line to peaks
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hspace_peaks.append(accum)
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dist_peaks.append(dists[dist_idx])
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angle_peaks.append(angles[angle_idx])
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hspace_peaks = np.array(hspace_peaks)
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dist_peaks = np.array(dist_peaks)
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angle_peaks = np.array(angle_peaks)
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if num_peaks < len(hspace_peaks):
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idx_maxsort = np.argsort(hspace_peaks)[::-1][:num_peaks]
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hspace_peaks = hspace_peaks[idx_maxsort]
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dist_peaks = dist_peaks[idx_maxsort]
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angle_peaks = angle_peaks[idx_maxsort]
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return hspace_peaks, angle_peaks, dist_peaks
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