mirror of
https://github.com/wassname/scikit-image.git
synced 2026-06-30 05:25:40 +08:00
Tony S Yu
130 lines
4.3 KiB
Cython
130 lines
4.3 KiB
Cython
"""
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Template matching using normalized cross-correlation.
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We use fast normalized cross-correlation algorithm (see [1]_ and [2]_) to
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compute match probability. This algorithm calculates the normalized
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cross-correlation of an image, `I`, with a template `T` according to the
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following equation::
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sum{ I(x, y) [T(x, y) - <T>] }
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-------------------------------------------------------
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sqrt(sum{ [I(x, y) - <I>]^2 } sum{ [T(x, y) - <T>]^2 })
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where `<T>` is the average of the template, and `<I>` is the average of the
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image *coincident with the template*, and sums are over the template and the
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image window coincident with the template. Note that the numerator is simply
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the cross-correlation of the image and the zero-mean template.
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To speed up calculations, we use summed-area tables (a.k.a. integral images) to
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quickly calculate sums of image windows inside the loop. This step relies on
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the following relation (see Eq. 10 of [1])::
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sum{ [I(x, y) - <I>]^2 } =
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sum{ I^2(x, y) } - [sum{ I(x, y) }]^2 / N_x N_y
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(Without this relation, you would need to subtract each image-window mean from
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the image window *before* squaring.)
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.. [1] Briechle and Hanebeck, "Template Matching using Fast Normalized
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Cross Correlation", Proceedings of the SPIE (2001).
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.. [2] J. P. Lewis, "Fast Normalized Cross-Correlation", Industrial Light and
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Magic.
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"""
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import cython
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cimport numpy as np
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import numpy as np
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from scipy.signal import fftconvolve
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from skimage.transform import integral
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cdef extern from "math.h":
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float sqrt(float x)
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float fabs(float x)
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@cython.boundscheck(False)
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cdef float integrate(np.ndarray[float, ndim=2, mode="c"] sat,
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int r0, int c0, int r1, int c1):
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"""
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Using a summed area table / integral image, calculate the sum
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over a given window.
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This function is the same as the `integrate` function in
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`skimage.transform.integrate`, but this Cython version significantly
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speeds up the code.
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Parameters
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----------
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sat : ndarray of float
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Summed area table / integral image.
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r0, c0 : int
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Top-left corner of block to be summed.
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r1, c1 : int
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Bottom-right corner of block to be summed.
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Returns
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-------
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S : int
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Sum over the given window.
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"""
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cdef float S = 0
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S += sat[r1, c1]
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if (r0 - 1 >= 0) and (c0 - 1 >= 0):
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S += sat[r0 - 1, c0 - 1]
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if (r0 - 1 >= 0):
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S -= sat[r0 - 1, c1]
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if (c0 - 1 >= 0):
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S -= sat[r1, c0 - 1]
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return S
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@cython.boundscheck(False)
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def match_template(np.ndarray[float, ndim=2, mode="c"] image,
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np.ndarray[float, ndim=2, mode="c"] template):
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cdef np.ndarray[float, ndim=2, mode="c"] corr
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cdef np.ndarray[float, ndim=2, mode="c"] image_sat
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cdef np.ndarray[float, ndim=2, mode="c"] image_sqr_sat
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cdef float template_mean = np.mean(template)
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cdef float template_ssd
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cdef float inv_area
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image_sat = integral.integral_image(image)
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image_sqr_sat = integral.integral_image(image**2)
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template -= template_mean
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template_ssd = np.sum(template**2)
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# use inversed area for accuracy
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inv_area = 1.0 / (template.shape[0] * template.shape[1])
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# when `dtype=float` is used, ascontiguousarray returns ``double``.
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corr = np.ascontiguousarray(fftconvolve(image,
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template[::-1, ::-1],
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mode="valid"),
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dtype=np.float32)
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cdef int i, j
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cdef float den, window_sqr_sum, window_mean_sqr, window_sum,
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# move window through convolution results, normalizing in the process
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for i in range(corr.shape[0]):
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for j in range(corr.shape[1]):
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# subtract 1 because `i_end` and `j_end` are used for indexing into
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# summed-area table, instead of slicing windows of the image.
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i_end = i + template.shape[0] - 1
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j_end = j + template.shape[1] - 1
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window_sum = integrate(image_sat, i, j, i_end, j_end)
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window_mean_sqr = window_sum * window_sum * inv_area
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window_sqr_sum = integrate(image_sqr_sat, i, j, i_end, j_end)
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if window_sqr_sum <= window_mean_sqr:
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corr[i, j] = 0
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continue
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den = sqrt((window_sqr_sum - window_mean_sqr) * template_ssd)
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corr[i, j] /= den
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return corr
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