mirror of
https://github.com/wassname/scikit-image.git
synced 2026-06-29 01:11:56 +08:00
Johannes Schönberger
76 lines
2.3 KiB
Python
76 lines
2.3 KiB
Python
import numpy as np
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from skimage.util import view_as_blocks, pad
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def block_reduce(image, block_size, func=np.sum, cval=0):
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"""Down-sample image by applying function to local blocks.
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Parameters
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----------
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image : ndarray
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N-dimensional input image.
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block_size : array_like
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Array containing down-sampling integer factor along each axis.
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func : callable
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Function object which is used to calculate the return value for each
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local block. This function must implement an ``axis`` parameter such as
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``numpy.sum`` or ``numpy.min``.
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cval : float
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Constant padding value if image is not perfectly divisible by the
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block size.
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Returns
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-------
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image : ndarray
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Down-sampled image with same number of dimensions as input image.
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Examples
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--------
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>>> from skimage.measure import block_reduce
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>>> image = np.arange(3*3*4).reshape(3, 3, 4)
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>>> image # doctest: +NORMALIZE_WHITESPACE
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array([[[ 0, 1, 2, 3],
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[ 4, 5, 6, 7],
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[ 8, 9, 10, 11]],
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[[12, 13, 14, 15],
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[16, 17, 18, 19],
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[20, 21, 22, 23]],
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[[24, 25, 26, 27],
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[28, 29, 30, 31],
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[32, 33, 34, 35]]])
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>>> block_reduce(image, block_size=(3, 3, 1), func=np.mean)
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array([[[ 16., 17., 18., 19.]]])
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>>> image_max1 = block_reduce(image, block_size=(1, 3, 4), func=np.max)
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>>> image_max1 # doctest: +NORMALIZE_WHITESPACE
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array([[[11]],
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[[23]],
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[[35]]])
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>>> image_max2 = block_reduce(image, block_size=(3, 1, 4), func=np.max)
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>>> image_max2 # doctest: +NORMALIZE_WHITESPACE
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array([[[27],
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[31],
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[35]]])
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"""
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if len(block_size) != image.ndim:
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raise ValueError("`block_size` must have the same length "
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"as `image.shape`.")
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pad_width = []
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for i in range(len(block_size)):
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if image.shape[i] % block_size[i] != 0:
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after_width = block_size[i] - (image.shape[i] % block_size[i])
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else:
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after_width = 0
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pad_width.append((0, after_width))
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image = pad(image, pad_width=pad_width, mode='constant',
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constant_values=cval)
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out = view_as_blocks(image, block_size)
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for i in range(len(out.shape) // 2):
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out = func(out, axis=-1)
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return out
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