Module: util
¶

Map a function in parallel across an array. 

Return an image showing the differences between two images. 

Crop array ar by crop_width along each dimension. 

Return intensity limits, i.e. (min, max) tuple, of the image’s dtype. 

Convert an image to boolean format. 

Convert an image to floating point format. 

Convert an image to singleprecision (32bit) floating point format. 

Convert an image to doubleprecision (64bit) floating point format. 

Convert an image to 16bit signed integer format. 

Convert an image to 8bit unsigned integer format. 

Convert an image to 16bit unsigned integer format. 

Invert an image. 

Assign unique integer labels to coordinates on an image mask 

Map values from input array from input_vals to output_vals. 

Create a montage of several single or multichannel images. 

Function to add random noise of various types to a floatingpoint image. 

Find n_points regularly spaced along ar_shape. 

Return an image with ~`n_points` regularlyspaced nonzero pixels. 
Remove repeated rows from a 2D array. 


Block view of the input ndimensional array (using restriding). 

Rolling window view of the input ndimensional array. 
apply_parallel¶

skimage.util.
apply_parallel
(function, array, chunks=None, depth=0, mode=None, extra_arguments=(), extra_keywords={}, *, dtype=None, compute=None, channel_axis=None, multichannel=False)[source]¶ Map a function in parallel across an array.
Split an array into possibly overlapping chunks of a given depth and boundary type, call the given function in parallel on the chunks, combine the chunks and return the resulting array.
 Parameters
 functionfunction
Function to be mapped which takes an array as an argument.
 arraynumpy array or dask array
Array which the function will be applied to.
 chunksint, tuple, or tuple of tuples, optional
A single integer is interpreted as the length of one side of a square chunk that should be tiled across the array. One tuple of length
array.ndim
represents the shape of a chunk, and it is tiled across the array. A list of tuples of lengthndim
, where each subtuple is a sequence of chunk sizes along the corresponding dimension. If None, the array is broken up into chunks based on the number of available cpus. More information about chunks is in the documentation here. When channel_axis is not None, the tuples can be lengthndim  1
and a single chunk will be used along the channel axis. depthint or sequence of int, optional
The depth of the added boundary cells. A tuple can be used to specify a different depth per array axis. Defaults to zero. When channel_axis is not None, and a tuple of length
ndim  1
is provided, a depth of 0 will be used along the channel axis. mode{‘reflect’, ‘symmetric’, ‘periodic’, ‘wrap’, ‘nearest’, ‘edge’}, optional
Type of external boundary padding.
 extra_argumentstuple, optional
Tuple of arguments to be passed to the function.
 extra_keywordsdictionary, optional
Dictionary of keyword arguments to be passed to the function.
 dtypedatatype or None, optional
The datatype of the function output. If None, Dask will attempt to infer this by calling the function on data of shape
(1,) * ndim
. For functions expecting RGB or multichannel data this may be problematic. In such cases, the user should manually specify this dtype argument instead.New in version 0.18:
dtype
was added in 0.18. computebool, optional
If
True
, compute eagerly returning a NumPy Array. IfFalse
, compute lazily returning a Dask Array. IfNone
(default), compute based on array type provided (eagerly for NumPy Arrays and lazily for Dask Arrays). channel_axisint or None, optional
If None, the image is assumed to be a grayscale (single channel) image. Otherwise, this parameter indicates which axis of the array corresponds to channels.
 multichannelbool, optional
If chunks is None and multichannel is True, this function will keep only a single chunk along the channels axis. When depth is specified as a scalar value, that depth will be applied only to the nonchannels axes (a depth of 0 will be used along the channels axis). If the user manually specified both chunks and a depth tuple, then this argument will have no effect. This argument is deprecated: specify channel_axis instead.
New in version 0.18:
multichannel
was added in 0.18.
 Returns
 outndarray or dask Array
Returns the result of the applying the operation. Type is dependent on the
compute
argument.
 Other Parameters
 multichannelDEPRECATED
Deprecated in favor of channel_axis.
Deprecated since version 0.19.
Notes
Numpy edge modes ‘symmetric’, ‘wrap’, and ‘edge’ are converted to the equivalent
dask
boundary modes ‘reflect’, ‘periodic’ and ‘nearest’, respectively. Settingcompute=False
can be useful for chaining later operations. For example region selection to preview a result or storing large data to disk instead of loading in memory.
compare_images¶

skimage.util.
compare_images
(image1, image2, method='diff', *, n_tiles=(8, 8))[source]¶ Return an image showing the differences between two images.
New in version 0.16.
 Parameters
 image1, image22D array
Images to process, must be of the same shape.
 methodstring, optional
Method used for the comparison. Valid values are {‘diff’, ‘blend’, ‘checkerboard’}. Details are provided in the note section.
 n_tilestuple, optional
Used only for the checkerboard method. Specifies the number of tiles (row, column) to divide the image.
 Returns
 comparison2D array
Image showing the differences.
Notes
'diff'
computes the absolute difference between the two images.'blend'
computes the mean value.'checkerboard'
makes tiles of dimension n_tiles that display alternatively the first and the second image.
crop¶

skimage.util.
crop
(ar, crop_width, copy=False, order='K')[source]¶ Crop array ar by crop_width along each dimension.
 Parameters
 ararraylike of rank N
Input array.
 crop_width{sequence, int}
Number of values to remove from the edges of each axis.
((before_1, after_1),
…(before_N, after_N))
specifies unique crop widths at the start and end of each axis.((before, after),) or (before, after)
specifies a fixed start and end crop for every axis.(n,)
orn
for integern
is a shortcut for before = after =n
for all axes. copybool, optional
If True, ensure the returned array is a contiguous copy. Normally, a crop operation will return a discontiguous view of the underlying input array.
 order{‘C’, ‘F’, ‘A’, ‘K’}, optional
If
copy==True
, control the memory layout of the copy. Seenp.copy
.
 Returns
 croppedarray
The cropped array. If
copy=False
(default), this is a sliced view of the input array.
dtype_limits¶

skimage.util.
dtype_limits
(image, clip_negative=False)[source]¶ Return intensity limits, i.e. (min, max) tuple, of the image’s dtype.
 Parameters
 imagendarray
Input image.
 clip_negativebool, optional
If True, clip the negative range (i.e. return 0 for min intensity) even if the image dtype allows negative values.
 Returns
 imin, imaxtuple
Lower and upper intensity limits.
img_as_bool¶

skimage.util.
img_as_bool
(image, force_copy=False)[source]¶ Convert an image to boolean format.
 Parameters
 imagendarray
Input image.
 force_copybool, optional
Force a copy of the data, irrespective of its current dtype.
 Returns
 outndarray of bool (bool_)
Output image.
Notes
The upper half of the input dtype’s positive range is True, and the lower half is False. All negative values (if present) are False.
img_as_float¶

skimage.util.
img_as_float
(image, force_copy=False)[source]¶ Convert an image to floating point format.
This function is similar to
img_as_float64
, but will not convert lowerprecision floating point arrays to float64. Parameters
 imagendarray
Input image.
 force_copybool, optional
Force a copy of the data, irrespective of its current dtype.
 Returns
 outndarray of float
Output image.
Notes
The range of a floating point image is [0.0, 1.0] or [1.0, 1.0] when converting from unsigned or signed datatypes, respectively. If the input image has a float type, intensity values are not modified and can be outside the ranges [0.0, 1.0] or [1.0, 1.0].
img_as_float32¶

skimage.util.
img_as_float32
(image, force_copy=False)[source]¶ Convert an image to singleprecision (32bit) floating point format.
 Parameters
 imagendarray
Input image.
 force_copybool, optional
Force a copy of the data, irrespective of its current dtype.
 Returns
 outndarray of float32
Output image.
Notes
The range of a floating point image is [0.0, 1.0] or [1.0, 1.0] when converting from unsigned or signed datatypes, respectively. If the input image has a float type, intensity values are not modified and can be outside the ranges [0.0, 1.0] or [1.0, 1.0].
img_as_float64¶

skimage.util.
img_as_float64
(image, force_copy=False)[source]¶ Convert an image to doubleprecision (64bit) floating point format.
 Parameters
 imagendarray
Input image.
 force_copybool, optional
Force a copy of the data, irrespective of its current dtype.
 Returns
 outndarray of float64
Output image.
Notes
The range of a floating point image is [0.0, 1.0] or [1.0, 1.0] when converting from unsigned or signed datatypes, respectively. If the input image has a float type, intensity values are not modified and can be outside the ranges [0.0, 1.0] or [1.0, 1.0].
img_as_int¶

skimage.util.
img_as_int
(image, force_copy=False)[source]¶ Convert an image to 16bit signed integer format.
 Parameters
 imagendarray
Input image.
 force_copybool, optional
Force a copy of the data, irrespective of its current dtype.
 Returns
 outndarray of int16
Output image.
Notes
The values are scaled between 32768 and 32767. If the input datatype is positiveonly (e.g., uint8), then the output image will still only have positive values.
img_as_ubyte¶

skimage.util.
img_as_ubyte
(image, force_copy=False)[source]¶ Convert an image to 8bit unsigned integer format.
 Parameters
 imagendarray
Input image.
 force_copybool, optional
Force a copy of the data, irrespective of its current dtype.
 Returns
 outndarray of ubyte (uint8)
Output image.
Notes
Negative input values will be clipped. Positive values are scaled between 0 and 255.
img_as_uint¶

skimage.util.
img_as_uint
(image, force_copy=False)[source]¶ Convert an image to 16bit unsigned integer format.
 Parameters
 imagendarray
Input image.
 force_copybool, optional
Force a copy of the data, irrespective of its current dtype.
 Returns
 outndarray of uint16
Output image.
Notes
Negative input values will be clipped. Positive values are scaled between 0 and 65535.
invert¶

skimage.util.
invert
(image, signed_float=False)[source]¶ Invert an image.
Invert the intensity range of the input image, so that the dtype maximum is now the dtype minimum, and viceversa. This operation is slightly different depending on the input dtype:
unsigned integers: subtract the image from the dtype maximum
signed integers: subtract the image from 1 (see Notes)
floats: subtract the image from 1 (if signed_float is False, so we assume the image is unsigned), or from 0 (if signed_float is True).
See the examples for clarification.
 Parameters
 imagendarray
Input image.
 signed_floatbool, optional
If True and the image is of type float, the range is assumed to be [1, 1]. If False and the image is of type float, the range is assumed to be [0, 1].
 Returns
 invertedndarray
Inverted image.
Notes
Ideally, for signed integers we would simply multiply by 1. However, signed integer ranges are asymmetric. For example, for np.int8, the range of possible values is [128, 127], so that 128 * 1 equals 128! By subtracting from 1, we correctly map the maximum dtype value to the minimum.
Examples
>>> img = np.array([[100, 0, 200], ... [ 0, 50, 0], ... [ 30, 0, 255]], np.uint8) >>> invert(img) array([[155, 255, 55], [255, 205, 255], [225, 255, 0]], dtype=uint8) >>> img2 = np.array([[ 2, 0, 128], ... [127, 0, 5]], np.int8) >>> invert(img2) array([[ 1, 1, 127], [128, 1, 6]], dtype=int8) >>> img3 = np.array([[ 0., 1., 0.5, 0.75]]) >>> invert(img3) array([[1. , 0. , 0.5 , 0.25]]) >>> img4 = np.array([[ 0., 1., 1., 0.25]]) >>> invert(img4, signed_float=True) array([[0. , 1. , 1. , 0.25]])
label_points¶

skimage.util.
label_points
(coords, output_shape)[source]¶ Assign unique integer labels to coordinates on an image mask
 Parameters
 coords: ndarray
An array of N coordinates with dimension D
 output_shape: tuple
The shape of the mask on which coords are labelled
 Returns
 labels: ndarray
A mask of zeroes containing unique integer labels at the coords
Notes
The labels are assigned to coordinates that are converted to integer and considered to start from 0.
Coordinates that are out of range of the mask raise an IndexError.
Negative coordinates raise a ValueError
Examples
>>> import numpy as np >>> from skimage.util._label import label_points >>> coords = np.array([[0, 1], [2, 2]]) >>> output_shape = (5, 5) >>> mask = label_points(coords, output_shape) >>> mask array([[0, 1, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 2, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0]], dtype=uint64)
map_array¶

skimage.util.
map_array
(input_arr, input_vals, output_vals, out=None)[source]¶ Map values from input array from input_vals to output_vals.
 Parameters
 input_arrarray of int, shape (M[, N][, P][, …])
The input label image.
 input_valsarray of int, shape (N,)
The values to map from.
 output_valsarray, shape (N,)
The values to map to.
 out: array, same shape as `input_arr`
The output array. Will be created if not provided. It should have the same dtype as output_vals.
 Returns
 outarray, same shape as input_arr
The array of mapped values.
montage¶

skimage.util.
montage
(arr_in, fill='mean', rescale_intensity=False, grid_shape=None, padding_width=0, multichannel=False, *, channel_axis=None)[source]¶ Create a montage of several single or multichannel images.
Create a rectangular montage from an input array representing an ensemble of equally shaped single (gray) or multichannel (color) images.
For example,
montage(arr_in)
called with the following arr_in1
2
3
will return
1
2
3
where the ‘*’ patch will be determined by the fill parameter.
 Parameters
 arr_in(K, M, N[, C]) ndarray
An array representing an ensemble of K images of equal shape.
 fillfloat or arraylike of floats or ‘mean’, optional
Value to fill the padding areas and/or the extra tiles in the output array. Has to be float for single channel collections. For multichannel collections has to be an arraylike of shape of number of channels. If mean, uses the mean value over all images.
 rescale_intensitybool, optional
Whether to rescale the intensity of each image to [0, 1].
 grid_shapetuple, optional
The desired grid shape for the montage (ntiles_row, ntiles_column). The default aspect ratio is square.
 padding_widthint, optional
The size of the spacing between the tiles and between the tiles and the borders. If nonzero, makes the boundaries of individual images easier to perceive.
 multichannelboolean, optional
If True, the last arr_in dimension is threated as a color channel, otherwise as spatial. This argument is deprecated: specify channel_axis instead.
 channel_axisint or None, optional
If None, the image is assumed to be a grayscale (single channel) image. Otherwise, this parameter indicates which axis of the array corresponds to channels.
 Returns
 arr_out(K*(M+p)+p, K*(N+p)+p[, C]) ndarray
Output array with input images glued together (including padding p).
 Other Parameters
 multichannelDEPRECATED
Deprecated in favor of channel_axis.
Deprecated since version 0.19.
Examples
>>> import numpy as np >>> from skimage.util import montage >>> arr_in = np.arange(3 * 2 * 2).reshape(3, 2, 2) >>> arr_in array([[[ 0, 1], [ 2, 3]], [[ 4, 5], [ 6, 7]], [[ 8, 9], [10, 11]]]) >>> arr_out = montage(arr_in) >>> arr_out.shape (4, 4) >>> arr_out array([[ 0, 1, 4, 5], [ 2, 3, 6, 7], [ 8, 9, 5, 5], [10, 11, 5, 5]]) >>> arr_in.mean() 5.5 >>> arr_out_nonsquare = montage(arr_in, grid_shape=(1, 3)) >>> arr_out_nonsquare array([[ 0, 1, 4, 5, 8, 9], [ 2, 3, 6, 7, 10, 11]]) >>> arr_out_nonsquare.shape (2, 6)
Examples using skimage.util.montage
¶
random_noise¶

skimage.util.
random_noise
(image, mode='gaussian', seed=None, clip=True, **kwargs)[source]¶ Function to add random noise of various types to a floatingpoint image.
 Parameters
 imagendarray
Input image data. Will be converted to float.
 modestr, optional
One of the following strings, selecting the type of noise to add:
‘gaussian’ Gaussiandistributed additive noise.
 ‘localvar’ Gaussiandistributed additive noise, with specified
local variance at each point of image.
‘poisson’ Poissondistributed noise generated from the data.
‘salt’ Replaces random pixels with 1.
 ‘pepper’ Replaces random pixels with 0 (for unsigned images) or
1 (for signed images).
 ‘s&p’ Replaces random pixels with either 1 or low_val, where
low_val is 0 for unsigned images or 1 for signed images.
 ‘speckle’ Multiplicative noise using out = image + n*image, where
n is Gaussian noise with specified mean & variance.
 seed{None, int,
numpy.random.Generator
}, optional If seed is None the
numpy.random.Generator
singleton is used. If seed is an int, a newGenerator
instance is used, seeded with seed. If seed is already aGenerator
instance then that instance is used.This will set the random seed before generating noise, for valid pseudorandom comparisons.
 clipbool, optional
If True (default), the output will be clipped after noise applied for modes ‘speckle’, ‘poisson’, and ‘gaussian’. This is needed to maintain the proper image data range. If False, clipping is not applied, and the output may extend beyond the range [1, 1].
 meanfloat, optional
Mean of random distribution. Used in ‘gaussian’ and ‘speckle’. Default : 0.
 varfloat, optional
Variance of random distribution. Used in ‘gaussian’ and ‘speckle’. Note: variance = (standard deviation) ** 2. Default : 0.01
 local_varsndarray, optional
Array of positive floats, same shape as image, defining the local variance at every image point. Used in ‘localvar’.
 amountfloat, optional
Proportion of image pixels to replace with noise on range [0, 1]. Used in ‘salt’, ‘pepper’, and ‘salt & pepper’. Default : 0.05
 salt_vs_pepperfloat, optional
Proportion of salt vs. pepper noise for ‘s&p’ on range [0, 1]. Higher values represent more salt. Default : 0.5 (equal amounts)
 Returns
 outndarray
Output floatingpoint image data on range [0, 1] or [1, 1] if the input image was unsigned or signed, respectively.
Notes
Speckle, Poisson, Localvar, and Gaussian noise may generate noise outside the valid image range. The default is to clip (not alias) these values, but they may be preserved by setting clip=False. Note that in this case the output may contain values outside the ranges [0, 1] or [1, 1]. Use this option with care.
Because of the prevalence of exclusively positive floatingpoint images in intermediate calculations, it is not possible to intuit if an input is signed based on dtype alone. Instead, negative values are explicitly searched for. Only if found does this function assume signed input. Unexpected results only occur in rare, poorly exposes cases (e.g. if all values are above 50 percent gray in a signed image). In this event, manually scaling the input to the positive domain will solve the problem.
The Poisson distribution is only defined for positive integers. To apply this noise type, the number of unique values in the image is found and the next round power of two is used to scale up the floatingpoint result, after which it is scaled back down to the floatingpoint image range.
To generate Poisson noise against a signed image, the signed image is temporarily converted to an unsigned image in the floating point domain, Poisson noise is generated, then it is returned to the original range.
regular_grid¶

skimage.util.
regular_grid
(ar_shape, n_points)[source]¶ Find n_points regularly spaced along ar_shape.
The returned points (as slices) should be as close to cubicallyspaced as possible. Essentially, the points are spaced by the Nth root of the input array size, where N is the number of dimensions. However, if an array dimension cannot fit a full step size, it is “discarded”, and the computation is done for only the remaining dimensions.
 Parameters
 ar_shapearraylike of ints
The shape of the space embedding the grid.
len(ar_shape)
is the number of dimensions. n_pointsint
The (approximate) number of points to embed in the space.
 Returns
 slicestuple of slice objects
A slice along each dimension of ar_shape, such that the intersection of all the slices give the coordinates of regularly spaced points.
Changed in version 0.14.1: In scikitimage 0.14.1 and 0.15, the return type was changed from a list to a tuple to ensure compatibility with Numpy 1.15 and higher. If your code requires the returned result to be a list, you may convert the output of this function to a list with:
>>> result = list(regular_grid(ar_shape=(3, 20, 40), n_points=8))
Examples
>>> ar = np.zeros((20, 40)) >>> g = regular_grid(ar.shape, 8) >>> g (slice(5, None, 10), slice(5, None, 10)) >>> ar[g] = 1 >>> ar.sum() 8.0 >>> ar = np.zeros((20, 40)) >>> g = regular_grid(ar.shape, 32) >>> g (slice(2, None, 5), slice(2, None, 5)) >>> ar[g] = 1 >>> ar.sum() 32.0 >>> ar = np.zeros((3, 20, 40)) >>> g = regular_grid(ar.shape, 8) >>> g (slice(1, None, 3), slice(5, None, 10), slice(5, None, 10)) >>> ar[g] = 1 >>> ar.sum() 8.0
Examples using skimage.util.regular_grid
¶
regular_seeds¶

skimage.util.
regular_seeds
(ar_shape, n_points, dtype=<class 'int'>)[source]¶ Return an image with ~`n_points` regularlyspaced nonzero pixels.
 Parameters
 ar_shapetuple of int
The shape of the desired output image.
 n_pointsint
The desired number of nonzero points.
 dtypenumpy data type, optional
The desired data type of the output.
 Returns
 seed_imgarray of int or bool
The desired image.
Examples
>>> regular_seeds((5, 5), 4) array([[0, 0, 0, 0, 0], [0, 1, 0, 2, 0], [0, 0, 0, 0, 0], [0, 3, 0, 4, 0], [0, 0, 0, 0, 0]])
unique_rows¶

skimage.util.
unique_rows
(ar)[source]¶ Remove repeated rows from a 2D array.
In particular, if given an array of coordinates of shape (Npoints, Ndim), it will remove repeated points.
 Parameters
 ar2D ndarray
The input array.
 Returns
 ar_out2D ndarray
A copy of the input array with repeated rows removed.
 Raises
 ValueErrorif ar is not twodimensional.
Notes
The function will generate a copy of ar if it is not Ccontiguous, which will negatively affect performance for large input arrays.
Examples
>>> ar = np.array([[1, 0, 1], ... [0, 1, 0], ... [1, 0, 1]], np.uint8) >>> unique_rows(ar) array([[0, 1, 0], [1, 0, 1]], dtype=uint8)
view_as_blocks¶

skimage.util.
view_as_blocks
(arr_in, block_shape)[source]¶ Block view of the input ndimensional array (using restriding).
Blocks are nonoverlapping views of the input array.
 Parameters
 arr_inndarray
Nd input array.
 block_shapetuple
The shape of the block. Each dimension must divide evenly into the corresponding dimensions of arr_in.
 Returns
 arr_outndarray
Block view of the input array.
Examples
>>> import numpy as np >>> from skimage.util.shape import view_as_blocks >>> A = np.arange(4*4).reshape(4,4) >>> A array([[ 0, 1, 2, 3], [ 4, 5, 6, 7], [ 8, 9, 10, 11], [12, 13, 14, 15]]) >>> B = view_as_blocks(A, block_shape=(2, 2)) >>> B[0, 0] array([[0, 1], [4, 5]]) >>> B[0, 1] array([[2, 3], [6, 7]]) >>> B[1, 0, 1, 1] 13
>>> A = np.arange(4*4*6).reshape(4,4,6) >>> A array([[[ 0, 1, 2, 3, 4, 5], [ 6, 7, 8, 9, 10, 11], [12, 13, 14, 15, 16, 17], [18, 19, 20, 21, 22, 23]], [[24, 25, 26, 27, 28, 29], [30, 31, 32, 33, 34, 35], [36, 37, 38, 39, 40, 41], [42, 43, 44, 45, 46, 47]], [[48, 49, 50, 51, 52, 53], [54, 55, 56, 57, 58, 59], [60, 61, 62, 63, 64, 65], [66, 67, 68, 69, 70, 71]], [[72, 73, 74, 75, 76, 77], [78, 79, 80, 81, 82, 83], [84, 85, 86, 87, 88, 89], [90, 91, 92, 93, 94, 95]]]) >>> B = view_as_blocks(A, block_shape=(1, 2, 2)) >>> B.shape (4, 2, 3, 1, 2, 2) >>> B[2:, 0, 2] array([[[[52, 53], [58, 59]]], [[[76, 77], [82, 83]]]])
Examples using skimage.util.view_as_blocks
¶
view_as_windows¶

skimage.util.
view_as_windows
(arr_in, window_shape, step=1)[source]¶ Rolling window view of the input ndimensional array.
Windows are overlapping views of the input array, with adjacent windows shifted by a single row or column (or an index of a higher dimension).
 Parameters
 arr_inndarray
Nd input array.
 window_shapeinteger or tuple of length arr_in.ndim
Defines the shape of the elementary ndimensional orthotope (better know as hyperrectangle [1]) of the rolling window view. If an integer is given, the shape will be a hypercube of sidelength given by its value.
 stepinteger or tuple of length arr_in.ndim
Indicates step size at which extraction shall be performed. If integer is given, then the step is uniform in all dimensions.
 Returns
 arr_outndarray
(rolling) window view of the input array.
Notes
One should be very careful with rolling views when it comes to memory usage. Indeed, although a ‘view’ has the same memory footprint as its base array, the actual array that emerges when this ‘view’ is used in a computation is generally a (much) larger array than the original, especially for 2dimensional arrays and above.
For example, let us consider a 3 dimensional array of size (100, 100, 100) of
float64
. This array takes about 8*100**3 Bytes for storage which is just 8 MB. If one decides to build a rolling view on this array with a window of (3, 3, 3) the hypothetical size of the rolling view (if one was to reshape the view for example) would be 8*(1003+1)**3*3**3 which is about 203 MB! The scaling becomes even worse as the dimension of the input array becomes larger.References
Examples
>>> import numpy as np >>> from skimage.util.shape import view_as_windows >>> A = np.arange(4*4).reshape(4,4) >>> A array([[ 0, 1, 2, 3], [ 4, 5, 6, 7], [ 8, 9, 10, 11], [12, 13, 14, 15]]) >>> window_shape = (2, 2) >>> B = view_as_windows(A, window_shape) >>> B[0, 0] array([[0, 1], [4, 5]]) >>> B[0, 1] array([[1, 2], [5, 6]])
>>> A = np.arange(10) >>> A array([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]) >>> window_shape = (3,) >>> B = view_as_windows(A, window_shape) >>> B.shape (8, 3) >>> B array([[0, 1, 2], [1, 2, 3], [2, 3, 4], [3, 4, 5], [4, 5, 6], [5, 6, 7], [6, 7, 8], [7, 8, 9]])
>>> A = np.arange(5*4).reshape(5, 4) >>> A array([[ 0, 1, 2, 3], [ 4, 5, 6, 7], [ 8, 9, 10, 11], [12, 13, 14, 15], [16, 17, 18, 19]]) >>> window_shape = (4, 3) >>> B = view_as_windows(A, window_shape) >>> B.shape (2, 2, 4, 3) >>> B array([[[[ 0, 1, 2], [ 4, 5, 6], [ 8, 9, 10], [12, 13, 14]], [[ 1, 2, 3], [ 5, 6, 7], [ 9, 10, 11], [13, 14, 15]]], [[[ 4, 5, 6], [ 8, 9, 10], [12, 13, 14], [16, 17, 18]], [[ 5, 6, 7], [ 9, 10, 11], [13, 14, 15], [17, 18, 19]]]])