How to apply a disc shaped mask to a numpy array?

问题:

I have an array like this:

>>> np.ones((8,8)) array([[ 1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.],        [ 1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.],        [ 1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.],        [ 1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.],        [ 1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.],        [ 1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.],        [ 1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.],        [ 1.,  1.,  1.,  1.,  1.,  1.,  1.,  1.]]) 

I'm creating a disc shaped mask with radius 3 thus:

y,x = np.ogrid[-3: 3+1, -3: 3+1] mask = x**2+y**2 

This gives:

>> mask array([[False, False, False,  True, False, False, False],        [False,  True,  True,  True,  True,  True, False],        [False,  True,  True,  True,  True,  True, False],        [ True,  True,  True,  True,  True,  True,  True],        [False,  True,  True,  True,  True,  True, False],        [False,  True,  True,  True,  True,  True, False],        [False, False, False,  True, False, False, False]], dtype=bool) 

Now, I want to be able to apply this mask to my array, using any element as a center point. So, for example, with center point at (1,1), I want to get an array like:

>>> new_arr array([[ True,  True,  True,  True,    1.,  1.,  1.,  1.],        [ True,  True,  True,  True,  True,  1.,  1.,  1.],        [ True,  True,  True,  True,    1.,  1.,  1.,  1.],        [ True,  True,  True,  True,    1.,  1.,  1.,  1.],        [ 1.,    True,    1.,    1.,    1.,  1.,  1.,  1.],        [ 1.,      1.,    1.,    1.,    1.,  1.,  1.,  1.],        [ 1.,      1.,    1.,    1.,    1.,  1.,  1.,  1.],        [ 1.,      1.,    1.,    1.,    1.,  1.,  1.,  1.]]) 

Is there an easy way to apply this mask?

Edit: I shouldn't have mixed booleans and floats - it was misleading.

>>> new_arr array([[ 255.,  255.,  255.,  255.,    1.,  1.,  1.,  1.],        [ 255.,  255.,  255.,  255.,  255.,  1.,  1.,  1.],        [ 255.,  255.,  255.,  255.,    1.,  1.,  1.,  1.],        [ 255.,  255.,  255.,  255.,    1.,  1.,  1.,  1.],        [ 1.,    255.,    1.,    1.,    1.,  1.,  1.,  1.],        [ 1.,      1.,    1.,    1.,    1.,  1.,  1.,  1.],        [ 1.,      1.,    1.,    1.,    1.,  1.,  1.,  1.],        [ 1.,      1.,    1.,    1.,    1.,  1.,  1.,  1.]]) 

This is more the result I require.

array[mask] = 255

will mask the array using centre point (0+radius,0+radius).

However, I'd like to be able to place any size mask at any point (y,x) and have it automatically trimmed to fit.

回答1:

I would do it like this, where (a, b) is the center of your mask:

import numpy as np  a, b = 1, 1 n = 7 r = 3  y,x = np.ogrid[-a:n-a, -b:n-b] mask = x*x + y*y 

回答2:

I just wanted to share with everyone a slightly more advanced application of this technique that I just had to face.

My problem was to apply this circular kernel to compute the mean of all the values surrounding each point in a 2D matrix. The kernel generated can be passed to scipy's generic filter in the following way:

import numpy as np from scipy.ndimage.filters import generic_filter as gf  kernel = np.zeros((2*radius+1, 2*radius+1)) y,x = np.ogrid[-radius:radius+1, -radius:radius+1] mask = x**2 + y**2 

Hope this helps!

回答3:

To put it one convenient function:

def cmask(index,radius,array):   a,b = index   nx,ny = array.shape   y,x = np.ogrid[-a:nx-a,-b:ny-b]   mask = x*x + y*y 

Returns the pixel sum within radius, or return(array[mask] = 2) for whatever need.

回答4:

You could use scipy's convolve function, which has the benefit of allowing you to place any particular mask, aka kernel, on any number of given coordinates in your array, all at once:

import numpy as np from scipy.ndimage.filters import convolve 

First create a coordinate array with the coordinate of where you want the mask (kernel) to be centered marked as 2

background = np.ones((10,10)) background[5,5] = 2 print(background)  [[ 1.  1.  1.  1.  1.  1.  1.  1.  1.  1.]  [ 1.  1.  1.  1.  1.  1.  1.  1.  1.  1.]  [ 1.  1.  1.  1.  1.  1.  1.  1.  1.  1.]  [ 1.  1.  1.  1.  1.  1.  1.  1.  1.  1.]  [ 1.  1.  1.  1.  1.  1.  1.  1.  1.  1.]  [ 1.  1.  1.  1.  1.  2.  1.  1.  1.  1.]  [ 1.  1.  1.  1.  1.  1.  1.  1.  1.  1.]  [ 1.  1.  1.  1.  1.  1.  1.  1.  1.  1.]  [ 1.  1.  1.  1.  1.  1.  1.  1.  1.  1.]  [ 1.  1.  1.  1.  1.  1.  1.  1.  1.  1.]] 

Create your mask:

y,x = np.ogrid[-3: 3+1, -3: 3+1] mask = x**2+y**2 

Convolve the two images:

b = convolve(background, mask)-sum(sum(mask))+1 print(b)  [[   1.    1.    1.    1.    1.    1.    1.    1.    1.    1.]  [   1.    1.    1.    1.    1.    1.    1.    1.    1.    1.]  [   1.    1.    1.    1.    1.  255.    1.    1.    1.    1.]  [   1.    1.    1.  255.  255.  255.  255.  255.    1.    1.]  [   1.    1.    1.  255.  255.  255.  255.  255.    1.    1.]  [   1.    1.  255.  255.  255.  255.  255.  255.  255.    1.]  [   1.    1.    1.  255.  255.  255.  255.  255.    1.    1.]  [   1.    1.    1.  255.  255.  255.  255.  255.    1.    1.]  [   1.    1.    1.    1.    1.  255.    1.    1.    1.    1.]  [   1.    1.    1.    1.    1.    1.    1.    1.    1.    1.]] 

Note that the convolve function entries do not commute, i.e. convolve(a,b) != convolve(b,a)

Note also that if your point is near an edge, the algo does not reproduce the kernel at the coordinate. To get around this you can pad the background by the largest axis of your kernel, apply the convolution, then remove the padding.

Now, you can map any kernel to any number of points in an array, but note that if two kernels overlap, they add at the overlap. You can threshold this if you need.

回答5:

Did you try making a mask or zeroes and ones and then using per-element array multiplication? This is the canonical way, more or less.

Also, are you certain you want a mix of numbers and booleans in a numpy array? NumPy, as the name implies, works best with numbers.

回答6:

To get the same result as in your example, you can do something like this:

>>> new_arr = np.array(ones, dtype=object) >>> new_arr[mask[2:, 2:]] = True >>> print new_arr array([[True, True, True, True, 1.0, 1.0, 1.0, 1.0],        [True, True, True, True, True, 1.0, 1.0, 1.0],        [True, True, True, True, 1.0, 1.0, 1.0, 1.0],        [True, True, True, True, 1.0, 1.0, 1.0, 1.0],        [1.0, True, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0],        [1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0],        [1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0],        [1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0]], dtype=object) 

文章来源: How to apply a disc shaped mask to a numpy array?