iPython numpy - How to change value of an array slice with a map

Question

I've got a 3-dim array [rows][cols][3] with values between 0 and X. I need to manipulate a specific dimension in the array. So I've taken a slice of the part I want to manipulate

arr_slice = array[:,:,0]

now I can make some manipulations like arr_slice *= 3 and that will change the original array, as I intended.

However, I need to change values according to a map, which is an array with size X that maps the values of the slice (0-X) to new values. the map is called mapping

so I know mapping[arr_slice] will do what I want, but using it like this: arr_slice = mapping[arr_slice] will of course change only arr_slice and not the original array I have.

So, How can I perform this task to change the original array?

The array is actually an image, that I'm trying to manipulate it's Y values in YIQ format:

im_eq = np.copy(im_orig)

if (rgb):
    im_eq = rgb2yiq(im_eq)
    im = im_eq[:,:,0]
else:
    im = im_eq

mapping = get_cumutative_histogram(im)
im = mapping[im.astype(int)] # the problematic line

Add a sample case?

Divakar
– Divakar

2016-11-22 09:21:54 +00:00
Commented Nov 22, 2016 at 9:21 — Divakar
– Divakar, Commented Nov 22, 2016 at 9:21

dnalow · Accepted Answer · 2016-11-22 10:09:32Z

2

You need to address the slice elements:

im[:] = mapping[im.astype(int)]

for example:

from pylab import *
a = rand(10)
sl = a[4:9]
print sl  # ->: array([ 0.97278179,  0.7894741 ,  0.38051133,  0.42684762, 0.82670638])

sl[:] = 1
print a #-> array([ 0.21125781,  0.4235981 ,  0.81950229,  0.93937973,  1.        ,  1.        ,  1.        ,  1.        ,  1.        ,  0.39047808])

answered Nov 22, 2016 at 10:09

dnalow

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1 Comment

Yoavhayun Over a year ago

Thanks, that's was just what I was looking for

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