Iterating over multidimensional arrays(images) with numpy array - python

Question

Hy! I have two images(same dimension) as numpy array imgA - imgB i would like to iterate each row and column and get somenthing like that:

for i in range(0, h-1):
  for j in range(0, w-1):
    final[i][j]= imgA[i,j] - imgB[i-k[i],j]

where h and w are the height and the width of the image and k is and array with dimension[h*w].

i have seen this topic: Iterating over a numpy array but it doens't work with images, i get the error: too many values to unpack Is there any way to do that with numpy and python 2.7?

thanks

edit I try to explain better myself. I have 2 images in LAB color space. these images are (288,384,3). Now I would like to make deltaE so I could do like that(spitting the 2 arrays):

 imgLabL=np.dsplit(imgL,3)
 imgLabR=np.dsplit(imgR,3)
 imgLl=imgLabL[0]
 imgLa=imgLabL[1]
 imgLb=imgLabL[2]
 imgRl=imgLabR[0]
 imgRa=imgLabR[1]
 imgRb=imgLabR[2]
delta=np.sqrt(((imgLl-imgRl)**2) + ((imgLa - imgRa)**2) + ((imgLb - imgRb)**2)   )

Till now everything is fine. But now i have this array k of size (288,384). So now i need a new delta but with different x axis,like the pixel in imgRl(0,0) i want to add the pixel in imgLl(0+k,0)

do you get more my problems?

yes sorry i wrote wrong but i meant that. My images are 384x288 but it goes in a infinity loop, also i can't write final[i][j] but just final because i get: valueError:output operand requires a reduction,but reduction is not enabled. — Gianfra
– Gianfra, Commented Dec 13, 2012 at 16:47
What's inside imgA[i][j]? Is it a tuple? If so, that could be your problem. The assignment expects one value. — Diego Basch
– Diego Basch, Commented Dec 13, 2012 at 16:48

Glorfindel · Accepted Answer · 2023-01-03 21:00:26Z

2

I'm pretty sure that whatever it is you are trying to do can be vectorized and run without any loops in it. But the way your code is written, it is no surprise that it doesn't work...

If k is an array of shape (h, w), then k[i] is an array of shape (w,). when you do i-k[i], numpy will do its broadcasting magic, and you will get an array of shape (w,). So you are indexing imgB with an array of shape (w,) and a single integer. Because one of the items in the indexing is an array, fancy indexing kicks in. So assuming imgB also has shape (h, w, 1), the return value of imgB[i-k[i], j] will not be an array of shape (1,), but an array of shape (w, 1). When you then try to substract that from imgA[i, j], which is an array of shape (1,), broadcasting magic works again, and so you get an array of shape (w, 1).

We do not know what is final. But if it is an array of shape (h, w, 1), as imgA and imgB, then final[i][j] is an array of shape (1,), and you are trying to assign to it an array of shape (w, 1), which does not fit. Hence the operand requires a reduction,but reduction is not enabled error message.

EDIT

You don't really need to split your arrays to compute DeltaE...

def deltaE(a, b) :
    return np.sqrt(((a - b)**2).sum(axis=-1))

delta = deltaE(imgLabL, imgLabR)

I still don't understand what you want to do in the second case... If you want to compare the two images displaced along the x-axis, I would suggest using np.roll:

deltaE(imgLabL, np.roll(imgLabR, k, axis=0))

will have at position (r, c) the deltaE between the pixel (r, c) of imgLabL and the pixel (r - k, c) of imgLAbR. Is that what you want?

edited Jan 3, 2023 at 21:00

Glorfindel

22.8k13 gold badges97 silver badges124 bronze badges

answered Dec 13, 2012 at 17:44

Jaime

67.7k19 gold badges128 silver badges164 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Gianfra Over a year ago

in this k i have the disparity of the two images. I m working with stereo images and i have found the disparity (docs.opencv.org/modules/calib3d/doc/…) So now i want to make the deltaE with the matching point and i should use this disparity. If i get right the point in the imgA(x,y) has a matching point in imgB(x+disp,y). therefore i have to move along the x-axis. I m gonna try this np.roll! btw the deltaE method work very good! thanks!

Gianfra Over a year ago

Using np.roll i get this error: only length-1 arrays can be converted to python scalars

James Porter · Accepted Answer · 2012-12-13 19:07:19Z

1

I usually use numpy.nditer, the docs for which are here and have many examples. Briefly:

import numpy as np
a = np.ones([4,4])

it = np.nditer(a)
for elem in a:
    #do stuff

You can also use c style iteration, i.e.

while not it.finished:
    #do stuff
    it.iternext()

If you need to access the indices of your arrays. In your situation, I would zip your two images together to create an array of shape [2,h,w] and then iterate over this, filling an empty array with the results of the computation.

answered Dec 13, 2012 at 19:07

James Porter

1,9532 gold badges19 silver badges30 bronze badges

Collectives™ on Stack Overflow

Iterating over multidimensional arrays(images) with numpy array - python

2 Answers 2

2 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related