Numpy multidimensional array, count inner arrays which are equal

Question

I'm trying to compare two 3-dimensional arrays and count how many inner arrays are equal.

I'm comparing 2 patches of a picture and want to know how many pixels are equal and not how many color values are equal. And it would be nice if it's efficient so I'm using numpy. I know how to make the comparison with for loops but it's too slow.

But I'm only able to count it element wise, here's my snippet:

import numpy as np

a = np.array([[[255, 255, 255],
           [255, 255, 255],
           [255, 255, 255],
           [255, 255, 255]],

          [[255, 255, 255],
           [255, 255, 255],
           [255, 255, 255],
           [255, 255, 255]],

          [[255, 255, 255],
           [255, 255, 255],
           [255, 255, 255],
           [255, 255, 255]],

          [[255, 255, 255],
           [255, 255, 255],
           [255, 255, 255],
           [255, 255, 255]]])

b = np.array([[[255, 255, 255],
           [255, 255, 255],
           [0, 0, 0],
           [0, 0, 0]],

          [[255, 255, 255],
           [255, 255, 255],
           [0, 0, 0],
           [0, 0, 0]],

          [[0, 0, 0],
           [0, 0, 0],
           [0, 0, 0],
           [0, 0, 0]],

          [[0, 0, 0],
           [0, 0, 0],
           [0, 0, 0],
           [0, 0, 0]]])
print(np.sum(a[:, :] == b[:, :])) 
# prints 12 and i would like to have a 4 in this example

Shouldn't the desired answer be 4? I see 4 sub arrays equal

Sheldore
– Sheldore

2018-12-16 19:42:35 +00:00
Commented Dec 16, 2018 at 19:42 — Sheldore
– Sheldore, Commented Dec 16, 2018 at 19:42
Yes of course :)

vanderub
– vanderub

2018-12-16 19:54:38 +00:00
Commented Dec 16, 2018 at 19:54 — vanderub
– vanderub, Commented Dec 16, 2018 at 19:54

akuiper · Accepted Answer · 2018-12-16 19:43:52Z

2

Check whether all three color values are equal for each pixel first and then count the pixels that are equal by summing up the trues:

(a == b).all(axis=-1).sum()
# 4

answered Dec 16, 2018 at 19:43

akuiper

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1 Comment

vanderub Over a year ago

i have another question, maybe you can answer it. I want to check if there are equal with a small difference. In other words there are equal if the value is for example 10% off.

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