CODEWITHSUNDEEP
pythonarraysrgb
A.LeBrun
A.LeBrun

Reputation: 161

Adding elements of RGB array in Python

Say I have a 2D RGB array that is show as:

[[[ 0.    0.   0.5]
  [ 0.5   0.   0.5]
  [ 0.    0.   0.5]
  ..., 
  [ 0.5   0.   0.5]
  [ 0.    0.   0.5]
  [ 0.    0.   0.5]]]

I want to add the individual elements in the R and B columns to the G part of the array so it looks like:

[[[ 0.    0.5   0.5]
  [ 0.5   1.0   0.5]
  [ 0.    0.5   0.5]
  ..., 
  [ 0.5   1.0   0.5]
  [ 0.    0.5   0.5]
  [ 0.    0.5   0.5]]]

What would be the easiest way to approach this problem?

Upvotes: 0

Views: 398

Answers (2)

unutbu
unutbu

Reputation: 879143

You could use 

arr[...,1] = arr[...,0] + arr[...,2]

or perhaps more readably,

R, G, B = arr[...,0], arr[...,1], arr[...,2]
G[:] = R + B

Since G is a basic slice of arr, it is a view. Modifying a view modifies the original array. The assignment G[:] = ... modifies G inplace. So the assignment modifies arr.

import numpy as np

arr = np.array([[[ 0.,    0.,   0.5],
                 [ 0.5,   0.,   0.5],
                 [ 0.,    0.,   0.5],
                 [ 0.5,   0.,   0.5],
                 [ 0.,    0.,   0.5],
                 [ 0.,    0.,   0.5]]])

R, G, B = arr[...,0], arr[...,1], arr[...,2]
G[:] = R + B

print(arr)

yields

[[[ 0.   0.5  0.5]
  [ 0.5  1.   0.5]
  [ 0.   0.5  0.5]
  [ 0.5  1.   0.5]
  [ 0.   0.5  0.5]
  [ 0.   0.5  0.5]]]

Upvotes: 1

Pierre Cordier
Pierre Cordier

Reputation: 504

I would convert your list of lists in an numpy.array, and then slice it.

import numpy as np

a=np.array([[ 0. ,  0. ,  0.5],
            [ 0.5,  0. ,  0.5],
            [ 0. ,  0. ,  0.5]])
a[:,1] += a[:,0] + a[:,2]
print(a)

Output :

array([[ 0. ,  0.5,  0.5],
       [ 0.5,  1. ,  0.5],
       [ 0. ,  0.5,  0.5]])

Upvotes: 2

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