How to use Numpy to replace values in a matrix with a color e.g.[0,255,0]

Question

I have a matrix which only contains 0 or 1. I also have a list of colors e.g. the below

class_colors = [[0,0,0], [0,255,0]]
m = np.random.random_integers(0,1,(5,5))

e.g. the m looks like:

0,1,0,1
0,1,0,1
0,1,0,1

How can I replace the 1-values in m with the class_colors[1] and 0-values in m with class_colors[0] , so the m will look something like:

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

I used to be able to do so with np.argmax() and np.take() but it requires the m looks like [class_num,w,h] and then I can do argmax with axis=0.

I know I could do it with for loop, but is there any better and faster approach to do this?

@Trilarion searched and found this:https://stackoverflow.com/questions/55949809/efficiently-replace-elements-in-array-based-on-dictionary-numpy-python and many others but not able to apply those methods.... — Franva, Aug 21 '21 at 05:07

score 2 · Answer 1 · answered Aug 21 '21 at 05:09

2

Not surprisingly the intuitive way works (using advanced indexing):

class_colors = np.array([[0,0,0], [0,255,0]])
m = np.array([0,1,0,1,0,1,0,1,0,1,0,1]).reshape((3,4))
class_colors[m]

answered Aug 21 '21 at 05:09

Julien

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thanks, I didn't know I could do it in this way. good learn. – Franva Aug 21 '21 at 05:24

Corralien · Accepted Answer · 2021-08-21T05:18:07.700

Is it what you expect:

>>> c[m]
array([[[  0,   0,   0],
        [  0, 255,   0],
        [  0,   0,   0],
        [  0, 255,   0]],

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

>>> c  # c = np.array(class_colors)
array([[  0,   0,   0],
       [  0, 255,   0]])

>>> m  # m = np.random.randint(0, 2, (2, 4))
array([[0, 1, 0, 1],
       [0, 1, 0, 1]])

By extension:

class_colors = [[0,0,0], [0,255,0], [255,0,0], [0,0,255]]
c = np.array(class_colors)
m = np.random.randint(0, 4, (2, 4))
# m <- array([[2, 1, 1, 3], [2, 3, 1, 0]])

>>> c[m]
array([[[255,   0,   0],
        [  0, 255,   0],
        [  0, 255,   0],
        [  0,   0, 255]],

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

thanks for the detailed explanation and extension example. :) — Franva, Aug 21 '21 at 05:24

How to use Numpy to replace values in a matrix with a color e.g.[0,255,0]

2 Answers2