Finding the (x,y) indexes of specific (R,G,B) color values from images stored in NumPy ndarrays

Question

I have an image stored in a numpy array, as yielded by imread():

>>> ndim
array([[[  0,   0,   0],
        [  4,   0,   0],
        [           


        

Answer 1

For some array colour array a and a colour tuple c:

indices = numpy.where(numpy.all(a == c, axis=-1))

indices should now be a 2-tuple of arrays, the first of which contains the indices in the first dimensions and the second of which contains the indices in the second dimension corresponding to pixel values of c.

If you need this as a list of coordinate tuples, use zip:

coords = zip(indices[0], indices[1])

For example:

import numpy
a = numpy.zeros((4, 4, 3), 'int')    

for n in range(4):
    for m in range(4):
        a[n, m, :] = n + m
        if (n + m) == 4:
            print n, m

c = (4, 4, 4)
indices = numpy.where(numpy.all(a == c, axis=-1))
print indices
print zip(indices[0], indices[1])

will output:

1 3
2 2
3 1
(array([1, 2, 3]), array([3, 2, 1]))
[(1, 3), (2, 2), (3, 1)]

which corresponds to all the pixels of value (4, 4, 4).