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Arrays of images: how to efficiently switch from RGB to Hex

I've used nested loops to include RGB images in an array of images with hex values, but it's too slow for large images. Does anyone know a quick way and / or a library that can help me switch back and forth from RGB to HEX?

edit: @ragingSloth

This is what I came up with, but this is too slow for what I need:

def rgb_to_hex(array):
    (x, y, z) = array.shape
    for v in range(0, x):
        for u in range(0, y):
            array[v, u] = int('%02x%02x%02x' % (array[v, u, 0], array[v, u, 1], array[v, u, 2]))
+3


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3 answers


Using the Depth of Market idea, you can also eliminate the double for loop:



def tohex(array):
    array = np.asarray(array, dtype='uint32')
    return ((array[:, :, 0]<<16) + (array[:, :, 1]<<8) + array[:, :, 2])
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String operations are probably pretty slow. A straightforward mathematical approach would be as follows:

array[v, u] = ((array[v, u, 0]<<16) + (array[v, u, 1]<<8) + array[v, u, 2])


This concatenates 3 bytes of RGB representation into one int:

>>> A = [123, 255, 255]
>>> B = (A[0]<<16) + (A[1]<<8) + A[2]
>>> B
8126463
>>> hex(B)
'0x7bffff'
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Not sure if this is much faster, but you can do something like this:

hexarr = np.vectorize('{:02x}'.format)

And then run it on an RGB array:

In [67]: a = (np.random.rand(2,5,3)*255).astype('u1')

In [68]: a
Out[68]:
array([[[149, 145, 203],
        [210, 234, 219],
        [223,  50,  26],
        [166,  34,  65],
        [213,  78, 115]],

       [[191,  54, 168],
        [ 85, 235,  36],
        [180, 140,  96],
        [127,  21,  24],
        [166, 210, 128]]], dtype=uint8)

In [69]: hexarr(a)
Out[69]:
array([[['95', '91', 'cb'],
        ['d2', 'ea', 'db'],
        ['df', '32', '1a'],
        ['a6', '22', '41'],
        ['d5', '4e', '73']],

       [['bf', '36', 'a8'],
        ['55', 'eb', '24'],
        ['b4', '8c', '60'],
        ['7f', '15', '18'],
        ['a6', 'd2', '80']]],
      dtype='|S2')

You can collapse the third dimension with view

:

In [71]: hexarr(a).view('S6')
Out[71]:
array([[['9591cb'],
        ['d2eadb'],
        ['df321a'],
        ['a62241'],
        ['d54e73']],

       [['bf36a8'],
        ['55eb24'],
        ['b48c60'],
        ['7f1518'],
        ['a6d280']]],
      dtype='|S6')

Unfortunately this doesn't look much faster (a little over twice as fast):

In [89]: timeit rgb_to_hex(a)
1 loops, best of 3: 6.83 s per loop

In [90]: timeit hexarr(a).view('S6')
1 loops, best of 3: 2.54 s per loop
+1


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