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_binary_array_to_hex gives wrong value

See original GitHub issue

Correct me if I am wrong, but _binary_array_to_hex gives wrong values.

Python 2.7.13 (default, Feb 16 2017, 19:11:00) 
[GCC 5.3.0] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from PIL import Image
>>> import imagehash
>>> img = Image.open('sample_image.png')
>>> ph = imagehash.phash(img)
>>> bool_array = ph.hash.flatten()
>>> bool_array
array([ True, False, False,  True,  True,  True,  True, False, False,
       False,  True,  True,  True,  True, False, False,  True,  True,
       False, False, False, False, False,  True,  True,  True,  True,
        True, False, False, False, False,  True,  True,  True,  True,
       False, False, False, False,  True,  True, False, False, False,
       False,  True,  True,  True,  True,  True, False, False,  True,
        True,  True,  True,  True, False, False, False, False, False, False], dtype=bool)
>>> bit_array = 1*bool_array
>>> bit_array
array([1, 0, 0, 1, 1, 1, 1, 0, 0, 0, 1, 1, 1, 1, 0, 0, 1, 1, 0, 0, 0, 0, 0,
       1, 1, 1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 1, 0, 0, 0, 0, 1, 1, 0, 0, 0, 0,
       1, 1, 1, 1, 1, 0, 0, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0])
>>> bit_string = ''.join(str(b) for b in bit_array)
>>> bit_string
'1001111000111100110000011111000011110000110000111110011111000000'
>>> int(bit_string, 2)
11402201597170935744L
>>> hex(int(bit_string, 2))
'0x9e3cc1f0f0c3e7c0L' # This is the expected hex value for the given bit array (phash)
>>> str(ph)
'793c830f0fc3e703' # and here is the actual value.

If I made no mistake, let me know and I will submit a PR.

Issue Analytics

State:
Created 7 years ago
Comments:12 (5 by maintainers)

Top GitHub Comments

1reaction

phretorcommented, Oct 26, 2017

I’d like to reopen this issue, because the implementation of the binary-to-hexadecimal conversion is not interoperable with other implementations of the same algorithm, and simply not interoperable with “standard” value-conversion functions.

Therefore, if I calculate the pHash with imagehash and then I take its hex value and interpret it with another “standard” hex-to-bin function, I obtain a completely different value.

When I say “standard” I mean something like “widely used”. Take for example the bitstring.BitArray class:

In [3]: BitArray(bin='1000111100001111000011110000111100001111000010110000101101111010')
Out[3]: BitArray('0x8f0f0f0f0f0b0b7a')

versus

In [13]: _binary_array_to_hex(np.array([bool(int(x)) for x in '1000111100001111000011110000111100001111000010110000101101111010']))
Out[13]: 'f1f0f0f0f0d0d05e'

Again, for Hamming-distance comparison, as long as the bits are the same, it doesn’t matter. However, if you use other libraries, you might get weird results.

0reactions

JohannesBuchnercommented, Dec 7, 2017

@phretor, could you check if the newest version (4.0) works OK for you?