dhash.py

from PIL import Image

# from http://blog.iconfinder.com/detecting-duplicate-images-using-python/
#

def dhash(image, hash_size = 8):
        # Grayscale and shrink the image in one step.
        image = image.convert('L').resize(
            (hash_size + 1, hash_size),
            Image.ANTIALIAS,
        )

        pixels = list(image.getdata())

        # Compare adjacent pixels.
        difference = []
        for row in xrange(hash_size):
            for col in xrange(hash_size):
                pixel_left = image.getpixel((col, row))
                pixel_right = image.getpixel((col + 1, row))
                difference.append(pixel_left > pixel_right)

        # Convert the binary array to a hexadecimal string.
        decimal_value = 0
        hex_string = []
        for index, value in enumerate(difference):
            if value:
                decimal_value += 2**(index % 8)
            if (index % 8) == 7:
                hex_string.append(hex(decimal_value)[2:].rjust(2, '0'))
                decimal_value = 0

        return ''.join(hex_string)