It would be pretty easy to assign any image an arbitrary value. Compare the image's current (unquantized) DCT values to the desired ones, find the value that differs from the target but is closest to being quantized as the desired bit, and apply a filter that just barely flips it. Each time you do this you reduce the Hamming distance score between the image and the target by 1.
If getting close enough results in unsightly blotches on the image, reduce the power of the low frequency luminance channel across the board, which will mask the changes by making the unmodified high frequency components more noticeable. That looks like what's being done in the catdog image, at least.
You could increase the saturation as well, as this fingerprinting system ignores color.
If getting close enough results in unsightly blotches on the image, reduce the power of the low frequency luminance channel across the board, which will mask the changes by making the unmodified high frequency components more noticeable. That looks like what's being done in the catdog image, at least.
You could increase the saturation as well, as this fingerprinting system ignores color.