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I found an image hash library for python and this blog

In the block Getting Funky With pHash, the author writes:

Further reduce the DCT. This is the magic step. Set the 64 hash bits to 0 or 1 depending on whether each of the 64 DCT values is above or below the average value. The result doesn't tell us the actual low frequencies; it just tells us the very-rough relative scale of the frequencies to the mean. The result will not vary as long as the overall structure of the image remains the same; this can survive gamma and color histogram adjustments without a problem.

Histogram equalization changes frequencies of pixel values. Could you explain why histogram equalization does not affect coefficients?

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  • $\begingroup$ why should it or shouldn't it? in the quote I see no claims referring to histogram equalization. $\endgroup$ Commented Jan 23, 2023 at 10:52
  • $\begingroup$ @ChristophRackwitz histogram equalisation doesn't relate with histogram adjustments? $\endgroup$ Commented Jan 29, 2023 at 8:59
  • $\begingroup$ how does that relate to DCT? $\endgroup$ Commented Jan 29, 2023 at 10:42

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