[WM]: Questions about JPEG compression attack

["Cheong David" <david1661@hotmail.com>]

Dear all,

I have implemented a PGM watermarking system and i want to test its
detection rate against JPEG compression attack. So after i watermarked
the image, i convert it into JPEG with certain percent quality, then
back to PGM image again for detection. However, i found that for the
detection rate of attack of 90% quality JPEG compression, it is lower
than 70% quality compression, and it seems quite irregular since the
detection rate is not directly proportional to the JPEG compression
quality.=20
Did anyone also encounter this situation before? Is it normal or is
there something wrong with any part of my system?
i used Cox's additive embedding method and embedded all AC coefficients.

Thank you!

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