Re: [WM]: Questions about JPEG compression attack

["Dr Johann A. Briffa" <jabrif@eng.um.edu.mt>]

Cheong David wrote:
> 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.

What are you using to JPEG-encode the images? Did you confirm that at 90% quality the compressed image size is larger than at 70%?
Johann

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