Re: [WM]: Questions about JPEG compression attack

["Kunal Bhandari" <kunal_bhandari@da-iict.org>]

dear cheong
Even i have encountered the same effect..but donno really what the phenomenon responsible for it.
The thing which i feel is that..consider it like a problem of signal to noise ratio...our watermarked image is signal and the
distortion due to JPEG is noise. We know that for all natural images the major content of image lies in low freq. and JPEG
compression cuts the high freq. for quality 70% signal is almost same as original as only high freq are cut down but the noise is
reduced as noise is in all freq. thus the denominator decreses without much decrese in numerator. thus SNR improves..while if u go
below 70 u'll see that the SNR reduces .  This is because the signal gets affected at such high compression...

If somebody can validate my understanding or rectify it then i'll be highly gratefull

--
Kunal Bhandari
M.Tech.(ICT)
DA-IICT,Gandhinagar-382009
Mobile +91-9426394751

Cheong David said:
> 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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