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From: ImageAnalyst <imageanalyst@mailinator.com>
Newsgroups: comp.soft-sys.matlab
Subject: Re: I need to help?
Date: Sun, 21 Sep 2008 06:34:32 -0700 (PDT)
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On Sep 21, 4:38=A0am, Walter Roberson <rober...@hushmail.com> wrote:
> ABUTALA wrote:
> > decibels =3D
>
> > =A0 =A029.4771
> > What is this number
>
> The number is an arbitrary value calculated by the program for no apparen=
t
> reason and accidentally printed out because the line that calculates it
> has a missing semi-colon at the end.
>
> > because i want to know (find the quality at different &#8216;strength&#=
8217; value.
> > Plot a graph of the quality of =A0the stego image and the reconstructed=
 image
> > =A0vs &#8216;strength&#8217;.)
> > Can you help to answer this?
>
> I would say that to answer that, you are going to have to choose a precis=
e
> definition for "quality", and you are going to have to choose a value
> range for "strength". At least the code -mentions- "strength" (in one of
> the comments) so you have a fighting chance of figuring out what the
> valid range is for "strength". Unfortunately there is no scientific defin=
ition
> for "quality" when it comes to images built from mathematical arrays.
> What is the "quality" associated with an image of a cat? What is the "qua=
lity"
> associated with an image of a dog? How does the image of the cat differ f=
rom
> the cat itself?

---------------------------------------------------------------------------=
----
Walter's right again as usual.  PSNR is not really a quality metric in
the context that you're using it.  It's meant to be used to determine
how close a reconstructed image is to the original image, For example
if you take an image and then compress it and decompress it, and then
compare that to the original.  I don't think it really makes too much
sense in watermarking.  PSNR will be max for no watermark at all and
get less as the watermark gets bigger or more contrasty, neither of
which is related to the "quality" ro effectiveness of the watermark in
doing its job.  I think you should use a different criteria to assess
the effectiveness of your watermarking.
Regards,
ImageAnalyst