Re: Quality factor and bits per pixel in JPEG

"fulltime" <PiaoChieh@xxxxxxxxx> wrote in message
news:1174714751.715832.153700@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Hi gentlemen,

I have another 2 questions regarding quality factor.

to quantize a coefficient with a quality factor, Qs, it is =
Coefficient / (Quant Matrix Value * Qs)

I don't think this is defined anywhere in the specification.
Different people make different assumptions, about
the meaning of quality factor (just to complicate things).

The JPEG FAQ has a good (though dated) discussion of
it in item [5].

http://www.faqs.org/faqs/jpeg-faq/part1/

Much of the earlier JPEG software was based on open software
created by the Independent JPEG Group (IJG), and so will use
the Quality scale used there. See the thread you started on
comp.compression back in 12/24/06 fro some good discussion.

1) i have been searching the net and people are talking about a
quality factor of 0 to 100 with 100 being the best quality. So for
quality of 100, does tat mean Qs = 1 in the eqn and Qs=0.5 for a
quality of 50?

No. In general it's going to be more complicated than this
(see, for example, your december post).

2) how is this Qs going to reduce the file size, since if Qs is small,
it will require more bits to quantise the coefficient?

It isn't. If that's the definition you use it would be the wrong
way round.

The IJG code is freely available, and you might find some
aspects of it help you. Also, the JPEG spec (ITU
Recommendation T.81) is freely available on the web
(although it doesn't deal with Quality scale).


.

Relevant Pages

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