Date : Mon, 09 Jul 2001 10:07:45 +0100
From : Paul Wheatley <p.r.wheatley@...>
Subject: Re: Magazine test scans - Update
> > Russell Marks wrote:
> > > Not true. From the JPEG FAQ:
> > >
> > > > Except for experimental purposes, never go above about Q 95; using Q 100
> > > > will produce a file two or three times as large as Q 95, but of hardly
any
> > > > better quality. Q 100 is a mathematical limit rather than a useful
setting.
> > > > If you see a file made with Q 100, it's a pretty sure sign that the maker
> > > > didn't know what he/she was doing.
> >
> > This isn't true either. I've worked pretty seriously with graphics for
> > several years now (having coded things like Delirium on the RiscPC). I've
>
> Ok, so you wrote a screensaver, and I've written a picture viewer. Are
> we even now? :-)
I made that point to illustrate that I've compressed a lot of JPEGS is my
time and I've never seen one bloated to 2or 2 times its original size. Simply
doesn't happen. The JPEG FAQ is seriously misleading.
Anyway, my screensaver program was better than your picture viewer *;-)
>
> > component of JPEG compression. However, in my experience a Q100 is virtually
> > identical to the uncompressed original if not exactly the same. Certainly
>
> You're missing the point. When talking of compression algorithms,
> "lossless" has a very specific meaning - that you can get bit-for-bit
> identical output from the decompression. (Which you can find
> explicitly mentioned in the JPEG FAQ, BTW.) So your claim that using
> quality 100 for a JPEG makes it lossless is false.
It looks like I claim that Q100 is virtually identical. The point is, via
this route you at least end up with an easily understandable format. This
djvu thing is simply a risky direction to take, and if you want longevity
this isn't a good strategy.
>
> > anyway). Q100 *is* useful if you want to use the JPEG format (because of its
> > common use and wide support) but don't want loss of image.
>
> The usefulness or otherwise of Q100 notwithstanding, the "loss of
> image" is demonstrably present:
>
> rus@.../home/rus/tmp>pgmramp -ellipse 640 480 >a
> rus@.../home/rus/tmp>cjpeg -q 100 a |djpeg >b
> rus@.../home/rus/tmp>cmp a b
> a b differ: char 265, line 4
>
> This is on the same platform, with the same JPEG implementation, with
> an integer DCT, with a mindlessly simple (yet JPEG-friendly) artifical
> image. Surely optimal conditions. And it's *still* not lossless.
I can't really comment on an example I can't see. I find it hard to believe
the two images aren't virtually identical. Given that you're scanning the
magazines and hence don't have perfect images anyway, I can't see the tiny
amount of loss some people can apparently see on JPEG 100's being a problem.
However, I could see a problem if someone chose a horrible proprietary format
to store everything in which requires alsorts of weird and rare bits of
software, and no one can actually view the images in 20 years times.
>
> > The guy that wrote
> > the JPEG FAQ really doesn't know what he's talking about.
>
> The author of the JPEG FAQ, Tom Lane, is also one of the main authors
> of the Independent JPEG Group's JPEG software, as used by most
> programs which read/write JPEG files.
>
> So, unless you have some pretty compelling evidence, I think you might
> want to retract that.
I stand by the evidence I've presented.
Paul
--
Camileon Project Officer
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http://www.leeds.ac.uk/camileon
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