Date : Fri, 06 Jul 2001 13:42:24 +0100
From : Paul Wheatley <p.r.wheatley@...>
Subject: Re: Magazine test scans - Update
Russell Marks wrote:
>
> Paul Wheatley <p.r.wheatley@...> wrote:
>
> > This is only an issue if you don't set the quality of compression high
> > enough. If you want to use JPEG as a handy, widely used, open format then
> > simply compress all images on 100 which makes them lossless. They will
still
>
> 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
never seen JPEGs bloated 2 or 3 times the size of Q95. This simply does not
happen. As an example I compressed a little diagram image in various formats
and on various JPEG compressions. Heres what I got:
JPEG80 27K
JPEG95 42K
JPEG99 60K
JPEG100 64K
PNG 24K
TIFF 45K
BMP(raw) 631K
All these can be found at :
http://www.personal.leeds.ac.uk/~issprw/jpeg/comp.htm
While this is a test with only one image (and different levels of detail will
obviously yield different compression results) it is still a useful
comparison.
JPEGs above 95/96 are slightly wasteful in size (compare with the TIFF which
uses the same (if I remember rightly) LZW compression as the non lossy
component of JPEG compression. However, in my experience a Q100 is virtually
identical to the uncompressed original if not exactly the same. Certainly
close enough for you guys not to worry about (as long as you don't repeatedly
uncompress and the recompress the images, which would be extremely foolish
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 guy that wrote
the JPEG FAQ really doesn't know what he's talking about.
I don't know anything about the compression PNG uses (does anyone have a good
URL of info on this?), but it does look like a good choice. What you really
need to know however, is how stable the format is. JPEG is pretty stable, but
something like TIFF is very open ended and in my opinion is potentially very
dangerous as a preservation format.
Paul
--
Camileon Project Officer
http://www.si.umich.edu/CAMILEON/
http://www.leeds.ac.uk/camileon
0113 233 5830