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000046_owner-lightwave-l _Mon Apr 3 13:41:49 1995.msg
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From: Richard Addison <addison@cs.usc.edu>
Message-Id: <199504030542.WAA23670@pollux.usc.edu>
Subject: Re: Various Info
To: lightwave-l@netcom.com
Date: Sun, 2 Apr 1995 22:42:19 -0700 (PDT)
In-Reply-To: <9504022140.AA00649@casablanca.d2.com> from "Feliciano di Giorgio" at Apr 2, 95 02:40:51 pm
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> Yes, I have heard that there are certain new versions of JPEG
> that are virtualy or completely lossless, but you will not see compression
> ratios on the scale of +20:1 as usual but more like 2:1.
The JPEG specification includes a variation for lossless compression
(at a typical compression ratio of 2:1). However, it is essentially
different from the common JPEG format, so implementing it is like
implementing a different file format.
Its compression technique is more general than run-length encoding
since it can capture trends in variation between pixels rather than
just recording horizontal runs of repeated pixel values. It can
also capture trends between scan lines.
I believe it still incorporates a color-space transformation, so some
round-off error can occur. However, that step might be optional. I
don't happen the specification or the FAQ handy at the moment.
By the way, a lossless compression ratio of 2:1 is about the best you
can expect for "real" or scanned images. Higher ratios, with various
techniques, can be achieved for artificial images (like those with
a lot of flat black or transparent regions).
--
Richard Addison