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From: jallen@netxdev.DHL.COM (John Allen)
Newsgroups: comp.graphics
Subject: Re: Comp.graphics.images
Message-ID: <2045@netxcom.DHL.COM>
Date: 6 Dec 89 20:29:43 GMT
References: <8911212014.AA06822@ironwood.cis.ohio-state.edu>
Sender: news@netxcom.DHL.COM
Reply-To: jallen@netxdev.UUCP (John Allen)
Organization: NetExpress Communications, Inc.
Lines: 29

In article <8911212014.AA06822@ironwood.cis.ohio-state.edu> dnwiebe@CIS.OHIO-STATE.EDU (Dan N Wiebe) writes:
>------
>The uuencoded and compressed versions of the GIF images, while smaller
>than the uncompressed, uuencoded versions, are still about 25% larger
>than the pure GIF data.
>------
>	It looks to me like a more profitable route to compress and *then*
>uuencode, producing file.gif.Z.uue instead of file.gif.uue.Z, but even
>this isn't too sensible because compress can rarely make a .GIF file
>any smaller.

The GIF standard uses a slightly modified 12 bit Lempel-Ziv algorithm
to compress the image data.  Since compress is also Lempel-Ziv, the size
of a GIF is about equivalent to a 12 bit compress.

For those familiar with Lempel-Ziv, I have found that a typical 320x
256x8 bit image will have something between three and five RESETs.
Since LZW compression gets the greatest "mileage" at the end of the
compressed stream, each RESET is very expensive to the overall file
size.  So any gain obtained from compressing a GIF file will come
from full 16 bit compression.

Perhaps it would be worthwhile to lobby for an enhancement to the GIF
standard which permits 16 bit LZW compression.  If GIF did use 16 bit
compression, the result should be much smaller.
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John Allen, NetExpress Communications, Inc.    usenet: jallen@netxcom.DHL.COM
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