Re: Widespread bug in "compress" and GIF compressors

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Article #667

#667

From: Jeff C. Glover
Date: Fri, 18 Dec 1992 14:53

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In article <BzEzAM.3Gv.2@cs.cmu.edu> tgl+@cs.cmu.edu (Tom Lane) writes:
>I've discovered a small bug in the standard Unix compress(1) utility.
>The bug is also present in many derived implementations of LZW compression,
>notably common GIF graphics file writers such as ppmtogif and xv.

I found the "bug" in ppmtogif and made the recommended change.

I then ran the fixed ppmtogif against 2763 GIF images, comparing the
resulting ppm files (using giftoppm).

Over the 2763 GIF images, using the fixed version would have saved
6894328 bytes by re-encoding.  However, about a dozen files reported
differences in the resulting ppm files.  I'm tracking down each, but
the differences are subtle, if visible at all (header diffs?).

Jeff
jeffg@sgi.com

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References: <BzEzAM.3Gv.2@cs.cmu.edu>

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Path: arc1-ca.pugleaf.net!sparky!uunet!zaphod.mps.ohio-state.edu!saimiri.primate.wisc.edu!ames!sgi!fido!loki.esd.sgi.com!jeffg
From: jeffg@loki.esd.sgi.com (Jeff C. Glover)
Newsgroups: comp.sources.bugs
Subject: Re: Widespread bug in "compress" and GIF compressors
Date: 18 Dec 1992 14:53:19 GMT
Organization: Silicon Graphics, Inc., Mountain View, CA
Lines: 17
Message-ID: <1gsokvINNb2@fido.asd.sgi.com>
References: <BzEzAM.3Gv.2@cs.cmu.edu>
NNTP-Posting-Host: loki.esd.sgi.com
Keywords: LZW, compress, GIF

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