RE: re[2]: [cgmo-webcgm] Z compression research

["Bezaire, Benoit" <bbezaire@ptc.com>]

gzip libraries usually work on fixed size buffers, so someone can uncompress say the first 4096 bytes... have a look at the header and determine if it's a file format it supports.

Benoit.

-----Original Message-----
From: Don Larson [mailto:dlarson@cgmlarson.com] 
Sent: Thursday, September 20, 2007 12:48 PM
To: Lofton Henderson; CGM Open WebCGM TC
Subject: re[2]: [cgmo-webcgm] Z compression research

Lofton,

 >  At 06:56 AM 9/20/2007 -0700, Cruikshank, David W wrote:
 >  >I guess I also raises the question of if you find the "magic number" for  >  >zcompress how do you know it's not an svgz file instead of a cgmz file?

There is really no way to tell. Both would have the same magic number.
The magic number is only an indicator that a file has been compressed according the GZIP standard.

Here is how I see this compression scheme working.

The sender of a CGM file that wishes to send a compressed CGM file would use GZIP software to compress a CGM and save that file with a "cgmz" file extension, e.g. xxx.cgmz.

Then on the receiving end, when that xxx.cgmz file is received by a CGM viewer ( that supports GZIP compressed CGM files) and it sees the "cgmz" extension it would decompress with GZIP software. This would yield a binary CGM file which could then can be loaded and viewed.

Don.

 >  Decompress it?

 >  While "file sniffing" is a common practice (to determine GZIP, or SVG vs. 
 >  CGM, or CGM encoding), it is not actually the recommended way of tracking  >  content type -- not supported by any standards or protocols.

File sniffing is not required if there is a unique extension, i.e. "cgmz" that indicates that the CGM file is GZIP compressed.


 >  -Lofton.

 >  >-----Original Message-----
 >  >From: DULUC, Franck [mailto:franck.duluc@airbus.com]  >  >Sent: Thursday, September 20, 2007 6:31 AM  >  >To: Don Larson; CGM Open WebCGM TC  >  >Subject: RE: [cgmo-webcgm] Z compression research  >  >  >  >i think it is the same for VRML.
 >  >
 >  >Franck
 >  >
 >  >-----Message d'origine-----
 >  >De : Don Larson [mailto:dlarson@cgmlarson.com] Envoyé : mercredi 19  >  >septembre 2007 23:07 À : CGM Open WebCGM TC Objet : [cgmo-webcgm] Z  >  >compression research  >  >  >  >  >  >All,  >  >  >  >After researching Z compression, I have determined that the file format  >  >used for compressed SVG is the same used by "gzip" an open source  >  >software. And that there indeed is a magic number in first 2 bytes  >  >indicating Z compression.
 >  >
 >  >The complete specification for the gzip file format can found here:
 >  >http://www.ietf.org/rfc/rfc1952.txt
 >  >
 >  >Further details on gzip and source code can be found at:
 >  >http://www.gzip.org/
 >  >
 >  >
 >  >Regards,
 >  >Don Larson
 >  >
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