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Subject: RE: re[2]: [cgmo-webcgm] Z compression research
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 > > > >This mail has originated outside your organization, either from an > >external partner or the Global Internet. > >Keep this in mind if you answer this message. > > > > > > > >This e-mail is intended only for the above addressee. It may contain > >privileged information. > >If you are not the addressee you must not copy, distribute, disclose or > >use any of the information in it. > >If you have received it in error please delete it and immediately notify > >the sender. > >Security Notice: all e-mail, sent to or from this address, may be accessed > > >by someone other than the recipient, for system management and security > >reasons. This access is controlled under Regulation of security reasons. > >This access is controlled under Regulation of Investigatory Powers Act > >2000, Lawful Business Practises.
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