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From: "Joris" <joris.at.lebbeke@skynet.be>
To: "TIFF mailing list" <tiff@lists.maptools.org>,
        "Bob Friesenhahn" <bfriesen@simple.dallas.tx.us>
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Subject: Re: [Tiff] creating JPEG file 
Date: Fri, 9 Sep 2005 01:08:32 +0200
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Status:   

Bob Friesenhahn wrote:
> More specifically, the file format that most people call "JPEG" is
> actually the JFIF format ("JPEG File Interchange Format").

Yes. Of course you're absolutely correct, Bob, but it's hard to mention
this too and not become too confusing...

Not every JPEG is necessarilly a JFIF, and a JPEG compressed strip/tile
is a JPEG but should not be a JFIF... Fun, isn't it? I think I need to
clarify, or at least try to.

In short, JFIF adds a little to the older JPEG specification, in order
to have good ways to communicate such properties as color space and
densitity and such in a JPEG file. But JFIF does not change nor
specificy the actual compression scheme, JPEG does.

The little that JFIF adds, is
- not important when talking about JPEG compression inside TIFF, since
it shouldn't be there in the tile/strip data, since it should only be
there on the TIFF IFD level.
- important when talking about good interchangable JPEG files, since
pre-JFIF JPEG may be hard to make sense of.
- essentially more or less what you need to add to a JPEG compressed
tile/strip data as coded inside a TIFF, to end up with a valid
independent JPEG file, if the tile/strip does include its own tables.




Joris Van Damme
info@awaresystems.be
http://www.awaresystems.be/
Download your free TIFF tag viewer for windows here:
http://www.awaresystems.be/imaging/tiff/astifftagviewer.html

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