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From: "Joris Van Damme \(AWare Systems\)" <info@awaresystems.be>
To: "Bob Friesenhahn" <bfriesen@simple.dallas.tx.us>,
	"Philip Watkinson" <pwatkins@crawfordtech.com>
References: <20090212230702.1D9AA1D202630@lists.maptools.org>
	<Pine.SOC.4.64.0902121747210.1127@freddy.simplesystems.org>
Date: Fri, 13 Feb 2009 09:21:06 +0100
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Cc: Tiff List <tiff@lists.maptools.org>
Subject: Re: [Tiff] warning and errors
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Errors-To: tiff-bounces@lists.maptools.org

Bob,

> TIFF is a very flexible format.  There are proprietary usages of TIFF
> which require special tags in order for the proprietary software to be
> able to read the file correctly.  If a program using libtiff opens one
> of these files then the result may be gibberish.  However, the user is
> likely to see a wrong looking image.

I humbly dissagree. TIFF only works if you separate the 'nominal' layer from 
the 'proprietary extension' layer. For example, say some app needs to store 
elevation data with a map encoded in normal RGB. What it should do is not 
apply some proprietary photometric RGBE, nor should it revert to a complete 
proprietary set of tags that totally excludes interchange. Instead, it 
should recognise that the 'nominal' and interchangeble way to communicate 
this is a normal TIFF with photometric RGB and four channels, the fourth 
channel being indicated as 'undefined'. Next, it might add an additional 
'proprietary extension' adding a proprietary tag that fully defines the 
'undefined' channel in a proprietary way. Unsuspecting software/users 
render/see the nominal RGB image. The specially aware software/user can make 
sense of the extra channel.

In short, 'extra' stuff should never obscure the 'standard' stuff, but fully 
and only build on from the latter. This way and only this way can an unknown 
tag and unknown channels always be safely ignored, and that is requirement 
for the extensibility of TIFF to actually work.


Best regards,

Joris

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