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From: "Philip Watkinson" <pwatkins@crawfordtech.com>
To: "'Joris Van Damme \(AWare Systems\)'" <info@awaresystems.be>,
	"'Bob Friesenhahn'" <bfriesen@simple.dallas.tx.us>
Date: Fri, 13 Feb 2009 10:15:34 -0500
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Cc: 'Tiff List' <tiff@lists.maptools.org>
Subject: Re: [Tiff] warning and errors
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Guys,

Let me explain my situation. Our company provides automated document factory
(ADF) solutions to many large financial institutions. For one client, they
receive logos from their clients in TIFF files. There are thousands of
files. When they run the application, they see warnings which scares them
and they shut down thinking there is an error. The common warning is
"unknown field with tag ... ". If this warning has nothing to do with the
core functionality of encoding and decoding images, it would be nice to have
a switch to suppress these types of warnings.

Regards,
Philip 

-----Original Message-----
From: Joris Van Damme (AWare Systems) [mailto:info@awaresystems.be] 
Sent: Friday, February 13, 2009 3:21 AM
To: Bob Friesenhahn; Philip Watkinson
Cc: Tiff List
Subject: Re: [Tiff] warning and errors

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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