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From: "Joris" <joris.at.lebbeke@skynet.be>
To: "Phil Harvey" <phil@owl.phy.queensu.ca>, <tiff@lists.maptools.org>
References: <200701311705.l0VH53f1030173@duke.maptools.org>
	<7D903658-0120-4113-9FD4-7DDC730E4BD1@owl.phy.queensu.ca>
Subject: Re: [Tiff] Interpreting SubIFDs tag
Date: Wed, 31 Jan 2007 20:54:14 +0100
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Status:   

Phil,

Phil Harvey wrote:
> I just scanned my set of test images.  Unfortunately I don't have a
> sample where the next IFD pointer is used in the SubIFD.  However, I
> did note there are a number of images where the next IFD pointer is
> not even written for Interop and Exif IFD's (ie. not zero but not
> meaningful either).  So it looks like one should ignore the next IFD
> pointer for these IFD's.
>
> So my question is:  For what IFD's is the next-IFD pointer
> meaningful? (IFD0 obviously, SubIFD apparently, but what is the
> official word on Interop and Exif IFD's, and what about GlobParam and
> GPS IFD's?)

...and what about private tags we don't know what they might mean and can 
have the IFD datatype?

The fact that this last question exists, does indicate an answer, to me. It 
proves we need to think of reading the IFD tree scheme independently of the 
meaning of tags. Only in a second stage or on a second layer of the code, 
can we make assumptions based on tag meaning, for example that a second IFD 
in an EXIF IFD list isn't quite meaningfull.


Best regards,

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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