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From: "Joris" <joris.at.lebbeke@skynet.be>
To: "Bob Friesenhahn" <bfriesen@simple.dallas.tx.us>,
        "Chris Losinger" <losinger@earthlink.net>
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Subject: Re: [Tiff] reading private subdirs
Date: Thu, 16 Sep 2004 18:02:05 +0200
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> some EXIF writers use tag offsets in their MakerNotes that are based on the
> start of the EXIF data subdir, rather than the start of the MakerNote
> subdir. so, changing the length of the non-MakerNote data will destroy all
> those offsets.

I'd like to add: SIGH. TIFF data blocks (tag values) should be self-contained.
Should not point to other tags, other subdirs, or inside other tags, or
whatever. Thus, TIFF data (tag values) should be completely relocatable. Thus,
enabling page joggling apps and TIFF editing apps and the like. Failure to
comply to this scheme leads to major trouble, most of which is bound to result
in corruption of *the offender's data* after these data joggling apps did their
thing.

Exceptions are the clearly documented TileOffsets/StripOffsets tags, the SubIFDs
tag. And also, the spec allows for private dir schemes (which is unfortunate,
but there seems little use in trying to battle with it, and damage can be
contained if those private dir tags at least get documented publicly). NO OTHER
EXCEPTIONS can be made. This is a very essential corner stone of the TIFF file
format philosophy.

Just wanted to add that hopefully clear comment, for hopefully at least some
future potential offenders to read.


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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