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From: "Joris" <joris.at.lebbeke@skynet.be>
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Subject: Re: [Tiff] Re: Copy custom TIFF tags
Date: Sat, 28 Aug 2004 15:48:11 +0200
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> ... as I want to copy *all* the
> tags in a Tiff Image. If libtiff doesnt supports this functionality, I
> can work on this to add this functionality to it, if someone can guide
> me on how to approach the problem.

Here are some scenario's for you to consider:
- I suppose your tag copying is part of tiff splitting or concattenating or
something. Now suppose the tag you blindly copy turns out to be a page number
(http://www.awaresystems.be/imaging/tiff/tifftags/pagenumber.html)
- Even if your tag copying holds on to all pages in their exact order and
positioning (but wouldn't be file copying serve that purpose?), things can still
go wrong. You could end up copying a tag of datatype Long that your code knows
nothing about, but that actually points to a private IFD, similar to the Exif
IFD tag (http://www.awaresystems.be/imaging/tiff/tifftags/exififd.html).
You'll copy the exact value, next when a reader comes along that reads your new
TIFF file and is Exif aware, it is pointed to a location that now no longer
contains the private IFD.

Things could be even more confusing, and tags being safe to copy could depend on
the circumstances. For instance, after applying a filter to an image, a
thumbnail tag would not be safe to copy, but a GeoTIFF geospatial location tag
would be.

This is the reason why no such massive tag copying is included in any of the
library or tools code, I believe. Tag copying, without inbuilt knowledge of the
tags you copy, is a dangerous matter. I've been discussing with Frank that the
'TIFF Tag Reference', http://www.awaresystems.be/imaging/tiff/tifftags.html, if
it turns out to lead to a good complete directory supported by the community,
could aim to include a) information about the tags being safe to copy in this or
that circumstances, and b) could be built to support an XML feed service,
allowing users and/or applications to always the latest version of this tag
data.

Untill that time, I can't see how you can solve these problems without first
duplicating my tiff tag investigation and directory building effort, and next
duplicating my continued updating effort as new/more information is found. Wich
is duplicate effort, either bound to be private and incomplete, or either be
public and decreasing the chances of any tag directory initiative succeeding
rapidly as community support gets divided.

Note that this isn't all bad news, since it means that, best case scenario, the
data needed by massive tag copyers and the like might be available some time
soon.


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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