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Date: Tue, 27 Sep 2011 16:03:52 +0200
From: Andreas Kleinert <info@ar-kleinert.de>
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Subject: Re: [Tiff] Write Exif data to Tiff file
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>> I would like to write Exif data (such as exposure time) to a tiff
>> file I am creating. Is this possible with libtiff, and if so is there
>> any information or examples on how to do it?
> 
> Would it be appropriate to post this as a bug/enhancement request to the bugzilla list? Is there anyone who might consider it, is there any development work happening with libtiff?

The problem with EXIF essentially is, that it originally was meant as
replacement for IJG-JFIF - i.e. a TIFF file with JPEG content and
a number of additional tags (including one being a thumbnail preview
in some other size/format).

This means, intending to incorporate JPEG into TIFF and make it the
standard.

Now, that didn't work. JPEG in form of IJG-JFIF became the de-facto
standard.

Then someone decided to nevertheless insert EXIF tags into JFIF files,
using the available App markers. Due to limitations of the JFIF file
format (64k-chunk-oriented) this resulted in a stripped EXIF format,
with only a thumbnail but no image (and no pointer to the real image).

So, if nowadays someone wants to do either of the following...

- convert a TIFF with EXIF tags into a JPEG with EXIF App marker
- convert a JPEG with EXIF App marker into a TIFF

He'll run into numerous issues. Some being:

- some ambiguous EXIF tags (content and meaning wise)
- endian format conversion
- one-way conversion limitation (64k)
- same/different IFDs being used for EXIF metadata, thumbnail and image
  compared to a plain TIFF

There are (partial) solutions for that (and I did one myself) but if you
try to add that entirely to the libtiff API, it's a bit like the box of
pandora being opened. If you only add part of it, however, I'm still
sure this won't make the box shut anymore ;-)

Best Regards,
Andreas (author of SView5)
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