2016.03.18 14:03 "Re: [Tiff] Wrong data type of GPS_IFD and EXIF_IFD tags and other issues", by Fran=c3=a7ois-Xavier_Thomas

Hello all,

I've recently spent a few hours debugging a really strange issue where some TIFF images we use had unreadable EXIF and GPS tags using Exempi (the tag processing library we are using).

After a while, I found out that the images were all produced by LibTiff 4.0, and the TIFFTAG_EXIFIFD and TIFFTAG_GPSIFD both had type TIFF_IFD (13) instead of TIFF_LONG (4), which Exempi doesn't support.

Here is what I could find in various specs:

TIFF 6.0[0] uses TIFF_LONG
EXIF 2.2[1] uses TIFF_LONG
EXIF 2.3[2] uses TIFF_LONG
Tech notes for PageMaker[3] uses TIFF_IFD8 (and is the only one)

Someone already sent an email about this a few months back. I'm not sure I can reply to the original thread[4] since I just subscribed to the mailing list, but I cc'd the poster.

Anyway, the following libraries don't support TIFF_IFD8 at all and were only able to read EXIF/GPS metadata with TIFF_LONG:

Adobe XMP SDK C++ (all versions I could find up to the latest)
libexempi (2.2 from Ubuntu)
libexempi (2.3 and not-yet-released 2.4, compiled myself)
libexif (0.6.21 from the Ubuntu repos)
exifprobe (2.0.1 from the Ubuntu repos)
exifread (2.1.2 from PyPi)
metadata-extractor (Android lib using the Java version of the XMP SDK)
As you can see, the current LibTiff produces TIFF tags that are unreadable for a wide range of libraries.
Is there a spec somewhere that mentions the TIFF_IFD tag and its use for EXIF metadata? I'd be more comfortable in knowing that this is somewhat official.
What's the opinion of the LibTiff developers regarding this? Are patches welcome that would allow LibTiff to support both types?
Can LibTiff be forced to output TIFF_LONG instead of TIFF_IFD without having to patch it?
At the moment I'm relying on a patched version of Exempi, which is definitely not ideal ; any help is appreciated!

Regards,
François-Xavier

[0] https://partners.adobe.com/public/developer/en/tiff/TIFF6.pdf [1] http://www.exif.org/Exif2-2.PDF [2] http://www.cipa.jp/std/documents/e/DC-008-2012_E.pdf [3] https://partners.adobe.com/public/developer/en/tiff/TIFFPM6.pdf [4] http://www.asmail.be/msg0055066050.html

François-Xavier Thomas -- Ingénieur traitement des images

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