RANDOM INFORMATION ON MAKERNOTES

This document is an appendix to the main manual page of the Image::MetaData::JPEG module, which the reader should refer to for further details and the general scope.

On the problem of MakerNote corruption

A widespread problem with Exif maker notes is that there is no common standard for how to parse and rewrite the information in the MakerNote data area. This is the reason why most programs dealing with Exif JPEG files corrupt the MakerNote on saving, or decide to drop it altogether (be aware that there exist programs known to hang when they try to read a corrupt maker note).

In fact, many maker notes contain a non-standard IFD structure, with some tags storing file offsets (see the documentation page describying the IFD structure). Therefore, saving a maker note without regard for internal offsets's adjustment reduces the note mostly to garbage. Re-dumping a maker note after changing the Exif APP1 segment endianness incurs the same problem, because no internal byte-swap is performed.

It is possible to try to cure some maker note corruption problems, if the reader program is a little "brave". For example, the number of tags in an IFD-like maker note is always in the range [1,255], so the two-bytes tag count has always the most significant byte set to zero: therefore, a simple check allows for the determination of the correct byte order (endianness). Also, most often the maker note data area begins just after the tag list, so, if the first detected offset is patently wrong, it can be rescaled, as all of the following ones, to point to this spot; alternatively, all data blocks can be read in sequence from this spot, independently of stored offset values.

AUTHOR

Stefano Bettelli, bettelli@cpan.org

COPYRIGHT AND LICENSE

Copyright (C) 2004 by Stefano Bettelli

This library is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License. See the COPYING and LICENSE file for the license terms.

SEE ALSO

The main documentation page for the Image::MetaData::JPEG module.