Maker note offset error

[Archive]

[Originally posted by cjsmall on 2006-09-19 21:58:26-07]

I am using exiftool 6.31 without any serious problems.  Recently I started using a development version of gimp which is built using version 0.6.13 of libexif.  When gimp writes a jpeg file it appears to include all of the meta data.  However, when I later use exiftool to update the User Comment field I always get the following message:

Code:

Warning: [minor] Possibly incorrect maker notes offsets (fix by -16?) - filename.jpg

Exiftool fixes this problem when it writes the new comment, but I am wondering if someone can provide a more detailed explanation of exactly what problem this is indicating in the jpeg file so that I can help get the issue examined and resolved with libexif.  Currently, the people looking at the libexif code see no problem.  Thanks.

--

Jeff

[Archive]

[Originally posted by exiftool on 2006-09-19 23:19:10-07]

Hi Jeff,

The problem is that any software that rewrites the EXIF and changes the position of the maker note information must also change any absolute offsets inside the maker notes or else they will point to invalid data.  (The maker notes frequently contain a TIFF IFD structure, and unfortunately the TIFF standard specifies that all offsets are absolute.  There are, however, some smart manufacturers like FujiFilm who ignore this standard and use relative offsets instead, and these relative offsets are not invalidated when the maker note block changes location, but  most others like Canon, Nikon, Pentax, etc use absolute offsets.)

This is a very common problem because the maker notes contain proprietary information and not many utilities are able to properly shift the offsets when editing the EXIF.  The EXIF editor must at least understand the IFD structure of the maker notes in order to properly adjust the offsets when the information is moved.

- Phil

[Archive]

[Originally posted by cjsmall on 2006-09-20 00:05:11-07]

Phil:

Thanks for the great explanation.  I'm working with jpeg files creatred with a Konica-Minolta 7D camera. The thing that has me puzzled is that I was not seeing this problem until I started using a newer version of gimp - although it is linked against the same version of libexif.  So I have been trying to figure out just exactly where the problem is cropping up.

If I sent you a small jpeg, would you be able to tell exactly what needed to be addressed so that I could pass this info along to the coders for libexif?  If so, let me know what would be the best address to use.

And as always, exiftool is a fantastic program!

--

Jeff

[Archive]

[Originally posted by exiftool on 2006-09-20 11:56:25-07]

Hi Jeff,

You can send me a jpg if you want (philharvey66 at gmail.com), but you can see for yourself
what the problem is, just run this command on the jpg:

Code:

   exiftool -htmldump image.jpg > out.html

Then take a look at the output html file in a web browser.  Anything red or pink is bad
(double-referenced data), as are large chunks of orange (unreferenced data).

And to see what the "fixed" structure should be, you can add the '-F' option:

Code:

   exiftool -F -htmldump image.jpg > fixed_out.html

- Phil