[Libexif-devel] Writing EXIF back to JPEG file

[Jim N. <ji...@yo...>]

I'm trying to write EXIF data back to the JPEG file it came from.  This
appears to be a problem for a number of people.  If there's a general
practice for this -- or available code! -- I'd love to know where to find
it.

My own experiments show that libexif is, for at least the photos I'm using,
reducing the size of the EXIF data by approx. 1800 - 2000 bytes, even before
my modifications.  I assume this is due to padding in the original JPEG file
that libexif bit-buckets at load time.

Here's a general strategy I'm considering:

1. Read EXIF from the JPEG.  Modify.
2. Flatten into buffer with exif_data_save_data().
3. Examine JPEG file's APP1 segment.  If APP1 length <= buffer length, write
buffer directly into the APP1 segment in the file *without* modifying APP1's
length field.  Zero out remaining bytes in file.  Done.
4. Otherwise, write out temporary file with SOI, APP1 with new EXIF and
proper EXIF length, then copy remaining original file over.  Move temporary
file on top of original.  Done.

Step #3 is kind of dodgy ... will this work?  I don't know enough about the
EXIF or JPEG spec to understand if this is a problem (especially using
zeroes as padding).  I'm not worried about wasting 2000 bytes in a 1MB file.

Thanks,

-- Jim Nelson
Yorba