Most EXIF metadata is in IFD0 and most EXIF payloads only contain
one IFD, but it is possible for there to be more IFDs after the
existing trailing one. exiftool and similar software report these IFDs
as IFD1, IFD2, etc. This commit reads those additional IFDs and attaches
them as dummy entries in the top-level IFD ranging from 0xFFFC down to
0xFFED, which are unused by the EXIF spec. The EXIF API is only able to
return and work with a single IFD, so by attaching it as a subdirectory
this metadata can be preserved.
This is done transparently through the read/write process. Upon parsing
an additional IFD1, it will be attached, but it will be written with
av_exif_write after IFD0 rather than as a subdirectory, as intended.
Existing files without more than one IFD, i.e. most files, will be unaffected
by this change, as well as API clients looking to parse specific fields, but
now more metadata is parsed and written, rather than simply being discarded
as trailing data.
Signed-off-by: Leo Izen <leo.izen@gmail.com>
784aa09fa8