[FFmpeg-devel] [PATCH v2 0/3] [RFC] Exif Overhaul

[Leo Izen]

I've made some changes to the last EXIF overhaul patch I sent,
notably I fixed up some bugs and added MakerNote parsing, so it
should not corrupt MakerNotes that are inside TIFF files.

MakerNote is supposed to be a binary blob but many camera manufacturers
treat it as an IFD (e.g. Canon) with offsets relative to the start of the file,
so it has to be parsed in order to move it from TIFF to another file or it will
be corrupted.

I also fixed the fate tests now that pngenc.c writes eXIf chunks, so those should
pass.

I haven't come up with a solution that Andreas proposed, which is:

> If I see this correctly, then these patches can lead to a situation
> where an input packet has rotation metadata in exif which gets exported
> twice -- as displaymatrix and as exif metadata side data. If the user
> changes the displaymatrix (e.g. applies the transformation to the image
> data and removes the displaymatrix side data before reencoding), the
> exif data (that the user would probably not be aware of) would still be
> there and get propagated into the output, corrupting it.

Zeroing out the EXIF orientation tag upon attaching the displaymatrix is
something I did consider, but it would corrupt encoding unless I also read
the displaymatrix data and attach it as EXIF orientation. This isn't too hard
if the Orientation tag exists but it's more difficult if it doesn't already
exist. It's also not clear what to do if the AVDisplayMatrix is not in the
dihedral group D4.

Currently there's no code that takes an AVDictionary and writes it back into
EXIF as these are not compatible structures. EXIF includes integers of varying
lengths, while AVDictionary contains strings only.

Leo Izen (3):
  various: change EXIF metadata into AVFrameSideData
  avcodec/pngdec: parse eXIf chunk
  avcodec/pngenc: write eXIf chunks

 fftools/ffprobe.c                        |  27 +-
 libavcodec/Makefile                      |   1 +
 libavcodec/exif.c                        | 394 +++++++++++++++++++++--
 libavcodec/exif.h                        |  32 +-
 libavcodec/exif_internal.h               |  55 ++++
 libavcodec/mjpegdec.c                    |  91 +-----
 libavcodec/mjpegdec.h                    |   2 +-
 libavcodec/pngdec.c                      |  35 ++
 libavcodec/pngenc.c                      |   4 +
 libavcodec/tiff.c                        |  19 +-
 libavcodec/tiff.h                        |   1 +
 libavcodec/version.h                     |   2 +-
 libavcodec/webp.c                        |  38 +--
 libavformat/avidec.c                     |   4 +-
 libavutil/frame.c                        |   2 +
 libavutil/frame.h                        |   6 +
 tests/ref/fate/cover-art-mp3-id3v2-remux |   6 +-
 tests/ref/fate/exif-image-embedded       |   5 +-
 tests/ref/fate/exif-image-jpg            |  91 +++---
 tests/ref/fate/exif-image-webp           |  91 +++---
 tests/ref/fate/mov-cover-image           |   6 +-
 21 files changed, 676 insertions(+), 236 deletions(-)
 create mode 100644 libavcodec/exif_internal.h

-- 
2.46.0