[FFmpeg-devel] [PATCH 0/8] *_metadata-bsf: Update AVCodecParameters

[Andreas Rheinhardt]

The *_metadata-bitstream filter (I only mean the ones based upon cbs:
prores_metadata is not included.) currently only update the bitstream
and not the AVCodecParameters. As a consequence, if one changes a
bitstream parameter, it might very well happen that ffmpeg adds
information at the container level contradicting and preceding
(hence nullifying) the new bitstream value even if the new container
value was not already present in the source container, but is only read
from the original bitstream.

The situation is even worse for h264_metadata and hevc_metadata: If one
uses the crop parameters, the dimensions of the bitstream change, but
the AVCodecParamters don't. This means that muxer write wrong dimension
values which means that some files violate the specifications.

I have therefore implemented updating the output AVCodecParameters for
the five *_metadata filters. Some things can only be reliably updated
when there is extradata available (e.g. the aforementioned crop values
in h264_metadata and hevc_metadata don't denote additional cropping;
rather these values are used to replace the existing cropping values
which means that one can't know by how much the dimensions change).

I noticed something odd about the aspect ratio: If one updates this part
of the AVCodecParameters, one runs into trouble in init_muxer in
libavformat/mux.c because of an aspect ratio mismatch between muxer and
encoder layer (the muxer information comes from the sample_aspect_ratio
member of AVStream, the encoder information is the aspect ratio
information from the AVCodecParameters embedded in AVStream). Is there a
way to successfully update the sample_aspect_ratio from a bitstream
filter? I would very much appreciate that.

My first version that did update the aspect ratio par_out value
can be found on github: https://github.com/mkver/FFmpeg/commits/par_out
(The reason for the improved check for invalid values in cbs_mpeg2 was
that this enabled me to rely on the dimensions being strictly positive
in the next patch; in the new version, this part of the code has been
removed.)

A last note: The hevc_metadata filter now updates the height to the
frame height (after all, the width and height refer to video frames);
this deviates from current behaviour for field-based HEVC (FFmpeg always
reports the picture dimensions for HEVC, even when said pictures only
contain fields). But if I am not mistaken (I only have a draft of the
third edition of ISO/IEC 14496-15) then muxers (at least mp4 and
Matroska) want the frame dimensions, too, so this seemed to be the right
thing to do.


Andreas Rheinhardt (8):
  cbs: Add function to update video codec parameters
  av1_metadata: Add option to update video parameters
  cbs_mpeg2: Improve checks for invalid values
  mpeg2_metadata: Additionally update AVCodecParameters
  vp9_metadata: Improve spec-compliance and warnings
  vp9_metadata: Update AVCodecParameters
  h264_metadata: Update AVCodecParameters
  hevc_metadata: Update AVCodecParameters

 doc/bitstream_filters.texi             | 35 +++++++++++++++
 libavcodec/av1_metadata_bsf.c          | 43 ++++++++++++++++--
 libavcodec/cbs.c                       | 35 +++++++++++++++
 libavcodec/cbs.h                       | 15 +++++++
 libavcodec/cbs_mpeg2.c                 | 17 ++++---
 libavcodec/cbs_mpeg2_syntax_template.c |  4 +-
 libavcodec/h264_metadata_bsf.c         | 48 +++++++++++++++++---
 libavcodec/h265_metadata_bsf.c         | 44 ++++++++++++++++---
 libavcodec/mpeg2_metadata_bsf.c        | 11 +++++
 libavcodec/vp9_metadata_bsf.c          | 61 +++++++++++++++++++++-----
 10 files changed, 278 insertions(+), 35 deletions(-)

-- 
2.19.1