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• avcodec/h26[45]_metadata_bsf : Use separate contexts for reading/writing

  6 juillet 2020, par Andreas Rheinhardt

  avcodec/h26[45]_metadata_bsf : Use separate contexts for reading/writing
  Currently, both bsfs used the same CodedBitstreamContext for reading and
  
  writing ; as a consequence, the state of the writer's context at the
  
  beginning of writing a fragment is exactly the state of the reader after
  
  having read the fragment ; in particular, the writer might not have
  
  encountered one of its active parameter sets yet.
  This is not nice and may lead to invalid output even when the input
  
  is completely spec-compliant : Think of an access unit containing
  
  a primary coded picture referencing a PPS with id id (that is known from
  
  an earlier access unit/from extradata), then a new version of the PPS
  
  with id id and then a redundant coded picture that is also referencing
  
  the PPS with id id. This is spec-compliant, as the standard allows to
  
  overwrite a PPS with a different PPS in between coded pictures and not
  
  only at the beginning of an access unit. In this scenario, the reader
  
  would read the primary coded picture with the old PPS and the redundant
  
  coded picture with the new PPS (as it should) ; yet the writer would
  
  write both with the new PPS as extradata which might lead to errors or
  
  to invalid data being output without any error (e.g. if the two PPS
  
  differed in redundant_pic_cnt_present_flag).
  The above scenario does not directly translate to HEVC as long as one
  
  restricts oneself to input with nuh_layer_id == 0 only (as cbs_h265
  
  does : it currently strips away any NAL unit with nuh_layer_id > 0 when
  
  decomposing) ; if one doesn't the same issue as above can happen.
  If one also allowed input packets to contain more than one access unit,
  
  issues like the above can happen even without redundant coded
  
  pictures/multiple layers.
  Therefore this commit uses separate contexts for reader and writer.
  Reviewed-by : Mark Thompson <sw@jkqxz.net>
  
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
  

  • [DH] libavcodec/h264_metadata_bsf.c
  • [DH] libavcodec/h265_metadata_bsf.c

• lavc/vvc : Fix derivation of inverse LMCS idx

  2 février, par Frank Plowman

  lavc/vvc : Fix derivation of inverse LMCS idx
  The clamping of idxYInv from H.266(V3) section 8.8.2.3 was missing.
  
  This could lead to OOB reads from lmcs->pivot or input_pivot.
  I also changed the derivation of the forward LMCS idx to use a shift
  
  rather than a division for speed and as this is actually how the
  
  variable is declared in the specification (8.7.5.2).
  Signed-off-by : Frank Plowman <post@frankplowman.com>
  

  • [DH] libavcodec/vvc/ps.c

• ffmpeg : is there a simple way to edit the video resolution, but keep all audio and subtitles

  21 avril 2022, par Arkeen

  I would like to lower video resolution - usually from .mkv files - but to keep all possible audio tracks (might be only one, might be several) and subtitles (might be none, might be several) from the original one. I also would like to keep as many encoding parameters as I can from the original video file (especially those I do not understand).

  


  I am still new to ffmpeg : at first the idea seemed simple, but after many attempts, it seems it is more complex than that. Do I have to use the -filter_complex option ? It seems to be an overkill (or overcomplex) for what I thought to be an easy conversion, but I might be wrong.

  


  I tried to combine -vf scale=-1:720 with -c copy -map 0, which gave me an error that I now understand, but I am stuck with the next step.

  


  Any lead on to achieve that ? Could it be done with ffmpeg only or would I need a script ?