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• avcodec/pthread_frame : Don't update the first thread ctx before freeing

  18 mai 2022, par Andreas Rheinhardt

  avcodec/pthread_frame : Don't update the first thread ctx before freeing
  Currently, ff_frame_thread_free() uses the last worker thread
  
  to updates the first worker thread via update_context_from_thread()
  
  immediately before freeing all these worker threads. This is
  
  a remnant of the time in which the first worker was special.
  
  (E.g. the first worker shared its AVCodecInternal with the public
  
  AVCodecContext.)
  But these times are over (none of the uses of is_copy matter
  
  for ff_frame_thread_free()) ; nowadays the only thing that
  
  update_context_from_thread() does is referencing a few
  
  buffers/frames and replacing them with other references instead.
  
  These new references will then be freed immediately thereafter
  
  when the first worker thread is freed. Ensuring that the code is
  
  free of double-frees is achieved by using reference-counted structures
  
  (or in case of AVChannelLayouts : by giving each worker its own copy).
  Some archaeology :
  
  a) Updating the first worker thread from the last one used
  
  has been done since frame-threading was added in
  
  37b00b47cbeecd66bb34c5c7c534d016d6e8da24.
  
  b) The precursor to ff_mpv_common_end() checked for is_copy
  
  before freeing pictures (i.e. it only freed them for the first
  
  worker thread).
  
  c) Commits c2dfb1e37cc72bf144545c4410a4621cbff5c4b1 and
  
  e33811bd2686411233cb0eb4a4ee45eb99d7e736 modified the
  
  update_thread_context function of the H.264 decoder
  
  so that it could fail before calling ff_mpeg_update_thread_context().
  
  d) This led to a double free/an assert violation with a H.264
  
  sample for which ff_mpeg_update_thread_context() is not reached
  
  for the final update_context_from_thread(). Commit
  
  a6e4796fbf0aa9b13451a8ef917ecc4e80d1d272 added code to fix this
  
  sample.
  
  e) This issue was fixed (even with the last mentioned commit reverted)
  
  when the H.264 decoder was deMpegEncContextized in commit
  
  b7fe35c9e50e1701274364adf7280bf4a02b092b (merging commit
  
  2c541554076cc8a72e7145d4da30389ca763f32f).
  
  f) mpegvideo.c stopped using is_copy when it was switched to refcounted
  
  frames in 759001c534287a96dc96d1e274665feb7059145d.
  
  g) 1f4cf92cfbd3accbae582ac63126ed5570ddfd37 removed the init_thread_copy
  
  callbacks ; now no FFCodec.close callback checks for is_copy at all
  
  any more.
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
  

  • [DH] libavcodec/pthread_frame.c

• ffmpeg - Adding and Removing Subtitles without Changing the Video

  28 septembre 2018, par MeCe

  I’m trying to embed subtitles into video and removing the subtitles back again without changing the video, meaning I want the output video to be the same with the original video.

  I’m using the following command to embed the subtitles

  ffmpeg -i original.mp4 -i original.srt \
  
  -c:v copy -c:a copy -c:s mov_text \
  
  -map_metadata 0:g -map_metadata:s:v 0:s:v -map_metadata:s:a 0:s:a \
  
  -movflags +faststart -threads 8 \
  
  output.mp4

  To remove the subtitles,

  ffmpeg -i output.mp4 \
  
  -c:v copy -c:a copy \
  
  -map_metadata 0:g -map_metadata:s:v 0:s:v -map_metadata:s:a 0:s:a \
  
  -movflags +faststart -threads 8 \
  
  -sn \
  
  removed.mp4

  The output is almost the same but I couldn’t figure out what would cause the difference. When I compare the binaries, almost all of the differences are

  original: 0xF3
  
  removed: 0xF4

  The bytes are incremented by 1, I think only in the header.

  Can you help ? Thank you in advance.

• how to play audio output to a device using ffmpeg's avdevice library ?

  18 septembre 2022, par sssssss

  How can I use the ffmpeg's avdevices c library to output audio to an audio device (specifically alsa). All I could find is its doxygen and the only useful thing I was able to take out of it is, quote

  


  "the (de)muxers in libavdevice are of the AVFMT_NOFILE type (they use their own I/O functions). The filename passed to avformat_open_input() often does not refer to an actually existing file, but has some special device-specific meaning - e.g. for xcbgrab it is the display name."

  


  but I don't understand where do I specify AVFMT_NOFILE and where do I specify which device I want use.I see how I can get an 'AVOutputFormat' pointer but then what do I do with it ?

  


  update :
so now i found the function 'avformat_alloc_output_context2' so my code looks like this :

  


  AVPacket pkt = av_packet_alloc();
avformat_alloc_output_context2(&ofmt_ctx, NULL, "alsa", NULL);
avformat_new_stream(ofmt_ctx, NULL);

while(av_read_frame(fmt_ctx, pkt) == 0){
    av_write_frame(ofmt_ctx, pkt);
}


  


  fmt_ctx is the input file's AVFormatContext.

  


  but I am still getting an error '[alsa @ 0x555daf361140] Invalid packet stream index : 1' what am I missing ?