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• 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 ?

  


• 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.

• Why does HTML5 video with very large h.264 encoded mp4 (with +faststart, ie metadata at beginning), take ages to load ?

  15 septembre 2015, par Tom Jenkinson

  The video is rendered with ffmpeg with the "faststart" flag added meaning the metadata should be at the start of the file, and the server appears to be handling partial content requests correctly, so why does it need to have downloaded so much of the video before the player becomes enabled and can play the video ? I am testing it in Google Chrome.

  Once the player becomes enabled I can seek around to various points in the video pretty instantly and see the new partial content requests being made.

  Here is a link to the video : http://iptv.la1tv.co.uk/unibrass.mp4

  Here is a jsbin with the video tag : https://jsbin.com/rahewidoru . It takes a few minutes but does work after it loads.

  Any suggestions ?

  I realise there are other methods like HLS and dash which use chunks, but I would like to know why it isn’t working this way because I can’t find anywhere that provides a reason as to why it doesn’t work well.