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• FFmpeg - generate x264 CBR video transport stream with C-API

  6 juillet 2020, par ZeroDefect

  Using various posts sprinkled around the Internet, including this one here on SO, I've been able to understand how to use the FFmpeg cli to generate a CBR video bitrate using the x264 codec (wrapped in an MPEG-2 transport stream). Note : I'm concerned with the video bitrate - nothing else.

  


  ffmpeg -i cbr_test_file_input.mp4 -c:v libx264 -pix_fmt yuv420p -b:v 6000000 -preset fast -tune film -g 25 -x264-params vbv-maxrate=6000:vbv-bufsize=6000:force-cfr=1:nal-hrd=cbr -flags +ildct+ilme x264_cbr_test_output.ts


  


  However, I'm trying to approach this from an FFmpeg C-API point of view. I'm having issues. I've knocked together some code to try do something very similar to what is being done in the FFmpeg CLI. I can generate a transport stream of what I think should be CBR, but the profile of the video bitrate is very different from what I thought was the FFmpeg cli equivalent :

  


  The initialisation of the AVCodecContext looks something like :

  


        av_dict_set(&amp;pDict, "preset", "faster", 0);&#xA;      av_dict_set(&amp;pDict, "tune", "film", 0);&#xA;      av_dict_set_int(&amp;pDict, "rc-lookahead", 25, 0);&#xA;&#xA;      pCdcCtxOut->width = pCdcCtxIn->width;&#xA;      pCdcCtxOut->height = pCdcCtxIn->height;&#xA;      pCdcCtxOut->pix_fmt = AV_PIX_FMT_YUV420P;&#xA;      pCdcCtxOut->gop_size = 25;&#xA;&#xA;      // Going for 6Mbit/s&#xA;      pCdcCtxOut->bit_rate = 6000000;&#xA;      //pCdcCtxOut->rc_min_rate = pCdcCtxOut->bit_rate;&#xA;      pCdcCtxOut->rc_max_rate = pCdcCtxOut->bit_rate;&#xA;      pCdcCtxOut->rc_buffer_size = pCdcCtxOut->bit_rate;&#xA;      pCdcCtxOut->rc_initial_buffer_occupancy = static_cast<int>((pCdcCtxOut->bit_rate * 9) / 10);&#xA;&#xA;      std::string strParams = "vbv-maxrate="&#xA;                              &#x2B; std::to_string(pCdcCtxOut->bit_rate / 1000)&#xA;                              &#x2B; ":vbv-bufsize="&#xA;                              &#x2B; std::to_string(pCdcCtxOut->bit_rate / 1000)&#xA;                              &#x2B; ":force-cfr=1:nal-hrd=cbr";&#xA;&#xA;      av_dict_set(&amp;pDict, "x264-params", strParams.c_str(), 0);&#xA;&#xA;      pCdcCtxOut->field_order = AV_FIELD_TT;&#xA;      pCdcCtxOut->flags = (AV_CODEC_FLAG_INTERLACED_DCT | AV_CODEC_FLAG_INTERLACED_ME | AV_CODEC_FLAG_CLOSED_GOP);&#xA;&#xA;      // WARN: Make some assumptions here!&#xA;      pCdcCtxOut->time_base = AVRational{1,25};&#xA;      pCdcCtxOut->framerate = AVRational{25,1};&#xA;      pCdcCtxOut->sample_aspect_ratio = AVRational{64,45};&#xA;</int>

  


  The output graphs appear very different :

  


  

  


  Above is the FFmpeg CLI output - video bitrate holds fairly steady.

  


  

  


  Above is the output of my sample application - some significant dips in the video bitrate.

  


  I've taken this a step further and created a git repo consisting of :

  


  

  • Code of sample application

  


  • Test input file (.mp4)

  


  • Outputs (.ts file) of tests

  


  • Graphs of output bitrates.

  


  


• avformat/au : Remove redundant av_freep()

  13 juillet 2020, par Andreas Rheinhardt

  avformat/au : Remove redundant av_freep()
  This av_freep(&key) in conjunction with the fact that the loop condition
  
  checks for key != NULL was equivalent to a av_freep(&key) + a break
  
  immediately thereafter. But given that there is an av_freep(&key)
  
  directly after the loop, the av_freep(&key) is unnecessary and the break
  
  can also be added explicitly.
  Signed-off-by : Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
  

  • [DH] libavformat/au.c

• Play MJPEG stream to v4l2loopback device

  28 juillet 2020, par ahoffer

  I have a RaspberryPi with a V1 camera.

  


  I want to use it as my camera for Zoom, Teams, and other virtual meetings.

  


  I have the RaspberryPi streaming motion JPEG over http using the application named "Motion".

  


  I created a v4l2 dummy device on my linux laptop as a virtual camera.

  


  However, I can't get the mjpeg stream to play to the v4l2 dummy device with the simple command :

  


  ffmpeg  -i "http://milan.local:8081" -map 0:v  -f v4l2 /dev/video4

  


  The result is :

  


  Input #0, mpjpeg, from 'http://milan.local:8081':
  Duration: N/A, bitrate: N/A
    Stream #0:0: Video: mjpeg, yuvj420p(pc, bt470bg/unknown/unknown), 640x480 [SAR 1:1 DAR 4:3], 25 tbr, 25 tbn, 25 tbc
Stream mapping:
  Stream #0:0 -> #0:0 (mjpeg (native) -> rawvideo (native))
Press [q] to stop, [?] for help
[v4l2 @ 0x55a04567bbe0] Unknown V4L2 pixel format equivalent for yuvj420p
Could not write header for output file #0 (incorrect codec parameters ?): Invalid argument
Error initializing output stream 0:0 --


  


  ---

  


  What options am I missing from my ffmpeg command ?