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• How to send encoded video (or audio) data from server to client in a way that's decodable by webcodecs API using minimal latency and data overhead

  11 janvier 2023, par Tiger Yang

  My question (read entire post for context) :

  


  Given the unique circumstance of only ever decoding data from a specifically-configured encoder, what is the best way I can send the encoded bitstream along with the bare minimum extra bytes required to properly configure the decoder on the client's end (including only things that change per stream, and omitting things that don't, such as resolution) ? I'm a sucker for zero compromises, and I think I am willing to design my own minimal container format to accomplish this.

  


  Context and problem :

  


  I'm working on a remote desktop implementation that consists of a server that captures and encodes the display and speakers using FFmpeg and forwards it via pipe to a go (language) program which sends it on two unidirectional webtransport streams to my client, which I plan to decode using the webcodecs API. According to MDN, the video decoder needs to be fed via .configure() an object containing the following : https://developer.mozilla.org/en-US/docs/Web/API/VideoDecoder/configure before it's able to decode anything.

  


  same goes for the audio decoder : https://developer.mozilla.org/en-US/docs/Web/API/AudioDecoder/configure

  


  What I've tried so far :

  


  Because this remote desktop will be for my personal use only, it would only ever receive streams from a specific encoder configured in a specific way encoding video at a specific resolution, framerate, color space, etc.. Therefore, I took my video capture FFmpeg command...

  


  videoString := []string{
        "ffmpeg",
        "-init_hw_device", "d3d11va",
        "-filter_complex", "ddagrab=video_size=1920x1080:framerate=60",
        "-vcodec", "hevc_nvenc",
        "-tune", "ll",
        "-preset", "p7",
        "-spatial_aq", "1",
        "-temporal_aq", "1",
        "-forced-idr", "1",
        "-rc", "cbr",
        "-b:v", "500K",
        "-no-scenecut", "1",
        "-g", "216000",
        "-f", "hevc", "-",
    }


  


  ...and instructed it to write to an mp4 file instead of outputting to pipe, and then I had this webcodecs demo https://w3c.github.io/webcodecs/samples/video-decode-display/ demux it using mp4box.js. Knowing that the demo outputs a proper .configure() object, I blindly copied it and had my client configure using that every time. Sadly, it didn't work, and I since noticed that the "description" part of the configure object changes despite the encoder and parameters being the same.

  


  I knew that mp4 files worked via mp4box, but they can't be streamed with low latency over a network, and additionally, ffmpeg's -f parameters specifies the muxer to use, but there are so many different types.

  


  At this point, I think I'm completely out of my depth, so :

  


  Given the unique circumstance of only ever decoding data from a specifically-configured encoder, what is the best way I can send the encoded bitstream along with the bare minimum extra bytes required to properly configure the decoder on the client's end (including only things that change per stream, and omitting things that don't, such as resolution) ? I'm a sucker for zero compromises, and I think I am willing to design my own minimal container format to accomplish this. (copied above)

  


• ffmpeg save video in segments, time does not match

  22 août 2022, par Yohann

  I use ffmpeg to save the rtsp live stream to the local, specify the fragment size of 2 seconds and use the current time to name the video file

  


  But the length of the returned video is not equal to 2 seconds and the interval between file names is not 2 seconds

  


  I have tried several command line arguments

  


  ffmpeg -i rtsp://192.168.18.119/live/test -f segment -strftime 1 -segment_time 2 -segment_atclocktime 1 -reset_timestamps 1 -an -vcodec copy ./%Y-%m-%d-%H.%M.%S.mp4

# execute result
2022-08-22-17.20.16.mp4  duration：00:03
2022-08-22-17.20.17.mp4  duration：00:03
2022-08-22-17.20.18.mp4  duration：00:07
2022-08-22-17.20.19.mp4  duration：00:03
2022-08-22-17.20.20.mp4  duration：00:03
# file names are not time aligned


  


  ffmpeg -i rtsp://192.168.18.119/live/test -f segment -strftime 1 -segment_time 2 -reset_timestamps 1 -an -vcodec copy ./%Y-%m-%d-%H.%M.%S.mp4

# execute result
2022-08-22-17.42.36.mp4 duration：00:03
2022-08-22-17.42.37.mp4 duration：00:03
2022-08-22-17.42.41.mD4 duration：00:03
2022-08-22-17.42.44.mp4 duration：00:03
2022-08-22-17.42.48.mp4 duration：00:03
2022-08-22-17.42.51.mp4 duration：00:03
# file names are not time aligned


  


  Expect to generate video

  


  2022-08-22-17.50.00.mp4 时长：00:02
2022-08-22-17.50.02.mp4 时长：00:02
2022-08-22-17.50.04.mD4 时长：00:02
2022-08-22-17.50.06.mp4 时长：00:02
2022-08-22-17.50.08.mp4 时长：00:02
2022-08-22-17.50.10.mp4 时长：00:02
...


  


• Revision 6b7ec50812 : fixed a bug of context overwritten by key frame recoding The recoding loop save

  15 mars 2012, par Yaowu Xu

  Changed Paths : Modify /vp8/encoder/ratectrl.c fixed a bug of context overwritten by key frame recoding The recoding loop save and restore frame coding context for recodes. However in recoding of key frames, some of the coding context saved was stale from last encoded inter frame. The (...)