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HLS super fast frame accuracy trimming (JS)

Image of HLS trimming

#Usage

  • Install: ffmpeg and nodejs
  • Ensure ffmpeg is in the PATH
  • Use this syntax: ./trim_hls_frame_acc source_hls dest_file tmp_dir trim_in trim_out
    • Example1: ./trim_hls_frame_acc /hls_test/ /out/test.mp4 /tmp 10.0 21.2
    • Example2: ./trim_hls_frame_acc /hls_test/test.m3u8 /out/test.mp4 /tmp 10.0 21.2

#Description

  • This code is an evolution of this code: [Trim HLS stream with frame accuracy using ffmpeg and Bash script](https://jordicenzano.name/2014/08/07/trim-hls-stream-with-frame-accuracy-using-ffmpeg-and-bash-script-2/)
  • The purpose of this code gets a group if HLS chunks (.ts) and input and output timestamps and creates an output mp4 file frame accuracy trimmed, AV perfectly aligned, and thanks to the underlying algorithm in a very small amount of time.
  • It can be useful to:
    • On the fly live streaming trimming (highlights)
    • VOD frame accuracy trimming
    • Increase dramatically the speed of video editors (mostly cloud video editors)

#How it works

  1. Based on the in an out points analyze the input files an establish which are the first and last segments
  2. Split A/V of the necessary chunks, creating a .ts (video only) and .acc files. It is just a parsing operation.
  3. For the first and the last chunks:
    • Get video frame position information from and calculate the video cutting point to preserve frame accuracy
    • Decode to YUV
    • Encode the trimmed YUV to the same codec as the original files
  4. Concatenate the video segments to a final video .ts
  5. For the first and the last audio chunks:
    • Get audio frame position information and calculate the audio cutting points based on video cutting points to preserve A/V alignment
    • Calculate AV delay necessary at the output file to preserve AV alignment
    • Trim the stream at packet level
  6. Concatenate the audio segments to a final audio .aac
  7. Mux final video and final audio file to MP4 preserving the previously calculated A/V delay

#Assumptions

  • The video codec is H264 and the audio coded is aac
  • All the chunks start with and I frame (wowza forces an I frame at start of chunks, event the transmuxed ones)
  • Only 1 video and 1 audio present in the input files
  • If you use hls manifest (.m3u8) as a source, only relative local paths are allowed
  • The A/V delay in the original stream is reasonable (<2s)

#Future work

  • Make the code usable for any input different than HLS
  • Apply multi-threaded processing in some stages to speed up the process (reasonable goal is to increase the speed x2)
  • Modify the code to process the input files at video GOP level (not chunk level)
    • Increase the processing speed (GOPs use to be smaller than chunks)
    • No need that chunks start with I frame
    • Easier to handle other formats different than HLS