278 lines
9.3 KiB
Markdown
278 lines
9.3 KiB
Markdown
Title: Package my video
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Date: 2024-06-20T13:19:03-04:00
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Draft: 1
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---
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## TODO
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* Some intro
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* Investigate key frames and seeking
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--------------------------------------------------------------------------------
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The goal of this project is to get a minimum viable product of:
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* support H264 video and AAC audio
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* experiment with AV1 and Opus if time permits
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* produce a .mp4 file that is optimized for: raw progressive playback and segment generation
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* generate a HLS playlist on demand
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* generate the Nth segment from a HLS file on demand
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## Setup
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### ffmpeg
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First I want to start with the latest available ffmpeg static build from: <https://johnvansickle.com/ffmpeg/>
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```shell
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❯ curl -sL -O https://johnvansickle.com/ffmpeg/releases/ffmpeg-release-amd64-static.tar.xz
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❯ sudo tar -C /opt -xvf ffmpeg-release-amd64-static.tar.xz
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```
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My preference is to then link my preferred build to some location (`/opt/ffmpeg-static`) that I will then add to my `PATH`.
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```
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❯ sudo ln -sf /opt/ffmpeg-7.0.1-amd64-static/ /opt/ffmpeg-static
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# then edit your shell rc or profile, reset shell
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❯ type ffmpeg
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ffmpeg is /opt/ffmpeg-static/ffmpeg
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❯ ffmpeg -version
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ffmpeg version 7.0.1-static https://johnvansickle.com/ffmpeg/ Copyright (c) 2000-2024 the FFmpeg developers
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built with gcc 8 (Debian 8.3.0-6)
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configuration: --enable-gpl --enable-version3 --enable-static --disable-debug --disable-ffplay --disable-indev=sndio --disable-outdev=sndio --cc=gcc --enable-fontconfig --enable-frei0r --enable-gnutls --enable-gmp --enable-libgme --enable-gray --enable-libaom --enable-libfribidi --enable-libass --enable-libvmaf --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-librubberband --enable-libsoxr --enable-libspeex --enable-libsrt --enable-libvorbis --enable-libopus --enable-libtheora --enable-libvidstab --enable-libvo-amrwbenc --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libdav1d --enable-libxvid --enable-libzvbi --enable-libzimg
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libavutil 59. 8.100 / 59. 8.100
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libavcodec 61. 3.100 / 61. 3.100
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libavformat 61. 1.100 / 61. 1.100
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libavdevice 61. 1.100 / 61. 1.100
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libavfilter 10. 1.100 / 10. 1.100
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libswscale 8. 1.100 / 8. 1.100
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libswresample 5. 1.100 / 5. 1.100
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libpostproc 58. 1.100 / 58. 1.100
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```
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And checking codec support
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```
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❯ ffmpeg -codecs 2>/dev/null | grep '\s\(aac\|h264\|av1\|opus\)'
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DEV.L. av1 Alliance for Open Media AV1 (decoders: libdav1d libaom-av1 av1) (encoders: libaom-av1)
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DEV.LS h264 H.264 / AVC / MPEG-4 AVC / MPEG-4 part 10 (decoders: h264 h264_v4l2m2m) (encoders: libx264 libx264rgb h264_v4l2m2m)
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DEAIL. aac AAC (Advanced Audio Coding) (decoders: aac aac_fixed)
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D.AIL. aac_latm AAC LATM (Advanced Audio Coding LATM syntax)
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DEAIL. opus Opus (Opus Interactive Audio Codec) (decoders: opus libopus) (encoders: opus libopus)
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```
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### Test video file
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Here are a few and open test video sources:
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* Sintel: <https://download.blender.org/durian/movies/>
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* License: [Creative Commons Attribution 3.0](https://web.archive.org/web/20240105060647/https://durian.blender.org/sharing/)
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* Big Buck Bunny: <https://download.blender.org/peach/bigbuckbunny_movies/>
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* License: [Creative Commons Attribution 3.0](https://web.archive.org/web/20240521095028/https://peach.blender.org/about/)
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I grabbed a 720p version of each
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```shell
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❯ du -h test-videos/*
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398M test-videos/big_buck_bunny_720p_h264.mov
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650M test-videos/Sintel.2010.720p.mkv
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```
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### Deciding on eventual segment size
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Target segment size will hold some influence over our progressive transcoding.
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Each segment will begin with at least 1 key frame, so our progressive output key frame placement should line up with where our segments will be extracted.
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Apple [suggests 6 second durations for each HLS segment][apple_hls_seg] for VOD playback with HLS.
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6s would be fine to use, but it's a choice with consequences.
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If there was a desire to use a 3s segment instead, the progressive file would need to re-transcode to insert more key frames.
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So for flexibility's sake will choose 3s for key frames in the progressive transcode, but eventual segments will be 6s.
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### Packaging the progressive file
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But first, let's produce v1 of the files with the target codecs applied (H264 and AAC).
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```
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❯ ffmpeg -i ./test-videos/big_buck_bunny_720p_h264.mov -acodec 'aac' -vcodec 'h264' ./test-videos/bbb_h264_aac.mp4
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❯ ffmpeg -i ./test-videos/Sintel.2010.720p.mkv -acodec 'aac' -vcodec 'h264' ./test-videos/sintel_h264_aac.mp4
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❯ du -h ./test-videos/*
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138M ./test-videos/bbb_h264_aac.mp4
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398M ./test-videos/big_buck_bunny_720p_h264.mov
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650M ./test-videos/Sintel.2010.720p.mkv
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201M ./test-videos/sintel_h264_aac.mp4
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```
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Now lets inspect the frames a bit closer with this script `dumpframes.sh`
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```bash
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#!/usr/bin/env bash
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ffprobe -select_streams v -show_frames -show_entries frame=pict_type -of csv $1
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```
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This should show what [picture type][pic_types] each frame is.
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> I‑frames are the least compressible but don't require other video frames to decode.
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> P‑frames can use data from previous frames to decompress and are more compressible than I‑frames.
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> B‑frames can use both previous and forward frames for data reference to get the highest amount of data compression.
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**I** frames are also called **key frames**.
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So given a dump of Big Buck Bunny (BBB):
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```
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./dumpframes.sh test-videos/bbb_h264_aac.mp4 > bbb_frames.csv
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```
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BBB is 24fps, so every 3 seconds we want to see a key frame. Here are the frame numbers where the key frame should be.
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```
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❯ python3
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>>> fps = 24
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>>> i_frame_s = 3
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>>> for i in range(0, 10): print(i * fps * i_frame_s + 1)
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...
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1
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73
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145
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217
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289
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361
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433
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505
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577
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649
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```
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* First segment should contain 3s of content which will be 72 frames.
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* First frame should be a key frame.
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* Then 71 non I frames.
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* Then the next I frame (frame 73) begins the next segment.
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But our frames are not quite right.
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```shell
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❯ grep -n I ./bbb_frames.csv | head
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1:frame,I,H.26[45] User Data Unregistered SEI message
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7:frame,I
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251:frame,I
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286:frame,I
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379:frame,I
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554:frame,I
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804:frame,I
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1054:frame,I
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1146:frame,I
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1347:frame,I
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```
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This is because we didn't tell ffmpeg anything about how to encode and where to place I frames.
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There are a few options to `libx264` that help control this:
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* `--no-scenecut`: "Disable adaptive I-frame decision"
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* `--keyint`: effectively the key frame interval. Technically it is the "Maximum GOP size"
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* `--min-keyint`: the "Minimum GOP size"
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> A GOP is "Group of Pictures" or the distance between two key frames.
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So lets re-encode with those options. Actually lets write a wrapper script to do this.
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I'll choose something besides bash because there will be a bit of math involved.
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```python
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#!/usr/bin/env python
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import sys
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import json
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import subprocess
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import logging
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logging.basicConfig(level=logging.DEBUG)
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def probe_info(infname):
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cmd = f'ffprobe -v quiet -print_format json -show_format -show_streams {infname}'.split(' ')
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res = subprocess.run(cmd, check=False, capture_output=True)
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logging.info('running cmd %s', ' '.join(cmd))
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ffprobe_dict = json.loads(res.stdout)
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v_stream = None
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for stream in ffprobe_dict.get('streams'):
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if stream.get('codec_type') == 'video':
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v_stream = stream
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break
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r_frame_rate = v_stream.get('r_frame_rate')
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num, denom = r_frame_rate.split('/')
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fps = float(num) / float(denom)
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logging.info('got fps %s', fps)
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return {
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'fps': fps,
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}
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def run_ffmpeg_transcode(infname, outfname, probeinfo, segment_length=3):
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# must be an integer
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keyint = int(probeinfo.get('fps') * segment_length)
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cmd = [
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'ffmpeg',
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'-i',
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infname,
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'-vcodec',
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'libx264',
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'-x264opts',
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f'keyint={keyint}:min-keyint={keyint}:no-scenecut',
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'-acodec',
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'aac',
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outfname
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]
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logging.info('running cmd %s', ' '.join(cmd))
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subprocess.run(cmd, check=True)
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if __name__ == '__main__':
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args = sys.argv
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prog = args.pop(0)
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if len(args) != 2:
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sys.exit(1)
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infname, outfname = args
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probeinfo = probe_info(infname)
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run_ffmpeg_transcode(infname, outfname, probeinfo)
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```
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* Use `ffprobe` to dump the streams as json.
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* Take first video stream.
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* Get the `r_frame_rate` which is a fraction. Eval the fraction as `fps`
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* Calculate the keyframe interval using a static 3s segment length.
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And if we run it, lets take a look at the cmds it executes:
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```
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❯ ./progressive.py ./test-videos/big_buck_bunny_720p_h264.mov ./test-videos/bbb_h264_aac.mp4
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INFO:root:running cmd ffprobe -v quiet -print_format json -show_format -show_streams ./test-videos/big_buck_bunny_720p_h264.mov
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INFO:root:got fps 24.0
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INFO:root:running cmd ffmpeg -i ./test-videos/big_buck_bunny_720p_h264.mov -vcodec libx264 -x264opts keyint=72:min-keyint=72:no-scenecut -acodec aac ./test-videos/bbb_h264_aac.mp4
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```
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Now regenerate the frame dump and check if our I frames match the expected: 1, 73, 145, 217 ...
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```shell
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❯ ./dumpframes.sh test-videos/bbb_h264_aac.mp4 > bbb_frames.csv
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❯ grep -n I ./bbb_vimeo_frames.csv | head
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1:frame,I,H.26[45] User Data Unregistered SEI message
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73:frame,I
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145:frame,I
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217:frame,I
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286:frame,I
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289:frame,I
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361:frame,I
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379:frame,I
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433:frame,I
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505:frame,I
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```
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Excellent!
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[pic_types]: https://en.wikipedia.org/wiki/Video_compression_picture_types
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[apple_hls_seg]: https://developer.apple.com/documentation/http-live-streaming/hls-authoring-specification-for-apple-devices#Media-Segmentation
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