Split youtube video's chapter accurately

Yt-dlp and chapters

My main source of music is (gasp) youtube, and often I put very long videos in the background. For example: this civilization VI arrangement.

One thing I do sometimes is to download the video, split by chapters, and randomise the order of the tracks with mpv --shuffle. For this, the following incantation will yield something almost correct:

yt-dlp "https://www.youtube.com/watch?v=fZA7ARptg0M" \
    --no-vid \
    --verbose \
    --split-chapters \
    --output "civ6-$(playlist_index)s-$(title)s.%(ext)s"

This yields a list of file with names following the given template, but they don't quite match the chapter boundaries. This is because of how the video is encoded, the underlying ffmpeg cannot split at other points than keyframes.

Audio only, easy

For audio, it's simple enough since the encoders aren't that expensive to run:

yt-dlp "https://www.youtube.com/watch?v=fZA7ARptg0M" \
    --no-vid \
    --verbose \
    --split-chapters \
    --force-keyframes-at-cut \
    --output "civ6-$(playlist_index)s-$(title)s.%(ext)s"

The option --force-keyframes-at-cut will tell ffmpeg to re-encode the file and then split. This takes around 3 minutes on my machine.

With video !

Now, that's another story.

Unfortunately, changing the keyframes requires re-encoding the video, which is very resource intensive. And using only the CPU for that can take an extremely long times (many hours).

This is where hardware acceleration comes in. Modern GPU comes with support for native decoding and encoding 😍. I'm using linux and a gpu AMD, the wiki has pretty good documentation about setting these things up.

For example, running vainfo gives me:

Trying display: wayland
Trying display: x11
vainfo: VA-API version: 1.20 (libva 2.20.1)
vainfo: Driver version: Mesa Gallium driver 23.3.4-arch1.2 for AMD Radeon RX 7800 XT (radeonsi, navi32, LLVM 16.0.6, DRM 3.56, 6.7.2-arch1-1)
vainfo: Supported profile and entrypoints
      VAProfileH264ConstrainedBaseline:	VAEntrypointVLD
      VAProfileH264ConstrainedBaseline:	VAEntrypointEncSlice
      VAProfileH264Main               :	VAEntrypointVLD
      VAProfileH264Main               :	VAEntrypointEncSlice
      VAProfileH264High               :	VAEntrypointVLD
      VAProfileH264High               :	VAEntrypointEncSlice
      VAProfileHEVCMain               :	VAEntrypointVLD
      VAProfileHEVCMain               :	VAEntrypointEncSlice
      VAProfileHEVCMain10             :	VAEntrypointVLD
      VAProfileHEVCMain10             :	VAEntrypointEncSlice
      VAProfileJPEGBaseline           :	VAEntrypointVLD
      VAProfileVP9Profile0            :	VAEntrypointVLD
      VAProfileVP9Profile2            :	VAEntrypointVLD
      VAProfileAV1Profile0            :	VAEntrypointVLD
      VAProfileAV1Profile0            :	VAEntrypointEncSlice
      VAProfileNone                   :	VAEntrypointVideoProc

VAEntrypointVLD means support for decoding this format, and VAEntrypointEncSlice means support for encoding.

A quick test taken from this page is to run:

ffmpeg -vaapi_device /dev/dri/renderD128 \
    -i input.mp4 \
    -vf 'format=nv12,hwupload' \
    -c:v hevc_vaapi \
    -f mp4 \
    -rc_mode 1 \
    -qp 25 \

To force yt-dlp to use hardware support I needed to add:

--postprocessor-args "-vaapi_device /dev/dri/renderD128 -vf 'format=nv12,hwupload' -c:v av1_vaapi"

Let's dissect that, with the help of the ffmpeg documentation

The final incantation to accurately split the chapters of a given video are:

yt-dlp \
  --verbose \
  "https://www.youtube.com/watch?v=fZA7ARptg0M" \
  --split-chapters \
  --force-keyframes-at-cuts \
  --postprocessor-args "-vaapi_device /dev/dri/renderD128 -vf 'format=nv12,hwupload' -c:v av1_vaapi"


To check if it's actually working, I manually ran the ffmpeg command doing the transcoding. In verbose mode yt-dlp will output the full command, and running it in a terminal will give a progress bar. The key point is that the playback speed should be far greater than 1. On my machine it was around 10x and it took about 10 minutes to transcode the video.

Also, using radeontop I could see some activity on the gpu, that I couldn't see if I omit the --postprocessor-args, as in this later case, everything is done on the CPU.

Additional formats

yt-dlp --list-formats <url> gives a list of available formats, and then one can download the preferred combo. For example: yt-dlp -f 123+456 would download the format 123 for the video and 456 for the audio (or vice versa). This way, one can control what comes in before transcoding.