Adaptive playback (hls/dash)

[zoriya]

Expected behavior of the wanted feature

One of the main selling point of hls/dash is adaptive playback: having the player automatically switch between variants/renditions as the network bandwidth changes. It would be great to have optional support for this in mpv.

First discussed in #7033

Alternative behavior of the wanted feature

Write a lua script to manually toggle renditions but i don't think we can currently change renditions without clearing the playback cache (which is a requirement for seamless adaptive playback)

Log File

N/A

Sample Files

https://playertest.longtailvideo.com/adaptive/elephants_dream_v4/index.m3u8

[kasper93]

Architecturally this should be implemented in ffmpeg hls/dash demuxer. Currently it exposes all streams as separate tracks, which are tagged with program_id which maps to renditions. There is no precedence for this, but it probably should expose virtual track, that would be fed data with variable quality.

Why not do that in mpv? While it would be possible to implement rate control based on cache health, the track switching is not seamless. We operate on AVPackets from demuxer and while we could make it preserve cache, join packets and so on, it seems like a workaround not a real solution. Especially when we are starved for cache it would be hard to join two tracks in seamless way. Unlike lavf we don't control which segments are (pre)loaded and how. Lavf can load segments and dynamically flip to lower quality, without any disruption and with possible lookahead to make sure there is no pauses. This is the only way to implement this correctly. Although, on ffmpeg side the rate control information may not be available, because it's not really designed for real time playback in mind, but rather as a processing library, so the fact something is "too slow" is the player (mpv) information. So, there is conflict of interest here. There could be API for that, but generally it's just not that compatible with current design of ffmpeg. Unless of course we implement hls and dash demuxer in mpv. Which is not that big of a task, but seems not needed if we have one available already.

So, in fact it probably would be easier to re-implement demxuers in mpv itself, instead of trying to plumb it to the current ones. All in all, it's fun task, but I myself am not that interested in this feature.