[Feature request] Add support for transcripts

[SamantazFox]

Add support for transcripts

They're accessed via the following InnerTube endpoint:
https://www.youtube.com/youtubei/v1/get_transcript?key=AIzaSyAO_FJ2SlqU8Q4STEHLGCilw_Y9_11qcW8
And requested via "params": <protobuf encoded video-id>

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Notes:

• research is required to know if this could replace the legacy caption endpoint (do their content differ?)
• A videoJS plugin need to be written for those
• They also require quite some work on the UI to integrate well.

Issue opened on the behalf of @syeopite, and thanks to @TiA4f8R for the screenshot :)

[MinePlayersPE]

For replacing captions, note that there wouldn't be any styling info (positioning, color, etc) with the transcriptions (also some videos like 0hEvBW2NFQU will give spammy transcripts with mismatched timestamps but this also happens on normal VTT captions)
semi-PoC for transcript => vtt: https://gist.github.com/MinePlayersPE/f645f15d477be694748df721492d8a38

[syeopite]

For supporting transcripts there's videojs-transcript though it doesn't appear to be maintained.

[nidhoggr-nil]

I imagine this would help both for people with disabilities and also for tools which use the transcript as a source for summarization.

[syeopite]

Yep transcripts are great for that.

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Since the issue opened in 2021, the logic for requesting transcripts has been added to the code to be used as a workaround for captions on larger instances.

Now all that's left to do is to add a proper API endpoint for video transcriptions, and a UI component.

[callum-gander]

Any movement on this since Jan?

[syeopite]

I've finished implementing a basic transcripts feature into Invidious.

You can find the code over at my branch here https://github.com/syeopite/invidious/tree/transcripts-support

The feature is pretty much done but I'd like to work on a couple more QOL improvements, particularly to people with JS enabled, before sending a PR upstream.