Add CMAF closed captions support

[victor-at-work]

This PR will...

Add closed captions support for CMAF streams. This is a modification of @gkolb's work in #2623 for master.

Why is this Pull Request needed?

Closed captions for CMAF are not currently supported.

Are there any points in the code the reviewer needs to double check?

Resolves issues:

Checklist

• changes have been done against master branch, and PR does not conflict
• new unit / functional tests have been added (whenever applicable)
• API or design changes are documented in API.md

[robwalch]

Hi @victor-at-work,

Could you provide a sample stream for testing these changes? Thank you.

[victor-at-work]

@robwalch I'll talk to our team and get back to you on it.

[victor-at-work]

@robwalch To clarify, do we want the test stream hosted somewhere? Or just the files for it are okay?

[robwalch]

@victor-at-work hosted please.

If the stream is ok to host with the hls.js demo page, I can upload and maintain it as part of our test suite, as long as the content is fair use and sfw. Thanks!

[victor-at-work]

@robwalch Our backend folks found this public test stream https://devstreaming-cdn.apple.com/videos/streaming/examples/img_bipbop_adv_example_fmp4/master.m3u8. Please let me know if this is okay.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.

[victor-at-work]

Hi @robwalch, by no means trying to rush you but just checking in to see if there are any updates here. Thank you!

[robwalch]

@victor-at-work The test stream you provided contains a WebVTT subtitles track. As far as I can tell, Safari is displaying the VTT track, not CMAF-608

https://devstreaming-cdn.apple.com/videos/streaming/examples/img_bipbop_adv_example_fmp4/master.m3u8

As an example, if we load only one of the variants in Safari, no captions are displayed:
https://devstreaming-cdn.apple.com/videos/streaming/examples/img_bipbop_adv_example_fmp4/v5/prog_index.m3u8

[robwalch]

Hi @robwalch, by no means trying to rush you but just checking in to see if there are any updates here. Thank you!

I'm not inclined to include this functionality in a minor update since it doesn't appear present in Safari. If I'm mistaken and you can demonstrate this in a sample stream that does not also contain a VTT subtitles track, I'll prioritize these changes over others.

[victor-at-work]

Hi @robwalch, I think Safari displays the VTT track because it prefers that to the CEA track? When I manually disabled the former and enabled the latter CEA captions were then rendered, which shows that Safari does indeed support CMAF CEA. I'm not sure what's up with the variant playlists as neither of the tracks showed up when I played any of them.

Let me know if you still want a sample stream without VTT. I can try and ask our backend team for one.

[robwalch]

Hi @victor-at-work,

When I manually disabled the former and enabled the latter CEA captions were then rendered, which shows that Safari does indeed support CMAF CEA.

OK.

Let me know if you still want a sample stream without VTT. I can try and ask our backend team for one.

Thanks. I would appreciate that. It would help avoid any confusion.

If you could also clarify what you expect to happen when a stream has only CEA caption as well as when a stream has both CEA CCs and VTT subs that would help; Should we display both tracks or are the supposed to be deduplicated based on some property of the MEDIA tags?

[victor-at-work]

@robwalch This should have just a CEA track https://video.twimg.com/ext_tw_video/1319433806244319232/pu/pl/cBRNj-zqFNAUPQm-.m3u8?tag=10.

The existing behavior is that the VTT is shown and CEA hidden if both tracks are present. This seems reasonable to me and aligns with Safari's behavior as well.