feat: Add an option to support forced subtitles

[amtins]

Description

Proposal to introduce a new useForcedSubtitles option to access forced subtitles when available.

Current State

Currently, VHS ignores forced subtitle tracks. This behavior makes it impossible to access such subtitles, when available, from player.textTracks.

As a result, content with occasional foreign language dialogue cannot be translated, forcing the user to enable subtitles for the entire playback time.

Safari

Referring to the question asked in the Apple Developer Forums. Forced subtitles, if available, should be displayed automatically either according to the system language or the selected audio language. This seems to be confirmed by the Advice about subtitles section.

However, it seems that Safari does not implement this behavior, as confirmed by an Apple Media Engineer in point 2 of the response.

Finally, although Safari does not appear to implement this behavior correctly, forced subtitles are available via document.querySelector('video').textTracks, allowing them to be enabled programmatically.

Http Streaming

The media-groups.js file mentions the implementation choice with reference to point 5.8 of the Apple's HLS Authoring Specification. However, this choice affects point 5.9 of the specification, especially when the content contains multiple audio and occasional translations are not burned into the video.

Specific Changes proposed

• Updated videojs-http-streaming.js by adding the useForcedSubtitles option which is false by default, which can be used as a initialization or source option.
• Updated the media-groups.js file to support the useForcedSubtitles option.
• Update the README.md file documenting the new option.
• Updated the demo to add a check box for the useForcedSubtitles option.
• Add a unit test case in the master-playlist-controller.test.js file to ensure that forced subtitles are loaded.
• Update the settings of the media-groups.test.js file so that it does not have an undefined value.

Use of the new option

• At initialization

  const player = videojs(videoEl, {
    html5 : {
      vhs : { useForcedSubtitles : true }
    }
  });
  
  // Or
  const player = videojs(videoEl, {
    html5 : {
      vhs : { useForcedSubtitles : !videojs.browser.IS_SAFARI }
    }
  });

• Through the src

  const player = videojs(videoEl);
  
  player.src({
    src :'https://example.com/playlist.m3u8', 
    useForcedSubtitles : true 
  });
  
  // Or
  player.src({
    src :'https://example.com/playlist.m3u8', 
    useForcedSubtitles : !videojs.browser.IS_SAFARI
  });

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Unit Tests updated or fixed
  • Docs/guides updated
  • Example created (starter template on JSBin)
• Reviewed by Two Core Contributors

[welcome]

💖 Thanks for opening this pull request! 💖

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[gkatsev]

Thanks for the PR. This seems reasonable to me. Thoughts @videojs/collaborators?

[codecov]

Codecov Report

Merging #1329 (1c42920) into main (c90863c) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1329   +/-   ##
=======================================
  Coverage   85.37%   85.37%           
=======================================
  Files          40       40           
  Lines        9953     9954    +1     
  Branches     2307     2308    +1     
=======================================
+ Hits         8497     8498    +1     
  Misses       1456     1456           

Impacted Files	Coverage Δ
src/media-groups.js	98.66% <100.00%> (ø)
src/videojs-http-streaming.js	90.63% <100.00%> (+0.02%)	⬆️

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[privaloops]

lovely, please merge !

[mister-ben]

Would we want the forced tracks to show up in the menu and be user-selectable, or just programatically selectable as Safari?

Safari uses a kind of "forced" for these tracks. This is not one of the standard kinds but it's worth discussing whether or not we'd want to follow Safari's approach.

[privaloops]

In my opinion forced captions must be displayed without any user decision. Safari's programmatic approach is better.

[amtins]

@mister-ben, @privaloops, thanks for the feedback.

My initial thought was to opt for the simplest approach based on what different players in the market are doing. See:

• hls.js
• jwplayer
• kaltura player
• theoplayer
• visualon

The idea of using a kind of "forced" is very interesting, even if it is not standard. It seems that there was a discussion on this subject within the whatwg(issue 4472) which tends to the same conclusion. This discussion is based on another W3C discussion.

The proposed amendment would be as follows kind: properties.forced ? 'forced' :'subtitles',

https://github.com/amtins/http-streaming/blob/91fa3082aeada6e7175e1dfa57790ca56eb5bfdb/src/media-groups.js#L599-L605

However, the use of "forced" for the kind implies a modification of video.js to add this value in the track-enum, otherwise all forced subtitles will continue to have a kind of "subtitles" even if this PR is updated.

Finally, I can also make a PR in the video.js repo to add the "forced" value. The result will be that the forced subtitles will not be visible in the subtitles menu but will remain available via the API player.textTracks() allowing developers to make their own UX.

[video-archivist-bot]

Hey! We've detected some video files in a comment on this issue. If you'd like to permanently archive these videos and tie them to this project, a maintainer of the project can reply to this issue with the following commands:

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[amtins]

Hi @mister-ben, @gkatsev, is there anything I can do to help this PR be merged?

[misteroneill]

Sorry for the delay on your PR @amtins! It looks reasonable to me, but I'm not a playback expert.

Maybe I could get @adrums86 or @dzianis-dashkevich to have a look?

[dzianis-dashkevich]

LGTM

[adrums86]

My apologies for coming in late with this review. I had previously looked over this PR and simply forgot the final step, my approval 😁 ! Nice work, your contribution is greatly appreciated.

[adrums86]

Updating the branch as it would be great to get this merged before the next release.

[welcome]

Congrats on merging your first pull request! 🎉🎉🎉