HLS / DASH support for forced subtitles

[vish91]

Does shaka-packager support setting forces subtitles in HLS and DASH. Mainly DASH ?

In HLS it's the stream media tag FORCED
and in DASH its a ROLE forced-subtitles

Dash-Industry-Forum/DASH-IF-IOP#165

Dash-Industry-Forum/Ingest#40

Based on the first issue it looks like Shaka-player and ExoPlayer do support it for HLS and DASH

[kqyang]

That is not supported right now.

To support DASH forced-subtitles, a new role needs to be added. You can look at other roles to see how to add a new role, e.g. https://github.com/google/shaka-packager/search?q=kRoleCommentary.

To support HLS FORCED media tag, a new stream descriptor needs to be added. You can checkout hls_characteristics to see how to add support for a new stream descriptor: https://github.com/google/shaka-packager/search?q=hls_characteristics. We may use the same stream descriptor tag to signal DASH forced-subtitles too.

@vish91 Do you want to work on it?

[raestrada95]

@kqyang i am also interested in this function it would be great if you could implement it

[vish91]

@kqyang for DASH I understood and made progress.
For HLS are you suggesting it should be a stream descriptor property like hls_characteristics and not a Media Playlist Tag property ?
Thinking because I see default and autoselect being defined as is_default and is_autoselect in
https://github.com/google/shaka-packager/blob/master/packager/hls/base/master_playlist.cc#L276
That will also help me decide what kind of data type I use if we decide to use it as a stream descriptor so like boolean vs enum string for YES/NO property values.

[kqyang]

Yes, it needs to be a stream descriptor property. The media playlist tag property is derived internally in the code.

I think you can define it as a boolean and parse it in the same way as dash_only: https://github.com/google/shaka-packager/blob/master/packager/app/stream_descriptor.cc#L224

[vish91]

@kqyang gotcha and if its a stream descriptor. Does it still need to be declared or set after parsing it in stream_descriptor.cc ? Like how the hls_characterisctics after parsing are still being set in media_playlist.cc and hls_notify_muxer_listener. Trying to gauge where the actual conversion would happen from when it's parsed as bool in stream_descriptor to where the media_playlist is getting created.
One more question: once we parse and define the FORCED flag as per spec we should also make sure that AUTOSELECT=YES always if the forced flag is set in the master_playlist.cc file ? Does that sound right ?

[kqyang]

Does it still need to be declared or set after parsing it in stream_descriptor.cc ?

Yes, I suggest defining an accessor in media_playlist.h similar to characteristics() function.

One more question: once we parse and define the FORCED flag as per spec we should also make sure that AUTOSELECT=YES always if the forced flag is set in the master_playlist.cc file ? Does that sound right ?

I don't think we should set AUTOSELECT to YES for FORCED subtitles.

[defagos]

I don't think we should set AUTOSELECT to YES for FORCED subtitles.

According to rule 5.11 of the HTTP Live Streaming (HLS) authoring specification for Apple devices forced subtitles SHOULD always have AUTOSELECT=YES.