A default text track in language X is preferred over a forced text track in language Y when audio language Y is selected

When filing a question:

1. What is the default behavior for forced-subtitle text tracks in exo-player, if no text track is selected then is it expected that the forced-subtitle track, appropriate for that particular audio track will be selected?
2. At present with the demo application, I am able to see repeated text tracks (As mentioned in Issue #9432), and selecting None doesn’t select the appropriate forced-subtitle track but it disables the renderer altogether?

I have shared a sample .mpd file in the mail.

cc @vinodkputta @LuGO0

[icbaker]

I added a comment to #9432 about whether the UI should expose that a forced track is selected at all.

While looking into this issue I also spotted what I think is a bug in DefaultTrackSelector when the content has both default and forced tracks in a variety of languages. Something like:

audio:
  english
  german
text:
  english, default
  english, forced
  german, forced

Current behaviour:

• When the english audio track is selected the default english text track is selected.
  • It’s unclear to me whether this is expected vs selecting the forced text track.
• When the german audio track is selected the default english text track is still selected.
  • This seems unexpected, because the user has expressed a preference for german audio and no overriding preference for subtitle language, so we should select the ‘accompanying’ forced german text track.

I have a pending change which changes this behaviour to:

• When the english audio track is selected the forced english text track is selected.
  • As above, it’s unclear if this is expected vs selecting the default text track.
• When the german audio track is selected the forced german text track is selected.
  • This seems more expected to me.

I’m going to re-open this issue and use it to track making this change.

[icbaker]

I just played around some more, and I think my second bullet was incorrect - I’ve removed it now. The best way to get ‘automatic’ behaviour is just to start playback from scratch. The two track selectors in the demo app interact in quite a confusing way.

I observe:

• Selecting a specific track or ‘None’ (i.e. not ‘Auto’) in the top (demo-app only) selector will cause the same track to be selected in the bottom (StyledPlayerView) selector (providing the StyledPlayerView selector is not already set to ‘None’).
  • However in this state it’s impossible to select a different specific track from the StyledPlayerView selector (tapping them has no effect, the ‘old’ track remains selected).
  • Selecting ‘None’ in the StyledPlayerView selector does work (subtitles disappear) - however the ‘top’ selector still shows the ‘old’ selection.
    • In this state (e.g. top=French, StyledPlayerView=None) then selecting any specific track in the StyledPlayerView will re-enable the top-selected track (i.e. French).
• Selecting ‘Auto’ in the top selector will re-select whatever track was selected by StyledPlayerView before. i.e. the following steps will have the following result:
  • Start playback of the bipbop 16x9 stream. English subtitles are shown. Tops selector says ‘Auto’, StyledPlayerView says ‘English’)
  • Select ‘French’ in StyledPlayerView. French subtitles are shown. Top selector still says ‘Auto’.
  • Select ‘Spanish’ in top selector. Spanish subtitles are shown. StyledPlayerView selector now says ‘Spanish’.
  • Select ‘Auto’ in top selector. French subtitles are shown. StyledPlayerView selector says ‘French’.

I think these are all UI problems resulting from having two different text track selectors in the demo app - with one intended to be a ‘real’ track selector that you might show to end users, while the other is a deliberately ‘advanced’ one that shows all available tracks, even if an end-user should never specifically select them. I don’t think any of this indicates a problem with the underlying track selection logic (which of course is mostly unrelated to the UI components: https://exoplayer.dev/track-selection.html).