Forced Subtitles - Support for IMSC model

[dsilhavy]

Description

Description provided by @nigelmegitt in #4105 (comment):

The Netflix model described is one where there is some subtitle track playing and everything in it is displayed; the player chooses either the "forced-subtitle" track or the "everything" track.

An alternative model to this is supported in IMSC:

• the subtitle document contains both "forced" and "non-forced" subtitles, each type identified by the computed value of itts:forcedDisplay.
• the player passes a displayForcedOnlyMode flag to the subtitle renderer to tell it whether to show only those subtitles where itts:forcedDisplay="true" or all the subtitles.

Ideally, both modes would be supported.

Either way, the presence of a subtitle file that contains forced subtitles in the same language as the user's chosen audio track needs to be identified so that the subtitle file can be loaded and played.

[dsilhavy]

@nigelmegitt Are you aware of any DASH testcontent for this?

[nigelmegitt]

@dsilhavy I am not aware of DASH test content, but https://github.com/w3c/imsc-tests/blob/main/imsc1/ttml/forcedDisplay/forcedDisplay1.ttml is the IMSC test for it.

[nigelmegitt]

@palemieux do you know of any DASH test content for IMSC forced display mode?