TTML: Support tts:lineHeight setting

[tpiDev]

[REQUIRED] Use case description

Configurable Subtitles line spacing (extra, multiplier)
Business requirement is that lines needed to be further apart (based on provided line spacing). With ExoPlayer it can be achieved currently by applying lineSpacingMultiplier to Activity theme, but it seems to be not the best approach, as it introduces global change.

Proposed solution

Currently line spacing logic is handled in SubtitlePainter - lineSpacingExtra and lineSpacingMultiplier is taken from Activity theme. Consider adding method to set expected values with SubtitleView, or add it to CaptionStyleCompat?

Alternatives considered

n/a

[icbaker]

Could you expand the use-case description? It would help our prioritization to understand the context a little more.

Some questions I have:

• Do you control the subtitles being displayed (i.e. they're being streamed from your servers) or are they user-specified (either as sidecar files or embedded in videos)?
• What do you need to change about these values? (I kinda assume you want to make the lines further apart, but would be good to know for sure)

[tpiDev]

Hi @icbaker We have DASH media with TTML subtitles.
Business requirement is that lines needed to be further apart (based on provided line spacing). With ExoPlayer it can be achieved currently by applying lineSpacingMultiplier to Activity theme, but it seems to be not the best approach, as it introduces global change.

[nigelmegitt]

In TTML you should be able to set tts:lineHeight to modify the line spacing; that does of course require that the presentation processor (i.e. ExoPlayer in this case) honours the setting appropriately. I haven't checked to see if ExoPlayer does so, but this would be the "standard" way to achieve a functionality other than the default "normal" line spacing.

[icbaker]

Thanks for the context.

I'm hesitant to add more configuration options to SubtitleView, and I don't think CaptionStyleCompat is the right place for this (since this option doesn't exist on CaptionStyle).

As @nigelmegitt says, TTML allows you to specify this using tts:lineHeight. ExoPlayer doesn't currently parse/present that setting correctly, but definitely could do. I think that would be a more reasonable change than adding additional options to SubtitleView.

[tpiDev]

@icbaker What is status of ticket, Is the improvement on roadmap ? :)

[icbaker]

We don't have any plans to implement this in the short term.

As always, we would accept a high quality pull request adding support.