TTML: Support referential styling

[evandipietro]

Issue description

Observed: MP4-Encapsulated TTML subtitles default to top position (with positionAnchor defaulting to Float.MIN_VALUE)
Expected: Subtitles appear at the bottom of the screen with a valid position

Reproduction steps

Using a dash steam w/ MP4-Encapsulated TTML subtitles, enable subtitles. Subtitles appear at the top.

Link to test content

Cannot provide (legal reasons)

Version of ExoPlayer being used

2.4.1 (Was not an issue in 2.4.0)

Device(s) and version(s) of Android being used

Device: FireTV
OS: Fire OS v5.2.4.1 (Android v5.1.1)

[ojw28]

Are you sure they're not TTML captions? I don't think we made any changes to WebVTT captioning between 2.4.0 and 2.4.1.

[evandipietro]

@ojw28 My mistake, it is TTML. Updating the issue.

[ojw28]

We fixed TTML positioning in 2.4.1 (at least in theory). It's quite possible that your TTML caption files are actually specifying that the captions should be rendered at the top of the screen (or that the implicitly specify this by not specifying something else), and that what the previous behavior was incorrect.

There's nothing we can do to look at this without test content (or at least the TTML part of it).

[soutua]

We also encountered this problem with the ExoPlayer 2.4.1/2.4.2. The subtitles we use do not define the alignment so the subtitles are displayed at the top of the screen. Would it be possible to add a way to change the default position from top to bottom, in case the position isn't defined in the subtitles?

[ojw28]

We need to see sample TTML files to investigate this properly.

[zharf]

I extracted a snippet from our ttml stream (and swapped out the texts just in case) and attached it here. My initial investigation suggests that ExoPlayer doesn't support (chained?) referential styling as described by TTML

sidenote: I work with @soutua

ttml.zip

[ojw28]

@zharf - Yes, that looks like the problem for your case. Thanks for the sample. We should fix that, after which you'll get proper positioning as is defined in your TTML file.
@evandipietro - There's no way we can verify whether your case is the same or different without a sample TTML file, so we'll use this issue to track supporting referential styling as in @zharf 's case unless we learn more.

Thanks!

[ojw28]

The best short term fix for this is probably to ignore regions that don't directly define origin and extent, which will cause the the captions to go back to being displayed at the bottom again. We currently set default origin and extent in this case, but that results in unexpected behavior when the origin and extent are actually defined in the chained styles that we're not currently resolving. Does that sound sensible?

[zharf]

I'm not fully familiar with how this thing works as of right now but from what I understand it sounds like it could work for now. We can also test any suggested fixes.

[jonhan]

I have the same issue so +1