WEBVTT - Text Alignment values missing from output

[vish91]

Issue and steps to reproduce the problem

Packager Command:
To reproduce from master branch try running on test input

$ packager 'in=SampleVideo_1280x720_30mb.mp4,stream=audio,segment_template=/tmp/Sample_video_1280x720_hls/seg_$Number$.aac,lang=en,hls_name=English' 'in=1835844163863-web.vtt,stream=text,language=en,segment_template=/tmp/1835844163863-ttml-hls/seg_$Number$.vtt,hls_group_id=cbsi_vtt,hls_name=English' \
--segment_duration 6 --hls_master_playlist_output /tmp/SampleVideo_1280x720_30mb_hls/master.m3u8

[0324/172546:INFO:demuxer.cc(89)] Demuxer::Run() on file '1835844163863-web.vtt'.
[0324/172546:INFO:demuxer.cc(155)] Initialize Demuxer for file '1835844163863-web.vtt'.
[0324/172546:INFO:demuxer.cc(89)] Demuxer::Run() on file 'SampleVideo_1280x720_30mb.mp4'.
[0324/172546:INFO:demuxer.cc(155)] Initialize Demuxer for file 'SampleVideo_1280x720_30mb.mp4'.
[0324/172549:INFO:mp4_media_parser.cc(345)] Ignore unused 'mdat' box - this could be as a result of extra not usable 'mdat' or 'mdat' associated with unrecognized track.
Packaging completed successfully.
$ cat /tmp/1835844163863-ttml-hls/seg_1.vtt
WEBVTT
X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:9000

00:00:03.670 --> 00:00:04.462 line:85% position:28% size:35%
>> Devon: Hey.

00:00:04.546 --> 00:00:05.088 line:79% position:10% size:73% align:start
>> Amanda: Naya backed out on
our meeting.

00:00:05.171 --> 00:00:07.424 line:85% position:43% size:65%
>> Devon: Did she say why?

with this fix

$packager 'in=SampleVideo_1280x720_30mb.mp4,stream=video,segment_template=/tmp/SampleVideo_1280x720_30mb_hls/seg_$Number$.ts,iframe_playlist_name=SampleVideo_1280/stream_iframe.m3u8,playlist_name=SampleVideo1280x720/stream.m3u8'\
'in=SampleVideo_1280x720_30mb.mp4,stream=audio,segment_template=/tmp/Sample_video_1280x720_hls/seg_$Number$.aac,lang=en,hls_name=English' 'in=1835844163863-web.vtt,stream=text,language=en,segment_template=/tmp/1835844163863-ttml-hls/seg_$Number$.vtt,hls_group_id=cbsi_vtt,hls_name=English' \
--segment_duration 6 --hls_master_playlist_output /tmp/SampleVideo_1280x720_30mb_hls/master.m3u8
[0324/172855:INFO:demuxer.cc(89)] Demuxer::Run() on file '1835844163863-web.vtt'.
[0324/172855:INFO:demuxer.cc(155)] Initialize Demuxer for file '1835844163863-web.vtt'.
[0324/172855:INFO:demuxer.cc(89)] Demuxer::Run() on file 'SampleVideo_1280x720_30mb.mp4'.
[0324/172855:INFO:demuxer.cc(155)] Initialize Demuxer for file 'SampleVideo_1280x720_30mb.mp4'.
[0324/172859:INFO:mp4_media_parser.cc(345)] Ignore unused 'mdat' box - this could be as a result of extra not usable 'mdat' or 'mdat' associated with unrecognized track.
Packaging completed successfully.

$ cat /tmp/1835844163863-ttml-hls/seg_1.vtt
WEBVTT
X-TIMESTAMP-MAP=LOCAL:00:00:00.000,MPEGTS:9000

00:00:03.670 --> 00:00:04.462 line:85% position:28% size:35% align:center
>> Devon: Hey.

00:00:04.546 --> 00:00:05.088 line:79% position:10% size:73% align:start
>> Amanda: Naya backed out on
our meeting.

00:00:05.171 --> 00:00:07.424 line:85% position:43% size:65% align:center
>> Devon: Did she say why?

What is the expected result?

• Expected VTT files should carry over styling and text properties based on the WEBVTT spec and input file
  What happens instead?
• Outputs omits certain properties.

[vish91]

@kqyang found a small bug. Please take a look when you can.
It's causing problems with mainly older version of ExoPlayer or other players who do not handle default webvtt text alignment missing on cue's.
Looking into the webvtt parser, the parser is doing things correctly just the webvtt utils that should be writing these bytes to out stream was missing what to do when text alignment is center.
PR linked ^

[kqyang]

Thanks @vish91 for troubleshooting and fixing the problem.

@TheModMaker Can you review @vish91's PR?

[vish91]

@kqyang and @TheModMaker some test cases have failed, I'll update that. Before I fix that, correct me if I am wrong here but I think the parser is initially written to consider align:center as default so it doesn't add align:center to outputs packager generates.
Which means we are not considering the fact that for backwards compatibility if my input vtt has align:center , my output files should have it too instead it's getting ignored today. So this change will add align:center which will fix the legacy player issues that don't automatically align because they still are getting align:middle instead of align:center or not seeing an alignment property and don't know what to make of it.
https://github.com/google/ExoPlayer/blob/release-v2/library/core/src/main/java/com/google/android/exoplayer2/text/webvtt/WebvttCueParser.java#L459-L478

[vish91]

@kqyang @TheModMaker any thoughts ?