Support Webvtt subtitles class colors

[danybony]

Problem

Colors defined in Webvtt cue Class tags were ignored.
For example in the screenshots below the cue text was

At the <c.red.caps>left</c> we can see...

Example asset: http://dash.edgesuite.net/akamai/test/caption_test/ElephantsDream/elephants_dream_480p_heaac5_1.mpd

Solution

WebvttCueParser already knows about the cue classes, but these were not used. So in case of a color class, we now apply a ForegroundColorSpan to the subtitle text.

Screenshots

Before	After

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[danybony]

I signed it!

[AquilesCanta]

I thought VTT required the red class to be defined for the style to apply. Would you kindly provide a specification reference for this behavior change?

[AquilesCanta]

Closing due to lack of activity.

[icbaker]

Relevant specs:
https://www.w3.org/TR/webvtt1/#default-text-color
https://www.w3.org/TR/webvtt1/#default-text-background

Once this has a signed CLA and has been rebased/merged into current dev-v2 I can take a more in-depth look at the code.

[icbaker]

I think this will likely also fix #6581

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[icbaker]

This is neat code re-use, but because the WebVTT list is so much shorter than the list of CSS colors I think it would be clearer to explicitly list the WebVTT default colors. Otherwise if someone defines a custom class called papayawhip in their style section that maybe has nothing to do with that color, we'll color the cue anyway (which is not in-line with the WebVTT spec).

So I think a private static final Map<String, Integer> DEFAULT_COLORS inside WebvttCueParser would be the best approach here. You'll probably have to populate it in a static initializer block the same as in ColorParser.

That will also make adding background color support less 'different' - we can just add a DEFAULT_BACKGROUND_COLORS map alongside. If you have the time/inclination, feel free to also add that in this same PR! :)

[danybony]

Sounds good, I'll take a look and update this soon

[danybony]

Updated the strategy by adding the two maps (adds support for background too) in WebvttCueParser and removing the dependency from the color parser.
Added relevant tests too

[icbaker]

Can you add some tests to WebvttCueParserTest for this?

[icbaker]

Thanks! I'll get this merged shortly.

[zegnus]

hi @icbaker what are the next steps on this PR?