SSA AlphaLevel support

[libass]

SSA AlphaLevel support

[astiob]

Frankly, I don’t see how the override tags relate to injecting something through player settings. But let’s get on to the main question.

AlphaLevel is a style property in SSA; it does not exist in ASS because alpha is specified in the colour fields. I think libass currently ignores AlphaLevel; this should probably be fixed. I suppose we can add support for AlphaLevel forced overrides at the same time. Note that you can already use colour field overrides to change the alpha, but, of course, you need to specify the colour too, so this is probably not very useful if you just want all subtitles to be semi-transparent.

Note also that changing AlphaLevel does not make the whole line equally translucent: it affects shadows separately from main glyphs and outlines, so if you have a line with some shadow, you will see a translucent shadow and overlaid on top of that translucent glyphs (with borders).

[bitingsock]

Sorry, I'm not actually very knowledgeable about a lot of this. Just seemed like libass might implement it this or some other way.
The specs said "AlphaLevel. This defines the transparency of the text." do you just have to specify which element it's affecting or what?
Anyway, do you know why they don't include it in ASS? Seems useful still. Or do you know of anyway to change alpha levels but keep the same colors the style dictates?