Add support for SSA OutlineColour (only background)

[egor-n]

#8435

OutlineColour should be treated as the background color if BorderStyle=3.

Since currently BorderStyle is ignored, we can always treat OutlineColor as the background color. EDIT: with the suggestions from @icbaker, the code no longer ignores BorderStyle. Instead, the background color is applied only if BorderStyle=3.

[icbaker]

OutlineColour should be treated as the background color if BorderStyle=3.

Can you please link to a citation/spec for this? The specs I'm looking at suggest there's a BackColour field which I'd expect would be used for the background.

I'm looking at:

• https://fileformats.fandom.com/wiki/SubStation_Alpha
• The .doc file linked from https://www.matroska.org/technical/subtitles.html

Since currently BorderStyle is ignored, we can always treat OutlineColor as the background color.

I'm not sure I agree with this reasoning. The current implementation ignores both - but if we start considering only one, when the other has such a huge impact on how it's interpreted, I think the result could be very surprising. People would expect to see an outline + dropshadow and instead they'd see a big solid-coloured box.

As an extreme equivalent: What if we handled ScaleX but not ScaleY? The result would be that people expecting to grow/shrink their subtitles equally in both directions would see them change width but not height - which is probably more surprising than them not changing size at all!

It's probably not that hard to extend this PR to read BorderStyle too, and check it's set to 3? You can continue to ignore it (and this field) if it's set to 1 (i.e. you don't need to also implement dropshadow and outline in this PR as well).

[egor-n]

@icbaker thank you for your input, it makes perfect sense to check that BorderStyle is set to 3 before applying the background color. I will update the PR right away 👍

OutlineColour should be treated as the background color if BorderStyle=3

I can't find a good reference for this claim, but it's what I understood from looking at how libass behaves.
https://fileformats.fandom.com/wiki/SubStation_Alpha#Data_types only mentions that BackColour is actually the color of the shadow.

[icbaker]

Thanks, looks good - I'll work on getting this merged (sorry for the delay at turning this round).