[MIRROR] [MDB IGNORE] Refactors how legs are displayed so they no longer appear above one-another when looking EAST or WEST

[TaleStationBot]

Original PR: tgstation/tgstation#66607

About The Pull Request

So, for over 5 years, left legs have been displaying over right legs. Never noticed it? Don't blame you.
Here's a nice picture provided by tgstation/tgstation#20603, that clearly displays the issue that was happening:

It still happened to this day. Don't trust me? See for yourself:


Notice how the two directions don't look the same? That's because the left leg is always displaying above the right one.

Obviously, that's no good, and I was like "oh, that's a rendering issue, so there's nothing I can do about it, it's an issue with BYOND".

Until it struck me.

"What if we used a mask that would cut out the parts of the right leg, from the left leg, so that it doesn't actually look as if it's above it?"

Here I am, after about 25 hours of work, 15 of which of very painful debugging due to BYOND's icon documentation sucking ass.

So, how does it work?

Basically, we create a mask of a left leg (that'll be explained later down the line), more specifically, a cutout of JUST the WEST dir of the left leg, with every other dir being just white squares. We then cache that mask in a static list on the right leg, so we don't generate it every single time, as that can be expensive. All that happens in update_body_parts(), where I've made it so legs are handled separately, to avoid having to generate limb icons twice in a row, due to it being expensive. In that, when we generate_limb_icon() a right leg, we apply the proper left leg mask if necessary.

Now, why masking the right leg, if the issue was the left leg?
Because, see, when you actually amputated someone, and gave them a leg again, it would end up being that new leg that would be displayed below the other leg. So I fixed that, by making it so that bodyparts would be sorted correctly, before the end of update_body_parts(). Which means that right legs ended up displaying above left legs, which meant that I had to change everything I had written to work on right legs rather than left legs.

I spent so much time looking up BYOND documentation for MapColors() and filters and all icon and image vars and procs, I decided to make a helper proc called generate_icon_alpha_mask(), because honestly it would've saved me at least half a day of pure code debugging if I had it before working on this refactor.

I tried to put as much documentation down as I could, because this shit messes with your brain if you spend too long looking at it. icon and image are two truly awful classes to work with, and I don't look forward to messing with them more in the future.

Anyway. It's nice, because it requires no other effort from anyone, no matter what the shape of the leg is actually like. It's all handled dynamically, and only one per type of leg, meaning that it's not actually too expensive either, which is very nice. Especially since it's very downstreams-friendly from being done this way.

Just look, both legs are now looking the same now:

Why It's Good For The Game

It fixes tgstation/tgstation#20603, an issue that has been around for over half a decade, as well as probably many more issues that I just didn't bother sifting through.

Plus, it just looks so much better. Look at this:

It works with Ethereal limbs

It works with Moth legs

It even works with digitigrade legs!

Changelog

🆑 GoldenAlpharex
refactor: Refactored how legs are rendered, using dynamically-generated masks to avoid left legs always displaying over right ones, or right legs above left ones.
/:cl: