aes

Actually, you know what? I think Lua-style single argument function calls would be awesome for all kinds of quasi-literals, but sure Path should be built in.

I... discovered that it works if I use a plane mesh and just position it manually. The conclusion then has to be that there is something screwy with how Decals get their textures. Maybe there's even a bug involved? Should I try to make a minimal repro?

I was talking to some people from a company that makes an eye-tracking thing and apparently it's really hard to do with precision, so it's unlikely that any hardware you're willing to pay for is going to do what you want.