So this is still being worked on?
I could help if there are problems.
Amazing project dare i say
One low hanging fruit that i can think of: it uses multithreading to fill out the light values. However it does not use it to propagate the sun. It was just because i was stupid and thought that we are doing this while df is running, but we are not. So that could be moved and it should speed up quite nicely.
On things that would be bit higher - there was an idea by Elone to process world in chunks. Maybe somehow mark which chunks changed, etc. This way it potentially can be very fast (even unnoticeable). However there are no good way to detect that. You could just limit how many chunks to update each iteration but it might be ugly (not sure though).
Next on things i would like to see (and probably no one else) is a better light transport system. Something that would bounce light around. However this was very slow in any form i tried.
Other thing i played around: HDR. Basically setting sun to be 10000 and simulating eye adaptation. This way when you are looking in caves lit by magma it looks okay, outside looks okay and if you see both outside and inside it looks dark inside. All the cool
kidsgames use it nowadays. Easyish to do...
Imho lack of last two points this makes forts very dark.
Oh and
BIG thing that is missing: TWBT compatibility. I tried to make it happen but i forget what was the problem. Also new twbt exists (twbt-next?) maybe mifki does it in different way? I personally only use the multi-level rendering but it would be nice to have everything from both mods.
Edit: also - shaders. Basically think of a way to best push everything to gpu and let it do the work. This way we could easily (comparatively) have full light ray tracing (imagine having shafts for light to come down, and then silver floor to bounce the light around in the throne room). This of course would depend on gpu and there is (was?) shader based renderer in df itself. So it could even integrate nicely.
I imagine full fort in gpu memory might not fit (i.e. now it only has light sources (3*float per tile) and opacity (3*float) for visible portion and hacks for sunlight) but somebody needs to think about this