Shadow Map Light Leaks

sullium · February 8

Bug tracker: https://bugs.thedarkmod.com/view.php?id=6676

Hi, I've noticed some light leaks that only appear when playing with shadow maps (as opposed to stencil shadows). I'll show two cases that I think are not simply errors in modeling/mapping, but rather may indicate a general bug with shadow maps.

The first case can be seen in many missions: both of these bucket models allow light to leak inside them from certain angles when shadow maps are enabled (see images below).

When playing with stencil shadows, the interiors of these buckets are fully shadowed as expected. I brought this up to some devs, but @Bikerdude found that the bucket light leaks couldn't be fixed at the shadow mesh level, and he suggested it was a general shadow maps issue instead.

Hearing this, I realized it could explain a tiny light leak I had seen in my WIP mission. I decided to make a simple test map with similar brushwork to clearly show the problem.

So, that test map is the second case: these grid-aligned worldspawn stairs leak light through the corners when playing with shadow maps (see image below).

When playing with stencil shadows, the corners between stairs block light correctly (see image below):

Out of curiosity I put the buckets in the test map too. The light volume is quite large (776x776x776), which seems to exacerbate the light leak (see comparison images below):

I'm playing TDM 2.13 on Windows 11. The test map file is attached to this post if you're curious.

Bug tracker: https://bugs.thedarkmod.com/view.php?id=6676

smt.map

Edited February 8 by sullium
Added bug tracker link

stgatilov · February 8

That's what shadow maps do: fail on thin objects and have low resolution on large distance.

I don't think this is fixable in the conventional sense.
Smaller lights are less susceptible to such problems in general.

I.C.H.I. · February 8

Yeah, this issue is as old as shadow maps themselves, blame the lack of precision. You either get peter-panning or shadow acne, and shadow acne looks much, much worse when games have it.

stgatilov · February 8

1 hour ago, I.C.H.I. said:

shadow acne looks much, much worse when games have it.

TDM has it actually

I think games with shadow maps are tweaked with them in mind.
The problem with TDM is that our shadow maps implementation is plain awful. I tried to rewrite it, but met a dead end and stopped there. I need to do another try.
And without solid basic implementation is does not make sense to give any tweaking abilities to the mappers, since they will all die on implementation change...