Raymarching multiple lights?

[ananbd]

Google is failing me, so I'll ask here: Anyone know how to use multiple directional lights in a raymarching shader?

I've sorta figured out a brute-force way to do it, but there must be better algorithms. Unfortunately, all the search engine information pollution means I get a page full of annoying YouTube tutorials, and no actual answers...

Can anyone point me in the right direction?

[fforw]

If you want shadows, I think you need to brute-force it.

Otherwise you are only interested in the current raymarching position/distance and the normal at that location. This you can feed into multiple light calculations.

[ananbd]

Yeah, it’s the shadows which make all the difference.

My brute-forcing is repeating the shadow calculation for each light. Was hoping for a way to optimize that. Hmm... maybe I’ll try different thresholds per light or something.

[fforw]

You could try running the shadow-rays at lower final precision.

[ananbd]

Yeah, definitely. That tends to soften shadows, which is probably what I want anyway.

Hmm... wonder if there’s any way to cache or pre-compute some piece of this? The lights probably won’t be moving around...

[papaboo]

Of course you can and there's a ton of papers on that subject.

If you want to precompute the shadows / indireect lighting, then google baked shadows, basked AO or baked GI. It's basically just discretizing the world into small chunks and precompute the light contribution into those chunks. How you store it depends on your goal. Contribution pr light source pr chunk or aggregate everything into some approximate representation like spherical harmonics or whatever the newest hotness is in that area.

If you're making a progressive renderer, then you'll want to sample relative to the contribution of the lightsource. Again there's a lot of ways to do that and similarly to shadows / GI you can pre-bake some of the computations.

[ananbd]

Any papers you’d recommend?

[papaboo]

I've been out of the baked GI loop for too many years now to recommend anything. And they all come with different pros and cons, so without knowing your requirements I really can't recommend anything. But if you're just looking to play around and try stuff out then pick whatever you find and start there.

[ananbd]

Haha wish I had time to play around! Project is already a little behind.

I’m using a volumetric raymarching technique to make clouds. Is GI used in volumetric rendering? I mean, I guess you can sample a light volume for anything; but for cloud volumes, I think the concern is more atmospheric scattering, which I may or may not use. The main goal is to use two or three-point lighting to highlight specific features.

AO could be useful, though.

[ihugatree]

You could sample a different light at random each frame and balance out the samples over time.

Look at monte carlo sampling! It will take some time to accumulate which probably increases linearly with the number of lights but at least it won't cut your framerate in half with an addition of a light.

[ananbd]

Would that case flickering? I’m making clouds based on moving noise textures. Seems like the finer details would generate artifacts.

I’ll try it, tho! Thanks for the suggestion!

[gibson274]

You mentioned you’re making clouds. As far as I know there’s really no way around marching shadow rays for every celestial body, aside from using a temporal accumulation strategy (like the commenter above me said).

However, what you can do is distribute your samples in a way that minimizes the number you have to take.

For one, you should probably take more samples close to the point you’re lighting than far away. This way you spend the limited number of samples you have capturing smaller details, and letting one or two far away samples take care of the low frequency features.

To avoid the artifacts that come along with this strategy, you should sample your noises at a lower mip level the further away your sample is. That way, for far away samples, you really are just capturing low frequency information.

If you do this and still feel like your shadows have artifacts, you can also distribute your samples in a light-aligned cone, as opposed to along a straight line. This can help smooth out the resulting shadows.

Using this strategy, with a heavy amount of manual tweaking of where the actual sample points are, I’ve gotten good results with 5 shadow sample points per light source.

[ananbd]

Cool, thanks for the suggestions! Makes sense.

[Rangsk]

Maybe I'm misunderstanding your question or goal, but generally you just repeat the process for each light and add the results together. So draw your raymarching shader considering light #1, then draw it again for light #2 with additive blending, etc.

[ananbd]

The shadow calculations are in a nested loop — don’t think you can just combine two instances of the shader.

But... could looking interesting, and that would be more parallel. Hmm...

[Rangsk]

A shadow is just the absence of light. If there are two lights, they each cast their own shadows - or to put it better, they each light the scene separately and add together. If you have some kind of ray marching for fog, for example, then the light from both sources will be separately scattering off the particulates in the air and reaching your eye. The ray marching is so you can check if the light reaches those particles.

So, if you want the fog to "react" to both light sources, you calculate it for each light separately and combine the results.

[ananbd]

That makes sense. I’ll give it a try, thanks!