Your thoughts on and comments to Volume Rendering in Unreal Engine 4.

[dokipen (Dan 'dokipen' Elliott)]

@;576404

Re: NoobsDeSroobs and JennyGore:

Re: dopiken:

Everything I have read about half angle slicing also talks about using geometry slices but from what I can tell, you don’t need the geometry slices and you could treat it just like any other ray marcher as long as you structure the trace to do it in the right order where it alternates between lighting and density samples in a way that shares them. Maybe its just to make it a bit faster to index into the correct locations?

From my understanding, the biggest drawback to half angle slicing over regular ray marching is that it doesn’t scale super well since you would need two more buffers for each instance in the world since each would have a different slice angle from different viewing angles. Technically shadow volumes have the same limitation where you’d need a different volume for each instance that has a different rotation but you could at least share un rotated instances.

The nice thing about regular old ray marching is that even though it is slow, it is fairly simple and doesn’t have many issues with scaling other than the brute force cost.

I’d really like to hear your ideas about how to share the samples to mimic the slicing in a raymarch. I havn’t seen that done anywhere before. although I cant quite see how that would be done in a pixel shader unless there is some kind of buffer used in multiple passes.

Thats true if you are using the half angle slicing technique. What I am proposing is to not actually do half angle slicing for the shadows but do slicing for the density pass only (in that case you don’t even need half angle, just slice towards the camera) and then do the shadows in raymarching. If raymarching, you have do do shadow samples inbetween every density sample. If I do the slicing the density sample is done up front and overdraw and shadow samples are the main cost. It seems like in that case the shadow samples would be very similar to doing just a density sample. This matches with the big 0 cost like you say.

I’m close to testing. I’m actually first re-creating my slicing code in blueprint (mainly out of curiosity to see how it performs, especially when it comes to nativizing it and also because I want to be able to share this around without users having to compile plugins/modules etc). I’m up to the point where I’ve detected start and end intersection points and have written the box/plane intersection function. Now I just need to build the vertices and indexes.

You’re right in that there is no opportunity for early exit. I’ll get this working and see what the difference is.