I'm looking for a solution to handle indirect lighting of dynamic objects for low-end hardware (integrated graphics and GPUs that end with __50). I've done hand-placed environment probe volumes in the past with pretty good results. These are basically a box volume of any dimensions inside which an object uses a cubemap rendered from the center of that box. The idea is to put one volume in each room, with the edges touching the inside walls, and it gives an okay approximation of diffuse and specular reflection.

I think id's current engine does something similar to this, mixing probe volumes with screen-space reflections. I read a little bit about it but they like to speak in flowery terms to obfuscate what they are actually doing.

All these papers about irradiance volumes just seem to be about rendering a 3D grid of cubemaps, and they don't seem practical outside of the authors' carefully-picked setups. You would need a huge amount of memory to extend very far. A 512x512x512 cubemap storing six colors would require 3 GB storage space. I can imagine all kinds of problems when a wall lies too close to a probe.

If you used a smaller number of probes and made them follow the camera around, you're still going to have a bad performance hit when the camera shifts one unit and 128x128 cells have to be updated.

I feel like all these papers are very dishonest about the usefulness of such an approach, unless I am missing something. For low-end hardware, is there something better than the hand-placed volumes I have described? I want to make sure there's no other good solution before I proceed.

JoeJ said:
I always wanted to know: When rendering, how do you know which probe affects a pixel? I mean, if there are hundreds of probes in the scene, you don't iterate all of them per pixel, no?

I did it by defining a box area. The probe is only used if the pixel is inside the box. You can also shift the cubemap coord to make the reflection match the camera position with this approach. However, it requires precise alignment of the volume to the edges of the room.

Which papers are you trying to follow here? I'm pretty sure none of them are being dishonest with you, but any technique has its limitation.

In general no games or engines are going to be working with a dense uniform 3D grid of irradiance probes at the sizes you're suggesting (512^3). They will generally have some sort of system that adds a degree of sparseness so that you have more probes where you need them, or they will have a very low density. Having multiple 3D grids that are hand-placed throughout playable areas is not uncommon, and neither are automatic solutions that attempt to analyze the scene geometry and place more probes in areas with higher scene complexity. It's also quite common to use something like L1 spherical harmonics for irradiance with 4 RGB coefficients, which if you can compress offline or at runtime to BC6H works out to 4 bytes per probe.

This article is good:
https://www.adriancourreges.com/blog/2016/09/09/doom-2016-graphics-study/

JoeJ said:

Just saw some Unity presentation of automated / manually placed volume bricks of SH probes: http://advances.realtimerendering.com/s2022/index.html

Thank you, anything with Unity's name on it is a good recommendation of what not to do.

Each probe should have a defined volume, then it all works out perfectly.

I'm just doing boxes right now. Here are my results. I added some screen space reflections and mixed them together. They blend pretty nicely, maybe not smoothly enough for a game level made out of flat mirrors, but if you add normal maps to break up the surface this would definitely work well: