So I've been reading

https://graphicscodex.com/app/app.html

and

https://mini.gmshaders.com/p/radiance-cascades

so I could have a basic grasp of classical rendering theory.

I made some assumptions:

1. There's an isometric top-down virtual camera just above the player

2. The Radiance Cascades stack on top of each other, incresing probe density as they get closer to the objects and players

I suspect part of the increased algorithm efficiency results from:

1. The downsampling of radiance measuring at some of the levels

2. At higher probe density levels, ray tracing to collect radiance measurements involves less computation than classic long path ray tracing

But I'm still confused about what exactly in the "virtual 3D world" is being downsampled and what the penumbra theory has to do with all thus.

I've gained a huge respect for game developers though - this is not eady stuff to grasp.