I didn't get it either, I found this though which seems to be a much better fundamentals of radiance cascades: https://mini.gmshaders.com/p/radiance-cascades

IIUC basically you have a quad/oct-tree of probes throughout the area of screen space (or volume of view frustum?). The fine level uses faster measurements, and the broad level uses more intensive measurements. The number of levels/fineness determines resolution.

I guess for comparison:

- Radiance cascades: complexity based on resolution + view volume; can have leakage and other artifacts

- Ray tracing: complexity based on number of light sources, screen resolution, and noise reduction; has noise

- RTX: ??

- Radiosity: complexity based on surface area of scene

Also not sure, but I guess ray tracing + radiosity are harder to do in GPU?