ERPT has always puzzled me, but today I got a good feel for it.
Tried these -
32 depth all round
Hilbert pixel sampler
2 samples per pixel
I tried chain length 8 and 16 so far for 15 mins each - the lower number means caustics don't develop as quickly, but it seems to render bigger chunks of the image with Hilbert on a semi-tiling basis when using a 3 sec screen update.
So it covers the whole render and develops quite quickly. Reaches higher samples per pixel much quicker.
First pass is sorta darker, and gets brighter every pass in a bucket rendering fashion.
If you use Vegas or Low discrepency pixel samplers it looks like a mess!
Tile and linear is not bad, it goes down in a scanline fashion, but is slower than Hilbert
All rendered in 15mins - different chain lengths. Lower chain length = higher samples per pixel, more refined, but lower caustic developement, also not as bright...