Global subdiv and IRR Hsph subdiv

[Lars_Madsen]

Hey everyone!

A quick GI question:
As far as my vray understanding goes, the global subdiv parameter also controls the IRR Hsph, so my questions (or rahter problem) is: I am rendering test-renders with IRR settings like Hsph subdiv = 80 and samples = 20 and global subdiv set to 1. When rendering the final image though, it was my understanding that if upping the global subdiv to, say 20, and likewise reducing the IRR hsph subdiv to 4 (...because 20 x 4 = 80) should give me the exact same GI quality... yet, obvious GI blotches appear in the final render. Is my understanding/math-theory of the affect of the global subdiv on the IRR flawed, or am I missing something someqhere else?

Thanks in advance

[Micha_cg]

So far I understand the global multiplier shouldn`t be used to get a higher quality. Better is you make your setup for high quality and lower the global subdiv multiplier for a fast raw preview.

[Lars_Madsen]

Aha, ok... that´s new to me, i actually didnt think about that.... so basically, i put all the factors to what i think it should be (like glossy steel subdiv to lets say 60) and set the global subdiv to 1/60 = 0.02 for material test renders? I dont mean to question your method, but isnt it a little like "working backwards", since default materials like steel and such come with subdivs set to only 20?
On another note, can you tell me where my math is flawed regarding the Irradiance map Hsph? Or how does the global influence the factor?

[Micha_cg]

Your math looks ok, but I'm not sure about the IM and the global multiplier. Also never used the default materials since I wasn't so happy with it. "Working backward" - right, this was the recommendation that I got in the past. So, you can fine tune the subdivs where needed without to get to much subdivs. Using a higher multiplier than 1 can cause speed problems. Using lower than 1 for preview will give you a bad tuned preview only in the worst case, but it doesn't matter so much.

[RobSteady]

I thought "global subdivs multiplier" can be used to quickly bump up the subdivs for glossy reflections and lights.
If I have a very complex scene with like 25 different glossy materials and 16 lights I don't want to get in there and set the subdivs for each material and light when I switch back and forth between preview/final settings.

I've seen many tutorials where they used the global subdivs multiplier in that particular way to achieve higher quality.
Why should a higher multiplier cause problems and what is this function good for in your opinion?

[Micha_cg]

For example if you use DOF than you can get extreme long render times if you increase the subdivs per global multiplier. I'm not sure, maybe the multiplier influence the subdivs of the lights too. Also IM - there is only one optimal value for speed and quality, to low you get splotches and to high the render times are much to long. Light blurry material effects need less subdivs and ... so how will you control it per single multiplier?
I would afraid to get to long render times and would use the multiplier only for lowering the quality.