FG: difuse bounces yes or no?

[nelpiper]

Hi guys. I've watched a tutorial recently that said that if we could keep the FG difuse bounces at 0, it would be ideal. The tutorial also said that, if set over 2 bounces, I would get excessive color bleed in the scene. when I watched the MR convention from Jennifer O'Connor, it seems that difuse bounces are welcome.
Does anyone know for sure if difuse bounces are really "good" or "bad"?
Thank you.

[greg]

there's really no right or wrong answer to this..

using more diffuse bounces will create a slightly brigher, more evenly lit scene at the expense of render time. areas not lit with direct light will fill out more when there are more bounces. if that's what you want then diffuse bounces are welcome!

for most of my work, i don't usually go above 1 bounce, and sometimes i'll use 0 when i want more contrast between direct-lit areas and shaded areas or when i want to keep render times especially low.

greg

[hoppergrass]

If your using Photons and FG then diffuse bounces will be @ 0. If they are set higher they are automatically discarded.

If using FG only, then definiterly add bounces, anything above 3 is high and problably a good time to switch photons on.

jhv

[nelpiper]

Thanks guys. That got my thoughts cleared up.

[Jenni722]

Cool, you saw the conference. :) I'm sure I got a couple things wrong, but for this we are both right. And I agree with hoppergrass and greg.

I've never seen anyone say that you shouldn't use FG bounces when using just FG, however it is correct that when you enable GI then you should set bounces to zero, so that is probably the confusion. mr ignores the setting when GI is enabled, anyways, so you are covered either way.

The Max Lighting Analysis preset actually sets bounces to 5 - a bit extreme. For most of my scenes I use 2. Higher numbers are perhaps more accurate, and slightly brighter, but I haven't seen that as necessary. If you need higher quality and a brighter image, add GI. Usually the combination is faster with much better results than using high settings for FG.

Jenni

[bart]

Actually, when you turn on GI, you don't _have_ to set FG bounces to 0, as they will get ignored as Jennifer said; the photons will be used near where the primary FG ray lands instead of bouncing to create secondary FG rays. You can think of FG trace depth as the number of levels of secondary FG rays. So when turning on GI, the equivalent for FG bounces will be how much trace depth you have enabled with your photons.

Note that FG bounces really means the number of levels for secondary FG diffuse interactions, either reflection or refraction interactions. It just so happens that diffuse refraction is pretty rarely implemented in shaders.

What I teach, is to use either FG+GI or FG with lots of bounces to get a realistic lighting level as a reference. Then, cut that down to FG with one or two bounces, and boost the effect up to match that lightling level with the FG scale multiplier. This way you get approximately the correct lighting level in a faster render.

[Jenni722]

Excellent tips, thanks!

[hoppergrass]

Actually, when you turn on GI, you don't _have_ to set FG bounces to 0, as they will get ignored as Jennifer said; the photons will be used near where the primary FG ray lands instead of bouncing to create secondary FG rays. You can think of FG trace depth as the number of levels of secondary FG rays. So when turning on GI, the equivalent for FG bounces will be how much trace depth you have enabled with your photons.

Note that FG bounces really means the number of levels for secondary FG diffuse interactions, either reflection or refraction interactions. It just so happens that diffuse refraction is pretty rarely implemented in shaders.

What I teach, is to use either FG+GI or FG with lots of bounces to get a realistic lighting level as a reference. Then, cut that down to FG with one or two bounces, and boost the effect up to match that lightling level with the FG scale multiplier. This way you get approximately the correct lighting level in a faster render.

sure you can leave the fg bounces greater than 0, but it still calculates them and only discards them at rendertime, thus making the calculation time longerand the FG file size bigger. So If you're using Photons, save yourself time and set FG to 0

Interesting methode with the FG scale, I'll have to give that a go

JHV

[MichaelB]

sure you can leave the fg bounces greater than 0, but it still calculates them and only discards them at rendertime, thus making the calculation time longerand the FG file size bigger.
I don't believe this is the case.
If you set FG bounces to >0, turn on GI and open the mental ray Message Window.
The first message that pops up is:


MSG 0.0 warn : GI Photons are used. Multiple diffuse bounces from Final Gather are ineffective and ignored.
This is immediate, before the heap size limit is set, as the scene is begining the translation process and prior to any calculations begining.

As far as file size of the FGmap goes, I don't see what you're describing happening at all.
In fact, it is much smaller (as one would expect with 0 bounces vs bounces>0.

[hoppergrass]

I have noticed that when the FG is set >0 it takes longer to calculate, of cause I am taking about using Photons as well. I havn't sat down and done a second for second comparison though :)

jhv