god rays

[Gruender]

Hey guys...

I have a question about adding god rays.
Usually I put a volume material to the environment slot and set absorption/scattering and let it do it's thing. So far so good.
BUT, the more god ray effect I wanna bring in, the darker the overall lighting becomes... Yes, it is kinda natural and probably physically correct.
In Vray you just add the fog effect on top, I never saw a darkening effect when I used Vray Fog.
So my question is:
Is there a way to just add the rays without darkening the overall lighting or render 2 images/sequences and comp it togehter in post?

Many thanks in advance!
Cheers...

[sirio76]

the more god ray effect I wanna bring in, the darker the overall lighting becomes... Yes, it is kinda natural and probably physically correct.

I've noticed that too and to be honest I'm not sure it feels natural nor physically correct. If I turn on/off the environment fog, to get the similar brightness using the fog I need to bring my exposure down 4 F-stop. I've never done real world test specifically on fog but it seems way too much since in real world I use such exposure level for night scene and we all know from experience that a daylight fog environment is brighter than night. Like you I've noticed that in Vray the difference is not that much and I tend to believe that Vray solution it's plausible since in most circumstances the engine proved to be quite correct in my experience.
IMO Corona fog absorb way too much energy.

[TomG]

To emphasize god rays, best to use Single Bounce. Also best to keep absorption (which is what darkens everything else, by absorbing light) at something like 254, 254, 254 (can't use 255 as that would mean zero absorption, which would mean no volumetric effect at all).

For getting the two separately, you could try the Components multipass with everything checked there (Diffuse, Indirect Diffuse etc.) and a Volumetrics pass, and then combine those in post (you could instead individually add passes for Direct, Indirect, Reflection, etc. but that would be more work, and I think the Component pass would probably get you what you need with less to recombine in post).

[sirio76]

Hi Tom,
I've already tried to bring absorption color to near white but this way you totally miss the fog effect. You should investigate this behavior because it doesn't feel right;)

[TomG]

So far everything is working same way in C4D as it has in Max for many years :) So I believe the behaviour is correct.

[Gruender]

The absorption colour, when keeping it in greyscale, is just a multiplier of the distance. So if I set 200,200,200 and the distance to 1000cm and then change it to 254,254,254 I almost loose my fog effect. To get more of this effect I need to lower the distance, so it won't change much.
 With the current workflow I need to know if I wanna use fog or not, because I have to set my lights etc. to get everything as I want it.

[TomG]

Yes, the final fog effect is a balance of absorption distance and color (and of course, absorption color can indeed be a color, for colored fog). Find that balance will depend on the scale of the scene and the scale of the fog (generally, for god rays, I keep the color at the 254 setting and just adjust absorption distance).