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For a long time I find it difficult to generate an interior God Ray (volumetric light) in octane, we always complain the fog of environment medium is too difficult to use,

Yet occasionally today I found an interesting issue probably related, it is not the medium but the balance of the direct and indirect influence of the sun light is the key.
A testing rendering To generate this image I tweaked the parameters in the medium of Environment
the result is like this quite disappointed,right? it seems nothing to do with the god ray
but interestingly the image in the sunlight direct pass shows a really strong effect yet probably the image in the indirect pass is more dominant so that the final image do not show the God ray effect at all， I cannot find a parameter which change the balance of the direct and indirect, so I can only do the post processing to get the final image,
and here is the question:
Anyone could let me know if it is possible to adjust it within Octane ( I was using PMC kernel)

Cheers and thanks,

Jim

The Raytype Contribution OSL shader might do exactly what you are looking for, letting you tweak the direct vs. indirect illumination. Maybe apply it to a quick-n-dirty god-ray/murk shader filter plane for simple, global application.

viewtopic.php?f=21&t=71597&p=365050&hil ... pe#p365050

viewtopic.php?f=44&t=72510&p=368103&hilit=murk#p368103

(edit: A quick test over my lunch break isn't behaving the way I expected, but I'll leave this suggestion up and try to follow up on the idea when I have some time to play around a bit.)

Hi,
have a look at this old discussion about interior fog:
viewtopic.php?f=24&t=55567&p=284654&hil ... ok#p284654
ciao Beppe

Thanks Beppe,
I did play around with the sample you attached in the above post,
I noticed it was using Direct lighting Kernel, the "God ray" effect is really sharp and clear, quite exciting,
Though when we are applying to an interior scene we may need better effect for glass or fur etc materials, we need PMC kernel,
when I turn the scene to the PMC kernel, the volume light becomes weak and blur again,
guess I am not using the kernel in a right way, Can I ask if you could kindly make another sample in the PMC kernel?
Many thanks again,

Jim

Hi Jim,
yes, the example was in Direct Light AO mode, so without GI.
In PMC, if you use the old technique of the cube with specular/air material, with Camera outside of it, everything should work as expected, since the fog is limited inside the room: If you use the Environment node with Medium, all the scene is rounded by fog, so the GI/indirect illumination has a great impact: You need to reduce the Sky light, or reduce the GI Clamp value, to have a similar effect with PMC: Note that you, by reducing the GI Clamp value, you are also cutting the caustics.
ciao Beppe

Wow, this is amazing, Thank you so much, Guru Beppe