Vray Next update 2 - HDRI reflections stored in specular channel

[Neilg]

I've only just started using next so I dont know if this has been like this for a while, but it didn't do this in 3.6.
If it's a next-specific change, any way to switch it back to how it used to behave?

[vlado]

It has been like that for a while (since Update 1) and is what all other render engines do (Corona, Arnold, RenderMan, Redshift). The direct reflective contribution of a light source goes into the specular channel regardless of the way it is calculated. This also prevents artifacts with the adaptive dome light in those render elements (and the adaptive dome light is default now).

In any case, you can revert to the old behavior if you turn off the "Consistent lighting elements" option in the Expert mode of the Global switches rollout (see the screenshot).

Best regards,
Vlado

[Neilg]

Thanks for the info!
I don't want to change it if it fixes artifacts - Is there a way to spit out a channel to mask it out in post?

e: I know I could use 'afffect all channels' in the material reflection and use a multimatte, but that breaks other post production things in our workflow.

[vlado]

Is there a way to spit out a channel to mask it out in post?

Which part do you want to mask? The clear reflections vs the glossy reflections?

Best regards,
Vlado

[Neilg]

The reflection of the HDRI only - everything in the scene is fine.

e: i'm going to open 2016 and 3.6 and take a look how this used to work... i guess this is related to not having separate specular and glossiness controls anymore.
Presumably because the HDRI is all light, it's not considered a reflection?

e2: i just checked self illumination materials and that still shows up in reflection, only picking up in the specular where i'd expect it.

[vlado]

You can use a light select render element in "Indirect specular" mode and it will give you just the reflections of the HDRI; would that help?

Best regards,
Vlado

[vlado]

How does self illumination work with this, does that also show up in specular instead of reflection now?

No, self-illumination is just regular reflection.

Best regards,
Vlado

[Neilg]

You can use a light select render element in "Indirect specular" mode and it will give you just the reflections of the HDRI; would that help?

Best regards,
Vlado

That seems to mirror the reflection channel - except without the self illuminated object. tried direct specular too.


I can probably just switch off consistent lighting elements. What artifacts are we likely to see with that disabled?

[vlado]

What artifacts are we likely to see with that disabled?

If you turn off the adaptive dome light as well, there won't be any. Otherwise there will be blocky artifacts in places where V-Ray changes the sampling of the reflections vs the direct specular component of the light. In some parts of the scene it may be more advantageous (=less noise) to give more weight to the reflections vs the specular component, while in other places in the scene it might be the reverse. The adaptive dome light uses this fact to speed up rendering, sometimes significantly.

Best regards,
Vlado

[Neilg]

You can use a light select render element in "Indirect specular" mode and it will give you just the reflections of the HDRI; would that help?

Best regards,
Vlado

This would help, but I cant get this to work - did you see my image above? indirect specular is near-identical to the reflection channel.

edit: see below. direct specular gives both reflection and specular of the domelight in the same pass. indirect specular looks the same as the reflection pass and is missing the HDRI reflection. Maybe a bug?

[Neilg]

Sorry to keep on this...

I cant figure out how to make what I want in one render, but I was able to out of 2 separate renders.
I'm looking for the very bottom render in the attached image - thats what the old specular pass looks like. Is that not actually possible to create in one shot anymore, unless we do a complete re-render with 'specular contribution' set to 0 in the hdri?

[vlado]

No worries, it would be good to get this cleared up; it helps for me to understand what you need and maybe modify the behavior if needed.

In general, as far as area light sources are concerned, there is only one effect - the reflection of the light source in other objects, regardless of whether those reflections are mirror reflections or glossy reflections. In the real world, there is no "specular" and "reflections", there is just one single effect.

In renderers, this one effect is calculated in two different ways - one is by sampling reflection rays from the material onward; and the other is by sampling shadow rays from the light source. Those two methods calculate the same thing, but have different noise levels; V-Ray balances the two calculations to produce the optimal noise level by giving them different weights. This is knows as "multiple importance sampling". What you saw in previous versions of V-Ray, was the two calculations with their respective weights. One was written in the "reflection" channel, and the other in the "specular" channel. The fact that you could get these two elements out of V-Ray was a somewhat lucky coincidence. Like I said, no other renderer actually does that so it made sense to align V-Ray to that.

With that said, I also understand your goal; other users have also expressed the opinion that the previous method was more convenient for them without having to set up additional passes. Would it work for you if we simply move the sharper reflections back to the "reflection" render element while more glossy reflections of the light source remain in the "specular" render element? I can also provide a build for you to play with and tell me if it would work.

Best regards,
Vlado

[Neilg]

With that said, I also understand your goal; other users have also expressed the opinion that the previous method was more convenient for them without having to set up additional passes. Would it work for you if we simply move the sharper reflections back to the "reflection" render element while more glossy reflections of the light source remain in the "specular" render element? I can also provide a build for you to play with and tell me if it would work.

Best regards,
Vlado

That sounds like it would work! Thank you.
We're yet to fully move over to 2019 and next (wrapping up some old projects) so it's ideal timing to test a new build.