Bump in diffuse render element when using reflection

[CGMas]

Hi,

I've just found a weird behavior of vray for maya and wonder how I could've missed it until now...

When a vrayMtl is used with bump map (or normal map) AND reflections, then the bump comes out in the diffuse render element...
If there's no reflection, there's no bump in diffuse.
If the relfection amount is low, the effect of bump in diffuse is low.
If the reflection comes from another material using a vrayBlend, there's no bump in diffuse.

You will find a sample scene in copy.
Check the diffuse element.

I think it may not just affects the diffuse. When using normal map, it brings a lot of dark areas that disappears when reflection is off.

Thanks in advance !

Max

bump_diffuseElement.zip

[ivaylo.ivanov]

Very strange indeed. It doesn't look like an intended behaviour. Thank you for the pointer.

[CGMas]

Thanks for the answer !

And let me know if you find a solution cause in the meantime I have to used the vrayBlend system to get proper render so it may be a bit slower...

[pe_tor]

Looks to me like it's an intersection of Maya's rather severe bump mapping and how reflections with the Fresnel effect turned on technically, because of energy preservation, multiply down the diffuse color as reflection based on incidence angle increases.
I turned Fresnel off and lowered the reflection amount and the modulation of the diffuse color disappeared in the diffuse pass. Granted, not using Fresnel might be undesirable, so using a less severe strength of bump mapping might help, but the only way I can think of off hand to get around this would be to separate your diffuse and specular components into separate Vray Materials that are combined additively using a Vray Blend material.

[ivaylo.ivanov]

Yes, you are right, it's the energy preservation. It depends on what you want. There are 3 choices:
- the existing one, which produces this effect
- don't include bump in the fresnel effect, so you will not get bump in the diffuse filter. This will change the reflections, but the previous method can't be called correct either.
- use the additive mode of the blend material, so you get fresnel bump effects, but will break the energy preservation.
We definitely don't want to allow option 3 in other context then the blend material and its additive mode since it's not physically correct. If option 2 is something useful, we can think about adding it in some advanced rollout.

[CGMas]

Yes I agree with you about the energy preservation.
But, is it the right behavior for vray ? Or it's a bug ?
Do you think you will do something to correct this ?
Or do we have to adapt to this behavior ?

By the way, I've tried with fastSSS or VraySkin and they seem to work in the way we expect.
And the problem is the same with normal maps.

[ivaylo.ivanov]

I think it's the "right" behaviour, in the sense that one of the possibilities was picked and there are a lot of legacy scenes done with it. Like I said, option 2 is also a valid one, but I don't know what is worse - fresnel not affected by the bumps or changed diffuse RE. Option 3 is the worse one, since it can cause even bigger issues, and it can be achieved with blend material if you really want. I will check how SSS is dealing with the issue, but it should be one of these 3 choices...