Additive (shellac) mode in BlendMtl adversely affecting BumpNormals ?

[brobins]

Hi there,

We are using additive (shellac) mode on one of our BlendMtl shaders which our pipeline relies on but have found that the BumpNormals element is affected by this. Please see the attachment showing before (normal) and after (broken) normals.

Is there a workaround for this to get correctly rendered BumpNormals when using additive mode in a BlendMtl?

Thanks in advance

[vladimir_krastev]

Hi, thanks for posting.
This is what the docs says:

Additive (shallac) mode – When enabled, VRayBlendMtl behaves like a multi-layered Shellac material. Note that this would often result in a physically incorrect material (e.g. a material that reflects more light than falls on it). It is not recommended to use this option unless you know what you are doing. See the Additive Mode example below.

We even have a pop message appearing when "shellac" is enabled


The VRayNormals render element seems unaffected. Could you use it instead? Why do you need the shellac? Could you share a bit more details about your material?

[brobins]

Thanks for your reply.

Leaving the first half of your message aside, as its not really relevant to our situation here, are you suggesting that the fact that BumpNormals is affected by additive is a bug? Thanks for the heads up on the regular 'Normals', I will test that, fingers crossed!

[vladimir_krastev]

Hi, using the Additive (shellac) mode has downsides as written in the docs and in the warning message you get when enabling it.
I spoke to the developers and they confirmed that the difference in VRayBumpNormals render element is expected.
Why do you need to enable the additive mode?

[brobins]

Hi, using the Additive (shellac) mode has downsides as written in the docs and in the warning message you get when enabling it.
I spoke to the developers and they confirmed that the difference in VRayBumpNormals render element is expected.
Why do you need to enable the additive mode?

Without going into too much technical detail, the answer is 'because we do'. and a large part of our 2D / post processing pipeline relies upon it. We know that it breaks energy preservation law and does not technically produce physically accurate shaders out of the box but this is not a concern of ours. Our goal is to create a kit of parts to use in our compositing pipeline to create the content we produce. Additive mode in our very specific scenario produces far less noise on the break out AOVs / Elements related to the BlendMtl for use in post. We dont care about energy preservation law here.

It is not recommended to use this option unless you know what you are doing.

As VRay users here since 2008, please assume we know what we are doing

Would be great to have some context as to why it is expected behavior for BumpNormals to be affected by this but not regular Normals (or any other Normals via SamplerInfo etc)?

[vladimir_krastev]

Hi again,

please assume we know what we are doing

I have never even had the slightest thought you do not know what you are doing. You quoted the documentation with your reply but the way you did it may lead someone to the confusion that these are my words. Please avoid this. The docs is written for a wide audience.
I will ask for more technical details and get back to the thread.