Material ID - not a default?

[ValeroStudio]

What's the reasoning that Material ID needs to be added as an attribute separately when making new materials? I was wondering why a lot of materials on the ID channel were rendering black and just recently realized that it is not a default material property. That doesn't seem like a good thing to me.

[konstantin_chaos]

Hi,

There is now a better way to generate random colors based on material assignments - with the Material Random Color element.
The material ID attribute is actually a shading effect that slows down the rendering calculations (a little bit but still).
That's why we implemented this new element in V-Ray Core that does a similar thing without any shader modifications.
It is similar to Cryptomatte but outputs an 8bit image, works in interactive and on GPU.

Hope that helps,
Konstantin

[ValeroStudio]

I have found that this also produces black for some materials though.Here is a render I did last week. Random Color did not create ID for 3 of the materials in this example.

[konstantin_chaos]

Now, that is strange.
Can you share the scene for investigation on our side?
Vrscene also works...

[konstantin_chaos]

Thank you for the scene Matthew.
I found the issue and it's not gonna be very trivial to fix.

The Random color does not work for meshes that have the front and back faces painted with a different materials.
One of the materials can also be inherited from the Group / Component - it's still a problem.

To find such situations in the scene you can use the Scene Interaction Tool.
The following 2 examples will cause issues:


You can sort of work around that by making sure that both front and back faces use the same material.
Note that a default material assigned to the back will not cause the problem - it's handled differently by V-Ray.

It also seems that the problem is not there on GPU - the GPU handles this situation just fine.

I'll log this but it'll require a change of V-Ray Core to be resolved.

Thank you for reporting it.

Regards,
konstantin

[ValeroStudio]

I understand. Thanks for looking into it.

[konstantin_chaos]

Hi Matthew,

The issue we discussed here is now fixed.
You can read more about this latest update here:
https://forums.chaosgroup.com/forum/...-now-available

This particular fix was challenging but I think we got it right this time around.
Let me know if you find anything else.

Regards,
Konstantin

[ValeroStudio]

That's great news. I also see the material colorize bug squashed, I experienced this on many occasions but I never got to the cause of it.