Hide specific polygons when rendering with Arnold

[Anonymous]

Hello,

Is it possible to hide some chosen faces when rendering with Arnold?

For example, setting some faces invisible with the command: Edit Mesh > Assign Invisible Faces works in the viewport and Maya renderer, but not in Arnold.

The problem is I want to hide some polygons from a character, behind the clothes (but cannot delete them because the clothes shapes may change later).

Any help greatly appreciated, thanks.

[mspeer]

Hi!

Assign a new material to those faces and make it transparent or/and use a alpha mask.

[Anonymous]

Thanks. Sounds a like a good idea but i'd rather avoid it for practical reasons: the model contains several materials assigned to various faces, so it would be mess up the assignation later when reverting to the original materials...

Sounds like a basic feature... i'm surprised this is not implemented 😞

[mspeer · Accepted solution]

Hi!

"mess up the assignation later when reverting to the original"

1. You could use a render layer override for this

Edit: This might be a bit more complicated than i actual thought

2. As said before you also can use a mask (that can easily be removed or enabled/disabled).

[Anonymous]

Thank you for the tip mspeer, I had not seen your post edition mentionning the mask technic.

I don't know how to use it yet, i'll investigate 🙂

EDIT: do you mean alpha texture mask? If so, yes it's possible this way... but modifying the texture would complicate the workflow... There are several textures for each materials, different UVs...

[mspeer]

Hi!

Sorry, i don't have a better option for you.

The best option would be to correct the scene, so it's no necessary to hide faces. The only other solution, would be to make a duplicate object (with the faces deleted or an other material assigned) and to replace the original with this at rendertime (automated or manual)

[mspeer]

Hi!

Couldn't you just render the object and cloth in 2 separate passes and solve the problem in compositing?

[Anonymous]

Thanks for your help once again. Well, if the feature is not there yet, seems the only solution is to use these workarounds... so i'll probably go for the opacity map option.

And if any Autodesk/Arnold developer is browsing the thread... feel free to give it a go, would be amazing!

[miguel146212]

Easy, use clip geo in arnold shaders...

This shader hide the faces that you want in render.

Good luck. 

[newtaov]

Another way is to assign to those faces an aiStandardSurface Shader with Geometry>Opacity set 0 and Matte>Enable set 0 as well. And you can hide those faces Ctrl H, so you don't see them at the Viewport. Most of the times, this works. 
But Arnold should have a Null shader to hide and don't render poly faces easierly.