control objects alpha

[joie]

Hi there;

Coming from Mental Ray, I was used to do things in a different way.

I used to render my scene with regular opaque alpha for hero objects, and zero alpha for background (surface background, like a photography studio background).

But I can't find a way to do this with Arnold.

I know there is a "matte" option in the object's shape, but that forces the alpha channel to be zero, but makes the object completely black too, and that's not what I want.

I want the object to render as normal, but write black alpha channel.

Is that possible?.

Thanks in advance.

[Anonymous]

Hey,

I am not sure if you can do this thing in alpha But i can give you a way around,

if you are working with "standardSurface"

• scroll down a bit in shader tab you will find "AOVs" sub-tab
• Keep ID-1 (or whichever id you want to use) color to white
• add id_1 aov to your AOV tab in render settings, you can also rename this Aov name

• If or if not you renaming that AOV name, copy that name and go to you shader and put that name in "ID 1 AOV (or whichever id you using) "

now you will get you main beauty and as AOV you will get your alpha as you wanted.

Hope this will be helpful for you!!

[joie]

I'm sorry I never answered to you!

I wanted this for a shadow matte shader, which unfortunately doesn't have such ID AOVs you talk about.

I ended up using a surfaceColor shader as a passthrough for the aiShadowMatte one, because that shader allows to control Alpha contribution at render time.

Cheers.