Rendering unpremultiplied Alpha

[PandaPanda]

Hello, in Mental Ray or Maya Software Render I can choose Premultiply or not. How can I render in in VRay Unpremultiplied?

[bigbossfr]

Hello, in Mental Ray or Maya Software Render I can choose Premultiply or not. How can I render in in VRay Unpremultiplied?

You can do this in compositing. Just divide RGB with alpha.

[vlado]

Hello, in Mental Ray or Maya Software Render I can choose Premultiply or not. How can I render in in VRay Unpremultiplied?

V-Ray does not output un-premultiplied alpha. As bigbossfr suggested, you can do this in post.

Best regards,
Vlado

[nomads]

I got a similar question.

I want to render it my scene on a black background for compositing

At the moment I have Vray Sky in the GI Environment slot and a gradient in the Environment Map (Environment and Effects Panel)

Is there a way I can have the scene rendered against a black background and also have the Environment clean in a different channel?

[vlado]

Is there a way I can have the scene rendered against a black background and also have the Environment clean in a different channel?

For the moment, no - you will have to render against black, and add the background later on.

Best regards,
Vlado

[Khazmadu]

Why should this not Possible?

Just to be clear with envirement you mean a Spherical Image like an HDRI?
In that case you can simple use a Vray Extra Texture an a Vray Placement node to make that.

[vlado]

Just to be clear with envirement you mean a Spherical Image like an HDRI? In that case you can simple use a Vray Extra Texture an a Vray Placement node to make that.

There is that, yes - I didn't think of it But I'm not sure this is what nomads wants...

Best regards,
Vlado

[nomads]

Thank you for getting back to me.

I thought that might be the case.

Are you planing to adding it as a feature in the future?

[vlado]

I'm not really sure what would be the use case of this?

Best regards,
Vlado

[nomads]

So you wouldn't have to render the scene out twice. The background could be accessed in a channel.

[Khazmadu]

What do you mean exaclty with background?

If you just want to use a image you can do this with an Extra-Tex. Than you have a clean render on Black and the Background in an Seperate Channel.

[nomads]

I'm doing my compositing in Nuke and I want an unpremultiplied alpha.

So I need to render my scene on a black background and render the enviroment map out separately.

To avoid fringing I unpremult my image, grade it etc and premultiply the alpha then composit it back together.,

I was trying to find out if there is a way of having environment map in (without any geometry) a separate channel.

[Khazmadu]

Thats exaclty what I meant.
You can do that with an extra texture like this:



Set the Vray Placement to Spherical and connect it with the UVs of the File Node

You need a Geometry in the Scene because the ExtraTex will be mapped on that.
So just make an Big Sphere around your Scene and give it a Black Shader.
You can turn off everything except Primary Visibility.

[nomads]

ok!

and exclude everything from extra texture except the sphere.

I'll try that and see if it works.

[Khazmadu]

...exclude everything from extra texture except the sphere.

No you don't need to exclude anything.
If you exclude something will it be rendered black in the Extra Texture. I can't test it Since I have only Vray 2.45 here but I thing this I what you get if you exclude them.