Glass Alpha Contribution

[Mike Truly]

Vray 2.4.04 & Vray 5


Imagine a simple building with glass windows. You can see inside the building and also out through the windows on the other side of the building.

The glass has an Alpha Contribution of something less than 1.0 (for example 0.1). When rendered... the resulting alpha shows opaque walls on the other side of the building with an alpha of less than 1.0. For proper compositing... I need the alpha of those far opaque walls to be 1.0. (Anything less than 1.0 will show the background through the walls when composited).

See the attached images.




If I make the glass hidden or I set the glass Alpha Contribution to 0, the far walls show properly as alpha = 1.0... but there is no proper alpha shown for the see-thru glass areas then.

Is there some way to have the far walls show as alpha = 1.0 while also showing the proper alpha for the see-thru glass areas?

Thanks!

[dlparisi]

Normally you leave the alpha contribution as 1 and set your refraction in the glass material Affect Channels to "Color+alpha". This will give you what you want.

[Mike Truly]

Thanks for the ideas! Unfortunately, that didn't work for me. The glass material was already Color+Alpha. When the glass Alpha Contribution = 1.0... I get this in the alpha:




The solution (so far) has been to comp two alpha passes together to get the proper alpha for compositing:



I was hoping there was a Vray setting which would do this directly but I guess not. For now, this workaround works.

Thanks again.

[dlparisi]

By chance do you have the reflection's "Affect Channels" set to Color+alpha as well? Setting this to "Color only" should fix this.

[fixeighted]

Also bear in mind that with gpu this 'feature' is borked
Cpu is fine.

[Mike Truly]

dlparisi

Thank you very much!!

Indeed, it was the glass material Reflection>Affect Channels>Color+Alpha setting causing this issue. When switched to Color Only, the alpha result is as desired.



Thanks again.

[dlparisi]

Glad to help!

[kosso_olli]

Also bear in mind that with gpu this 'feature' is borked

Wait, what?

[fixeighted]

As in https://forums.chaosgroup.com/forum/...-alpha-channel