VRay for SketchUp Manual
With alpha contribution you
have the capability to obtain the alpha channel of each material on the scene.
This works by using the scale between 1 and 0, this means 1 no alpha (all White)
and 0 full alpha.
Adding Alpha contribution
Open the material editor and click on the "material option" option tab, then
change the number of alpha contribution for each material you want an alpha
Working with a Alpha Contribution
You can find the alpha channel on the V-Ray frame buffer.
Here are examples of different Alpha Contribution settings.