The Alpha channel is a grayscale image that stores the transparency of objects in the scene.
Overview
The Alpha channel shows the transparency of objects in the scene as grayscale colors. Solid geometry is represented by solid white, while those areas not blocked by geometry within the scene are black. Pixels at anti-aliased edges have varying shades of gray, as do semi-transparent surfaces.
The Alpha pass is generated automatically when rendering with V-Ray for NUKE.
Properties
The Alpha channel has no controllable properties.
Notes
When using transparent materials such as glass, make sure that the material is set up to treat the material as transparent with regard to the Alpha channel. For example, when using V-Ray Material (VRayMtl) as the material, set the Affect channels parameter for both Reflection and Refraction to Color+alpha or All channels. Otherwise, the transparent surface will not be treated as transparent in the Alpha Render Element.