The Alpha channel is a grayscale image that stores the transparency of objects in the scene.
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 Maya.
The Alpha channel has no controllable attributes.
The Alpha channel is primarily used as a mask to enable other elements to be placed behind the Beauty composite, such as the Background Render Element.
Beauty Render Element without Background Render Element
New Beauty composite with the Background Render Element behind the original Beauty composite
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.