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 Houdini.
Attributes
The parameters for this render element appear in the V-Ray RenderChannelColor render channels node.
Denoise – Enables the render element's denoising, provided the V-Ray Denoiser render element is present.
Consider For AA – When enabled, anti-aliasing is used where possible.
Filtering – Applies the image filter to this channel. Image filter settings are in the Image Sampler tab of the Sampler tab of the V-Ray Renderer node.
Color Mapping – When enabled, the Color Mapping options in the render settings are applied to the current render channel.
Derive Raw Channels – Generates data in the raw channels by combining the respective color and the filter color channels.
VFB Color Corrections – Applies the post render color adjustments made from the VFB.
Common Uses
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. The Alpha channel shown below takes into account the transparency of the glass in the windows and masks them. See how the render looks before and after compositing.
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 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.