The Background Render Element includes just the background as set in the scene, with the rest of the image rendered as black.
The Background Render Element stores an image of the texture used in the background behind geometry in a scene as seen from the camera. Geometry that blocks the background image renders as black.
This render element interprets a background texture, such as the texture set in the Environment Overrides, as the background; other types of manually created backgrounds, such as a plane mapped with a texture set outside a window, is not interpreted to be the "background" for the purposes of this render element.
UI Path: ||Render Settings window|| > Render Elements tab > Background
The parameters for this render element appear in the Attribute Editor under Extra V-Ray Attributes.
Enabled – When enabled, the render element appears in the V-Ray frame buffer.
Filename suffix – The text added to the end of the rendered file, when saved as a separate file (e.g. myrender.background.vrimg).
The Background Render Element is primarily used for layering the Beauty element (or combined render elements) on top of the background image. The render's Alpha channel can be used to create a matte for such a composite. In the following example the atmosphere and depth of field have been turned off for the sake of clarity.
Beauty Render Element without Background Render Element
Background Render Element
Beauty element with Background Render Element
Beauty element with Background Render Element darkened
and colors tinted slightly in composite
When the background is behind a transparent or semi-transparent material such as glass or clear plastic, make sure the Alpha channel is enabled for the material for all render elements; otherwise, the background will not be visible in the render element. For example, when using VRayMtl for a glass material, set the Affect channels parameter for both Reflection and Refraction to All channels to cause the background to appear in the Background Render Element behind the glass.