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 a background.
Other types of manually created backgrounds, such as a plane, mapped with a texture set outside a window, will not interpreted to be a "background" for the purposes of this render element.
To use render elements, select from the Available render elements column and click the Add button to add the passes you wish to generate at render time. When you render the scene, the added render elements listed in the column on the right, will also render, which can be viewed from the VFB window's channel drop-down menu.
For more information on the parameters on this dialog, see the Render Elements tab page.
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).
Denoise – Enables the render element's denoising, provided the Denoiser render element is present.
The Background Render Element is primarily used for layering the Beauty element (or combined render elements) on top of the background image. The 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.