For those pixels or parts of pixels that are not covered by any objects, intensity is specified by means of a background shader. A background shader may be as simple as one providing a single, uniform color, as well as one producing graduated and complex patterns.
A background shader can also be used to perform simple compositing functions by overlaying the computed image on top of a previously computed image or a scanned photograph.
We provide herewith names and descriptions of background shaders:
"clouds"
A cloudy appearance.
"graduated"
A graduation between two colors from top to bottom of the image.
"image"
An image read from a file.
"mixed"
Allows effects of two other background shaders to be mixed.
"plain"
A plain colored, uniform background.
"ray cube"
Background shader which takes two background shaders as parameters and uses one when the background is directly visible, and the other when the background is visible in a reflection.
"scaled image"
An image read in from a file, scaled to fit the viewport (rotation may also be done).
"two planes"
Background shader which handles reflections and refractions by placing two background planes front and back of the observer.
"environment"
Environment-mapped background.