A wallpaper or background (also known as a desktop wallpaper, desktop background, desktop picture or desktop image on computers) is a digital image (photo, drawing etc.) used as a decorative background of a graphical user interface on the screen of a computer, mobile communications device or other electronic device. On a computer it is usually for the desktop, while on a mobile phone it is usually the background for the 'home' or 'idle' screen. Though most devices come with a default picture, users can usually change it to custom files of their choosing. "Wallpaper" was the term used in Microsoft Windows before Windows Vista (where it is called the "desktop background"), while Mac OS X calls it "desktop picture". (Previously, the term desktop pattern was used to refer to a small pattern that was repeated to fill the screen.)
Chroma key compositing, or chroma keying, is a special effects / post-production technique for compositing (layering) two images or video streams together based on color hues (chroma range). The technique has been used heavily in many fields to remove a background from the subject of a photo or video – particularly the newscasting, motion picture and videogame industries. A color range in the foreground footage is made transparent, allowing separately filmed background footage or a static image to be inserted into the scene. The chroma keying technique is commonly used in video production and post-production. This technique is also referred to as color keying, colour-separation overlay (CSO; primarily by the BBC), or by various terms for specific color-related variants such as green screen, and blue screen – chroma keying can be done with backgrounds of any color that are uniform and distinct, but green and blue backgrounds are more commonly used because they differ most distinctly in hue from most human skin colors. No part of the subject being filmed or photographed may duplicate the color used as the backing.