In traditional festive legend, Santa Claus's reindeer pull a sleigh through the night sky to help Santa Claus deliver gifts to children on Christmas Eve. The commonly cited names of the eight fictional reindeer are Dasher, Dancer, Prancer, Vixen, Comet, Cupid, Donder and Blitzen; although Donder is often called Donner. They are based on those used in the 1823 poem "A Visit from St. Nicholas" (commonly called "The Night Before Christmas") by Clement Clarke Moore, arguably the basis of the reindeers' popularity.
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.