A horror film is one that seeks to elicit fear in its audience for entertainment purposes. Horror films additionally aim to evoke viewers' nightmares, fears, revulsions and terror of the unknown and macabre. Initially inspired by literature from authors such as Edgar Allan Poe, Bram Stoker, and Mary Shelley, horror has existed as a film genre for more than a century. Horror may also overlap with the fantasy, supernatural fiction, and thriller genres. Plots within the horror genre often involve the intrusion of an evil force, event, or personage into the everyday world. Prevalent elements include ghosts, extraterrestrials, vampires, werewolves, demons, Satanism, evil clowns, gore, torture, vicious animals, evil witches, monsters, giant monsters, zombies, cannibalism, psychopaths, natural, ecological or man-made disasters, cults and serial killers.
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.