has no body), because the clothing may be replaced with the background image/video. An example of intentional use of this is when an actor wears a blue covering over a part of his body to make it invisible in the final shot. This technique can be used to achieve an effect similar to that used in the Harry Potter films to create the effect of an invisibility cloak. The actor can also be filmed against a chroma-key background and inserted into the background shot with a distortion effect, in order to create a cloak that is marginally detectable. Difficulties emerge with blue screen when a costume in an effects shot must be blue, such as Superman's traditional blue outfit. In the 2002 film Spider-Man, in scenes where both Spider-Man and the Green Goblin are in the air, Spider-Man had to be shot in front of a green screen and the Green Goblin had to be shot in front of a blue screen. The colour difference is because Spider-Man wears a costume which is red and blue in colour and the Green Goblin wears a costume which is entirely green in colour. If both were shot in front of the same screen, parts of one character would be erased from the shot. For a clean division of foreground from background, it is also important that clothing and hair in the foreground shot have a fairly simple silhouette, as fine details such as frizzy hair may not resolve properly. Similarly, partially transparent elements of the costume cause problems. Background colour Blue was originally used for TV chroma keying special effects because the film industry used blue backgrounds for similar purposes. The colour blue was tied to the blue emulsion layer of film having comparable grain and detail in comparison to the red and green layers of the emulsion. In television and digital filmmaking, however, green has become the favoured colour because of some practical considerations, and because lossy compression algorithms used in both analog and digital TV distribution retain more detail in the green channel. The choice of colour is up to the effects artists and the needs of the specific shot. Whereas the blue screen was introduced to the television industry by the film industry, the use of green screen visual effects has been introduced to the film industry as electronic imaging has augmented and replaced the use of chemical film