Advanced Techniques for Chroma Keying
Chroma keying was first used in the middle of the 1990s, and the creator won an academy award for the use of this effect. However, since then technology has greatly improved, and chroma keying has many advanced techniques that can be applied. Chroma keying is also known as green screen effects. A scene is shot in front of a green screen, and then the green background is replaced with a different image. This is not overlaying or superimposing images or words onto the film. The areas are actually cut out of the film and then replaced with another image.
Normally in a scene, the actor is moving around and incorporating motion tracking within the scene is important. Basically, the software will track the actor as he moves around the scene and then the key frame follows the actor. So, the actual actor walked across the green screen backdrop, but in the finished version, he was crossing the street in Hong Kong.
It is also possible to do something called match moving, where the individual is moving within the background. Many times, some additional software may be needed as this creates a virtual camera that matches the movement of the real camera within the actual film.
It is also possible to replace a person or image with another person or image by tracking their features and then replacing them. This is normally done for very extreme situations or science fiction movies, where creatures that do not exist are used. This can be difficult if the right color is not used. Many people will use blue dots on the face. However, it is best to track the dots and replace those with the new image, and then to replace the background image. Or, do the background image and then the face. Doing this together can cause problems. With enough practice, this can be very easy.
One of the most difficult issues with chroma keying is the edges around the person or image that are being cut out of the green screen and replaced on another image. There are many filters that can be used to blur the edges between the new background and the object. However, it is also a good choice to try to reduce the edges with other features within the editing software. Color bleeding can also help, as well as light wrapping and other effects. Try to use color correction to eliminate the green edge and to de-saturate the existing green color. It is all about creating a new image that looks real.
The many techniques used with green screen are endless and can create some truly interesting and creative films. Some of the best effects are created in green screen, and entire films have been done on a blue or green screen-- some examples being Sky Captain and the World of Tomorrow, Sin City and 300. As technology advances more, the green screen effect will be easier to create and be almost impossible to detect in the film. Additionally, it will be able to integrate other post-production effects with the green screen effect at the same time.