How is Matting done ?
1. Creating mattes is the first step to matte-print compositing.
This can be done in several ways :
The earliest versions of mattes were made with cut-out cardboards that were held in front of the camera to block out certain parts of the frame from getting exposed. Then it would be replaced with the opposite matte that covers up the exposed part and exposes only the unexposed part of the film.
Hand drawn mattes are created with a technique called Rotoscoping , where mattes are manually created by drawing the outlines of elements from live action plates.
One of the most common ways of creating mattes for television or motion pictures is Chroma Keying. In this color keying technique particular color from the image can be removed and made transparent to a background placed behind it. The action is shot against a background with a single color or a limited range of colors, usually blue or green screens since they are farthest away from skin color. In compositing the portions of the video that match a preselected color are replaced by an alternative video. Therefore a chroma key subject should not wear the color of the background.
Blue screen – In the days when films was extensively used, blue screens were preferred because it is complementary to skin color and blue emulsion of the film has the finest grain.
Green screen - Lately green has become more popular because digital cameras retain more detail in the green channel. Also it does not require much lighting to be bright.
The biggest challenge of setting up blue or green screening is getting even lighting and avoiding shadow. In order to get a clear key it is better to create at least a two stop difference between the subject and the background.
Here is a history of green and blue screen effects and how they have infiltrated most films:
Yellow Screen - Also called sodium vapor technique, it solves some of the problems with blue/green screens. A white screen is lit with a powerful source of sodium vapor lights. Sodium light has a narrow spectrum source which falls between the sensitive layers of the color film. So complete range of colors can be used on the set.
Cameras with a beamsplitter prism is used to expose two separate films simultaneously. The first one is a regular color negative film which is less sensitive to sodium light. The other is a black and white film that is sensitive to sodium vapor light color. So the live action plate and the matte are created simultaneously.