week 6- eric

Creating a blue screen composite image starts with a subject that has been photographed in front of an evenly lit, bright, pure blue background. The compositing process, whether photographic or electronic, replaces all the blue in the picture with another image, known as the background plate.

Blue screen composites can be made optically for still photos or movies, electronically for live video, and digitally to computer images. Until very recently all blue screen compositing for films was done optically and all television composites were done using analog real time circuits.

Another term for Blue Screen is Chroma-Key. Chroma-Key is a television process . The Chroma Key process is based on the Luminance key. In a luminance key, everything in the image over a set brightness level is "keyed" out and replaced by either another image, or a color from a color generator. Primarily this is used in the creation of titles. A title card with white on black titles is prepared and placed in front of a camera. The camera signal is fed into the keyer's foreground input. The background video is fed into the keyer. The level control knob on the keyer is adjusted to cause all the black on the title card to be replaced by the background video. The white letters now appear over the background image.

Chroma Key creates keys on just one color channel. Broadcast cameras use three independent sensors, one for each color, Red, Green and Blue. Most cameras can output these RGB signals separately from the Composite video signal. So the original chroma key was probably created by feeding the blue channel of a camera into a keyer. This works, sort of, but soon manufacturers created dedicated chromakeyers that could accept all 3 colors, plus the background composite signal and the foreground composite signal. This made it possible to select any color for the key and fine tune the selection of the color.

As keyers became more sophisticated, with finer control of the transition between background and foreground, the effect became less obvious and jarring. Today's high-end keyers can make a soft key that is basically invisible.