Why blue screen?

The reason is based on the "color difference theory" which is covered in good detail in the American Cinematographers manual. It has to do with the way FILM emulsions see color. It has nothing to do with whether or not there is any blue pigment in human skin as I sometimes read... In film, after an image is photographed on a blue screen, a series of high contrast black and white negatives are produced from the color negative. These black and white negatives are the result exposing the color negatives with a single color of light (ie red) onto high contrast negative film stock. Through color filtration, two negatives are produced; one with a black foreground object and clear background and one with a clear foreground object and black background. These "mattes" are sandwiched along with the original blue screen footage and also with the background footage in an optical printer, where the final image is recorded on fresh filmstock. I have over simplified the process a bit but I hope you get the ideal.

Your statements about video blue screening are half-accurate. Yes, you could put a person in a yellow shirt in front of a yellow screen, as long as it was a different yellow, but you need to understand that it would produce a very bad composite. Most likely you would be able to see through parts of the actor, and the edges of the shirt would be very blocky. The background color, ideally, must be as different as possible from any color on the subject, and bright blue or green have become the most succesful colors for that. You can't really understand completely how important it is to keep these colors as seperate as possible from the subject until you actually try to do it. (I once tried to film a grey model spaceship against a light yellow background.. and it was a nightmare. I could not produce a composite that was anywhere near decent, and I had to reshoot the whole thing with a blue screen instead, which produced great results.)

Of course, if your subject is saturated with blue and green color, then you would use, say, a red screen. It just depends on your situation. Just try to keep you foreground color as different from your background color as possible.

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Mr. Hutt

-defender of neenerheads

Yes, it is true that optical printers have given way to digital composition but the fact remains that we use bluescreen based on concepts that revolve around generating mattes in the lab and not in the computer.

But, I think the reason blue and green are still used for video compositing, is because those colors provide the sharpest contrast between the reds and yellows that define human skin. I think we take the flexibility of modern programs from granted, but early video compositors were using only the analog RGB signals, probably manipulated with hardware, so they needed to use that color difference.

Plus, if it really didn't matter what color you use, then howcome blue and green are still the international standard colors?

Sorry if spelling is off, I had to type this one quickly.

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Mr. Hutt