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Chroma key backgrounds can be found in many colors. Please tell me:

1) Why green is mostly popular?

2) What special purposes of using black chroma key? When it is very useful?

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2 Answers 2

1) In principal any colour will work, it is of course helpful to chose a colour that is rare in your subject, so red and yellow are out as they collide with skintones. Green has a specific advantage when it comes to digital photography as most digital cameras have twice as many green photosites as red or blue.

2) Black is less useful as black is not a colour. Surfaces of any colour will look black if no light hits them, thus using a black as a chroma key is a bad idea as if any part of your subject is in shadow it will collide with the chroma key.

There is one advantage of black, however, and that is it is very easy to achieve if you have a large space and controlled (strobe) lighting on your subject. Simply make sure none of your key lights hit the background and it will go black.

An example of this is any outdoor space can yield a perfect black background, with enough rim lighting you could get a decent mask with one.

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Why is rim lighting important here? Shouldn't it be enough that the subject is well lit? –  Unapiedra Jul 16 '13 at 10:59
Without rim lighting odds are at least some of your subjects outline will be in shadow and will fade into the background. This actually works well if you are planning on compositing them into a dark scene, but will look off if you are designing something brighter. –  Patrick Hurley Jul 16 '13 at 11:13
@Unapiedra the key is that the subject is very well lit at the edges such that even dark things like hair are well distinguished from the background, which is easiest to achieve with back/hair/rim lighting. But if the subject doesn't have dark hair or clothing other lighting schemes could work. –  Matt Grum Jul 16 '13 at 11:14

Any color can be used, the reason for green is that it is a color that isn't found much in most subjects and tends to give a bit less reflection than the old blue screen (which could lead to blue highlights on the subject.) You can still get that with green screen, but it's less common.

As for black, that's a special case, when using a black backdrop it is no longer a chroma key(which keys on a particular color) but rather a luma key, which keys on the luminance of the scene. It has the advantage of allowing any colors, but isn't as permissive of dark shadows, so the subject needs to be well lit while still keeping the background black.

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