Chroma-Keying in Photoshop

Ingenious photo-editing will take you places you’ve never been. This is done by chroma-keying. In chroma-keying, your background is simply replaced by a different one, usually the background of your choice. This is a bit different from other editing techniques because the subject needs to be photographed against a background with a solid color. Nevertheless, once you master chroma-keying, you can put yourself and your friend in places where you’ve never been to like outer space, deep beneath the sea and the town library. Kids born in the digital age may scoff at this rather outdated technique, but it is a helpful technique to learn if you’re a photography enthusiast.

Making the foreground

One of the disadvantages of chroma-keying is that the subject needs to be photographed or filmed against a solid-colored, evenly lit background like green or dark blue. Uneven lighting will decrease the authentic look of the picture. The background’s size should be proportional to the foreground. That is, if the subject (set as the foreground) is small, the background should be small. If, on the other hand, the subject is big, the background should also be big enough to frame it. Tall subjects also require tall backgrounds. Replicas of a certain object, a plastic car model for example, will entail a smallish background while a basketball player shot from a low angle will need a tall background. The thing to remember when chroma-keying is that the background should have enough proportion to surround the foreground.

Eliminating the Key Color

In the old days, the person performing chroma-keying would need to remove the background color from the film chemically. The ‘cleaned’ film is then placed over a second picture which will become the new background. In this digital age, it is a simple matter of eliminating the chroma key by digitally replacing the luminance and color of the background. The colored background is then deleted and the transparency to lay over the second background is made.

Compositing the Image

Digital chroma-keying is done by layering a foreground image with a transparent background over a pre-generated background. As long as the subject is evenly lighted in the original photograph, it can be placed against just about any background. An expertly crafted composite image can be put anyone or anything, anywhere. The edges of the subject’s image may need to be blended, blurred or smoothed to increase the authentic look of the finished product.

Chroma-keying techniques for the Amateur

There are a lot of photo-editing programs that make chroma-keying nearly obsolete. Adobe Photoshop is the software commonly used by amateurs and professionals alike to crop one image and layer it into another. The effect is very much like chroma-keying; at times the result is even better. With Adobe Photoshop, photos that don’t have a solid-colored background can still be used as a composite with another photograph. That being said, chroma-keying still has the advantage over commercial photo-editing software since it can extensively be used in film videos.

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One Response to "Chroma-Keying in Photoshop"

  1. Reader says:

    This photo is actually a good example of the difficulties of Chroma-keying in PS. As you’d expect, there’s a lot of green hue in the hair that is impossible to remove in a single step of “Replace Color” or “Select Color Range,” at least in PS CS2.

    I’d like to see discussion of the full process for dealing with areas like the hair here. Hair is the main reason I don’t use Chroma Green or Chroma Blue backgrounds.

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