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When To Use Digital Key vs. Chroma Key

 

Large Green Screen

 

As high-grade recording equipment becomes more affordable and non-linear editing software becomes more robust, even grade-schoolers can now post high quality video content to YouTube on a regular basis. The improvement in the technology and its affordability has also enabled video producers, at every level of expertise, to incorporate special effects into their videos. One of the more popular special effects techniques is called "keying." Keying, or compositing, is the combining of visual elements from separate sources into single images, often to create the illusion that all of those elements are parts of the same scene.

The technique requires the use of a uniformly-colored keying backdrop such as a "green screen" to achieve the effect. Through the magic of keying and compositing, the green colored item/s in the scene (i.e., the keying backdrop) becomes transparent, enabling an editor to select a different background in its place (e.g., a street scene, a mountain, etc.)

Keying backdrops come in two main types and colors: Chroma Key (Green or Blue) and Digital Key (Green or Blue). But how do you know which type and color to choose to best suit your purpose?

The decision is usually determined by what is being shot and how you’re shooting it. You’ll need to consider: the subject’s proximity to the backdrop screen, the amount of reflected light coming off of the screen, and the colors of the costume, wardrobe and other elements in the shot.

Let’s take a situation where you have a limited space in which to shoot and so the actors will be performing very close to the green screen. Chroma Key Green will probably be the best alternative under these circumstances. Due to its darker shade, the Chroma Key Green will not reflect back as much light on your subject as will Digital Key Green. (We’ll deal with the selection of the green or blue color later.)

You’ll want to avoid any reflected green light back onto your subject because it will potentially cause a "halo" effect around the edges of the subject’s clothing or skin, thereby spoiling the intended effect. The more reflected green light there is on the subject, the more likely a halo effect will occur. In some circumstances, it can also make it look as if the subject’s hair has disappeared. This unwanted effect is most likely to occur with performers who have fine blonde hair. The hair is so fine that it can easily pickup the color being reflected off of the backdrop, and now that the hair appears "green," it disappears when you "key" the green out of the scene.

Just about every TV station will make use of Chroma Key Green for their weather reporting. The meteorologist or reporter traditionally stands in front of a small wall. The small wall is usually a green screen, which is replaced by a weather map as the keying effect works its magic. It is a shot that is used daily and the lighting is set up knowing exactly where the weather person is going to stand, which is usually pretty close to the screen. Even so, from time to time you will see the talent move around a bit and step away from their "mark." When this occurs you will see shadows fall onto the green screen, causing part of the map to fade or disappear. And on even more rare occasions, the weather person wears something odd e.g., a green blouse or green striped tie, which then turns portions of their body transparent.

While Chroma Key Green is often used in tight quarters, Digital Key Green (a hot lime color) is preferred in other circumstances because you can use it with just about any kind of wardrobe. (The odds of someone wearing a hot lime article of clothing are pretty low.) It also enables you to more easily use a keying effect when shooting a person who has naturally green eyes. The added flexibility is one of the biggest reasons that Digital Green has generally become the "standard" for film and video use. The danger in using this color will again be the amount of reflected light. This color reflects much more easily than Chroma Key Green and will quickly show up on skin tones, hair and possibly clothing. Just keep the talent far enough away from the screen to avoid the reflected green light off of the backdrop. The exact distance you’ll need to position the talent away from the backdrop is dependent on the amount of light used in the shot.

The current blockbuster film, "Avatar," used enormous airplane hangars with interiors that were entirely draped in Digital Key Green fabrics. In this case, they had lots of space to play with and didn’t worry about reflected light bouncing back.

Producers of the movie "Superman Returns" used Digital Key Green to create the effect of a superhero flying around in a blue and red suit. However, they used Chroma Key Blue for shots involving kryptonite due to kryptonite’s green color. (Chroma Key Blue was also sufficiently lighter than Superman’s blue costume, to help avoid any transparency issues with the costume as well.) Choosing the keying color, green or blue, obviously depends on what you are trying to hide and the colors of the objects that are in the scene.

If you have any questions about the best keying backdrop to use for your shot, please don’t hesitate to send us an email or give us a call at 800-223-1624.

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