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Chroma Keying Primer: Part One

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Chroma keying has been part of the moviemaking special effects arsenal for decades. In fact, you can trace the origins of the idea right back to the early days of film. In the 1930s, back projection was used to make actors appear as if they were on location, when in fact they were still in a Hollywood studio. The principle is the same today, but the technology used has transformed considerably. Nowadays, instead of projecting the desired scene onto a screen behind the actors, a background of a single bright colour is used instead. This can then be subtracted electronically, and a different background put in its place.

This process is called chroma keying, because the subtraction is matched to the chroma (colour) values of the background. Keying of this nature used to be something only high-end professional software could do. But in the last few years, all the leading video editing applications have acquired the facility. Adobe Premiere Elements has keying, as do the Plus versions of Pinnacle Studio and Ulead VideoStudio, as well as CyberLink’s PowerDirector. So, this week, we explain how the chroma keying effect works and how to shoot video for it. Next week, we finish off by discussing how to use the filters in various editing applications.

The chroma keying filters provided by editing applications are very powerful, but they can only go so far. If you don’t shoot video correctly in the first place, the results can still give the game away. This primarily focuses on the kind of background you use, how you arrange it, and how it’s lit.

First of all, the background colour needs to contrast with the foreground objects, which is why bright blue or green are generally chosen. These are colours not usually found in human skin, so you won’t have your actors’ faces disappearing along with the backdrop. For the same reason, the subjects need to avoid clothing of a similar hue. Otherwise, their limbs could become disembodied in dubious ways!

So your first step is to get a coloured background. You may already have a sheet or large roll of paper which would be adequate to get you started. But the best results will be obtained by using material specifically designed for the purpose. This will be of the most effective colour for keying. The top Ultimate version of Pinnacle’s Studio 12 actually includes a piece of greenscreen material in the box. This is a very bright green, and we have found it is very good at its job. You can also buy this separately from Pinnacle for £10 and Pinnacle also sells a T-shirt made of the same material for £9.99, which is worth considering for some fun effects.

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