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Chroma Keying Primer: Part Two


Chroma Keying Primer: Part Two

Last week, we discussed how to shoot footage for the infamous bluescreen effect, which has become increasingly ubiquitous in Hollywood movies. This week, we complete the series with the other end of the process – video editing.

Although different editing applications offer a variety of features in their keying filters, they all work in the same basic way. As we discussed last week, the background is supposed to be one uniform colour, which is why we stressed how important it is to keep the lighting as even as possible. The first step is therefore to set the chroma key filter to the background colour, which is then subtracted from the frame and made transparent. Once the footage has been imported into the application and placed on the timeline, in most cases this will be a simple task involving an eyedropper colour picker. This is used to select the background in a sample frame from your video footage.

If the background really is one single colour, this would be enough to completely remove it, leaving just the foreground subject matter. With the new systems from Reflecmedia and Holdan it is, which is why these systems have won so much favour recently – and can charge a hefty premium for the privilege. But this is rarely the case with the traditional coloured backcloth approach. Small variances in the brightness of the background will almost always mean there are residual grey smears all over the background, or some of the backcloth still visible.

The eyedropper tool is used in Premiere Elements’ Chroma Key filter to select the background colour, but by default this will only make a small part of it transparent.

This is where the features of a particular chroma key filter come in, and where simplistic versions are separated from professional-grade options. Further adjustment can make for a much cleaner key. Your next port of call is the Similarity slider, although it may have a different name in some applications. For example, Pinnacle Studio Plus calls the same feature Tolerance.

This setting behaves in a similar fashion to the similarity slider used with the bucket fill tool in a photo editing app. The wider the range of similarity, the more areas will be included. Sliding Similarity (or Tolerance) upwards widens the range, removing more of the background. This slider should be adjusted until just before the foreground objects begin to disappear around the edges.

With the Similarity slider adjusted, now all of the background has become transparent…

…but push things too far and your foreground will start disappearing too!

Now it is time to insert your background, which can be a still image or even moving video, although much more care will need to be taken with the latter to make the superimposed foreground look like it fits. Editing software almost universally takes a track-based approach here. Depending on the application, tracks further up the screen or further down will be visible on tops of those below or above. For example, Adobe Premiere Elements stacks tracks upwards, whereas Pinnacle Studio Plus and Ulead VideoStudio Plus stack them downwards. Either way, you will need your keyed track on top of your background track, so the latter shows through beneath the former.

With your key adjusted, you can place a background image, or even full-motion video, behind your foreground subject.

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