At this point, the capabilities of different applications start to diverge. For example, Premiere Elements offers Blend, Threshold and Cutoff settings. The Blend slider is useful for softening the edges around objects slightly. Increasing the Threshold setting retains shadows, if desired, whilst Cutoff darkens them. For a completely invisible background, you will probably want to leave these at their minimum settings. Pinnacle Studio Plus offers Minimum Saturation, Softness and Spill Suppression. Minimum Saturation performs a similar function to Premiere Elements’ Threshold, whilst Softness is similar to Blend, but neither behaves in precisely the same way. Spill Suppression helps reduce video noise and fringing around the foreground subject, but most of the time you won’t see much benefit from adjustment.
However, if there is considerable intensity variation in the background, or dark areas caused by folds, there’s a good chance that no amount of adjustment of these settings will help. In fact, a point will come where parts of the foreground start disappearing, particularly around their edges, or shimmers appear in certain areas. Similarity adjustment can only be pushed so far, and if this isn’t enough then another option will need to be called upon.
Some chroma key filters offer built-in edge cropping, or you can add a crop filter. But this will only work if the troublesome portions of the background are around the edges, away from the subject matter. If not, then a more sophisticated alternative will be required, which not every application offers. This is called the Garbage Matte, a name which is fairly self-explanatory because it excludes rubbish from the frame. Only Adobe Premiere Elements offers this in the sub-£100 consumer video editing market.
A garbage matte provides moveable points around the edge of the frame, which can be pulled into the frame to cut out unwanted areas. Only objects inside the shape defined by these points will be visible. The number of points available is usually indicated by the name, so a 4-point garbage matte has four, whereas a 16-point version has 16.
To use a garbage matte, apply the filter to your chroma key clip, then drag the points around your subject to cut out as much of the background as possible, without cutting out any areas where your actors move – otherwise weird disappearing effects will ensue! In some software, it’s possible to animate the matte from frame to frame, so it moves with the subject matter. Obviously, this can be very time consuming. But using a garbage matte to remove a background blemishes will mean you can relax the similarity slider a bit, and thereby avoid making the edges of your subjects invisible in odd ways.
Over the course of these two articles, it will hopefully have become clear that chroma keying may have become ubiquitous, but it’s far from a doddle. Nevertheless, if you’re careful how you shoot, and use your editing software’s keying filters correctly, you can create some amazing effects, and open a whole new world of fun and imagination.