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Choking
Under Effects>Matte Tools, you will see two choking methods to further refine your key. These two tools were added to After Effects to help fix really problematic areas of a key. The Simple Choker is ideal for cleaning up or tightening a key. The drawback is it does not give any softness control and acts as an all or nothing choke to the matte. This is great if you want hard edges particularly on a product shot. But for more organic subjects (like a human), this hard edge will give away that you are keying a subject over a background.
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| The Simple Choker (top) is good for getting hard edges on your key, but it can cause some rather strange results. The Matte Choker (bottom), let's you blur or soften the area between your subject and the keyed background. |
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A better solution is the Matte Choker. The Matte Choker gives you two ways to tighten or spread your matte and also gives you options to soften the matte at the same time. Softening the matte can be another way to help blend your foreground and background elements more naturally.
Choke 1 can be thought of as your rough choker, and Choke 2 can be thought of as the fine tune choker. You can use positive or negative numbers for the choke values. A negative value will spread the matte out, and a positive number will contract.
Gray Level Softness creates a natural gradation between the all white of the foreground object and the black of the alpha channel. Gray Level Softness also chokes the matte more when you apply it, so it may be helpful to apply a negative number to Choke 1, soften the transition and then use a positive number for Choke 2, increasing or decreasing the Gray Level Softness 2 as needed.
Chances are this give you significantly better results than the Color Range Key by itself.
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