NewTek TriCaster 8000 - Manual (Page 242)

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On the other hand, it may be that there is (usually unplanned for) detail in the
foreground that is somewhat similar to the key color. Reducing Tolerance may
allow you to prevent unwanted holes appearing when the subject opts to wear his
St. Patrick’s Day tie.
13.2.3 SMOOTHNESS
Smoothness defines a further tolerance factor, and a very useful one. We want our
keyed foreground to blend smoothly into the background – rather than to stand out
in hard relief like a postage stamp or decal. Smoothness serves this purpose. Don’t
overdo it though, as aggressive settings can cause the foreground to become
unnecessarily ‘muddy’.
13.2.4 LUMA LIMIT
When working with poorly-lit backgrounds (or poor quality footage), the color
Tolerance range separating the foreground (talent) from the background can be
extremely narrow.
This problem can be aggravated by the subject’s choice of clothing, or when there
are harsh shadows. There is often a strong chroma component (and associated
chroma noise) in dark foreground areas. The ‘noise’ may be partially or completely
transparent when tolerances are critical. Because the noise varies over time, ‘holes’
in the foreground can result, and even worse these may flicker on and off from one
frame to another.
LiveMatte’s Luma Limit control makes it possible to overcome this issue. In essence,
it restricts the chromakey operation based on luminance (brightness) values. Dark
foreground areas which typically cause the problems just described normally have
quite different luminance values from the background color. In simplest terms,
problem areas of this type can be decisively ‘pulled’ back into the foreground by
pre-filtering the chromakey effect around a luminance threshold.
Generally, try to set up the best key you can before raising the Luma Limit from its
default value of zero (no effect). Then gradually raise the limit until you are pleased
with the result.