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.

Section 13.2.4

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