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• Luma Rolloff: Use this slider to adjust the linearity of the luma rolloff slope (the ends
of the bell-shaped luma curve displayed in the Luma control). Luma rolloff modifies
the softness of the matte around the edges of regions that are affected most by the
Luma control. Lowering this value makes the slope between the upper and lower
handles in the Luma control more linear, which increases edge softness in the matte.
Raising this value makes the slope steeper, sharpening the edges of the matte and
making them more abrupt.
• Fix Video: Select this checkbox to apply subpixel smoothing to the chroma components
of the image, reducing the jagged edges that result from keying compressed media
using 4:2:0, 4:1:1, or 4:2:2 chroma subsampling. Although selected by default, this
checkbox can be deselected if subpixel smoothing degrades the quality of your keys.
Matte Tools: Click the disclosure triangle in the Matte Tools row to reveal controls for
post-processing the transparency matte generated by the previous sets of parameters.
These parameters don’t alter the range of values sampled to create the keyed matte.
Instead, they alter the matte generated by the Keyer filter’s basic and advanced controls
(the Color Selection parameter group), letting you shrink, expand, soften, or invert the
matte to achieve a better composite.
• Levels: Use this grayscale gradient to alter the contrast of the keyed matte, by dragging
three handles that set the black point, white point, and bias (distribution of gray values
between the black point and white point). Adjusting the contrast of a matte can be
useful for manipulating translucent areas of the key to make them more solid (by
lowering the white point) or more translucent (by raising the black point). Dragging
the Bias handle right erodes translucent regions of the key, while dragging the Bias
handle left makes translucent regions of the key more solid.
• Black, White, Bias: Click the disclosure triangle in the Levels row to reveal sliders for the
Black, White, and Bias parameters. These sliders, which mirror the settings of the Levels
handles described above, allow you to keyframe the three Levels parameters (via the
Add Keyframe button to the right of each slider). Keyframing the Black, White, and Bias
parameters may yield a better key, one that adapts to changing blue screen or green
screen conditions.
646 Chapter 13 Keying