The Matte Magic filter lets you manipulate the edges of a matte by shrinking, feathering, and eroding them to improve difficult keys. The Keyer and Luma Keyer filters have much of the same functionality. Matte Magic is a refinement filter to let you manipulate mattes you create via other means. After you apply a Matte Magic filter, you can adjust its controls in the Filters Inspector:
Shrink: A slider to manipulate the contrast of the matte in a way that renders translucent regions of the keyed matte more translucent while shrinking the matte.
Feather: A slider to blur the keyed matte, softening the edges by a uniform amount.
Erode: A slider that, when dragged to the right, increases transparency from the edge of the solid portion of the key inward.
Levels: A grayscale gradient control to alter the contrast of any matte or alpha channel. Drag the 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.
Mix: A slider to set the percentage of the original image to be blended with the filtered image. 100% is the filtered image, while 0% is the original, unfiltered image.