Edge Selection

Edge Selection

Earlier in this chapter the three-pass method for deriving
a matte was introduced; using this method is one way to
derive an edge matte. However, the simplest means to this
end is probably as follows:
1. Apply Shift Channels. Set Take Alpha From to Full On
and all three color channels to Alpha.
2. Apply Find Edges (often mistaken for a useless psychedelic
effect because, as with Photoshop, it appears in
the Stylize menu). Check the Invert box for an edge
highlighted in white.
Minimax is useful to help choke or spread this edge
matte. The default setting under Operation in this
effect is Maximum, which spreads the white edge
pixels by the amount specifi ed in the Radius setting.
Minimum chokes the edge in the same manner. If the
result appears a little crude, an additional Fast Blur
will soften it.

3. Apply the result via a luma matte to an adjustment
layer. You should not need to precomp before doing so.

You can then use Fast Blur to soften the blend area between
the foreground and background, which often works better
than simply softening a chewy matte. A Levels adjustment
will darken or brighten the composited edge to better blend
it. Hue/Saturation can be used to desaturate the edge, similar
to using a gray edge replacement color in Keylight.