Apple Shake 4 - Manual (Page 970)

970 Chapter 32 The Cookbook
A luminance key is used here. The blue channel has less contrast than the red channel,
so first insert a Color–Monochrome node, then boost the rWeight up and the g- and
bWeight down before you key.
A Color–LookupHLS node manipulates the luminance, then switches that into the alpha
with Color–Reorder (a macro begging to happen) to key the deep part of the clouds in
the lower-right corner. The curve looks like the following:
The LumaKey gets the edges of the cloud, and is enhanced by a DilateErode, and then is
subtracted from the soft key.
Red channel Blue channel
DilateErode3 LumaKey1