Chroma Keying from Simon Ubsdell using Blackmagic Fusion.
Simon shows how to make a chroma keyer from scratch.

In summary, Simon uses Channel Booleans for matte making, inverting, and filtering (using the Maximum operation). He also uses a sequence of Curves/Blur/Curves which he combines with a more delicate matte using the Minimum operation to create garbage mattes.
Another trick for recovering whispy hair is to use a less contrasty matte to colour correct the background to create hair-like grading on the background image. This technique eliminates spill problems.

sin for vertical oscillations
cos for horizontal oscillations
atan for a rotation at a distance
% for getting the remainder from a calculation