Blackmagic Design DaVinci Resolve Advanced Panel - Manual (Page 212)

COLOR 212
The Add Alpha Output menu item, next to the actual Alpha Output,
located underneath the Output bar.
2. Add a serial node after node 1 and connect its Key output to the Alpha Output bar. This
is a fast way to build this composite if you’re confident that the corrections you’ve made
in node 1 won’t adversely affect the key, or if you know they’ll improve it by expanding
the contrast of a flat-contrast source clip.
Alternately, you could create a second processing branch by adding a disconnected
node (node 2), connecting the Source bar to its RGB input, and then connecting its Key
output to the Alpha Output bar you just added. Creating a second image processing
branch from which to pull your key lets you key directly from the Source, avoiding any
problems that grading the first node could introduce to the key.
3. Use the Qualifier controls of node 2 to key out the green background, and then turn on
the invert checkbox to create the proper composite.
4. If necessary, adjust the primary correction being applied in node 1, and make whatever
corrections are necessary to the background plate clip to make the composite blend
together more seamlessly.
Node 1 grades the clip, expanding contrast to improve both the image, and its ability to be keyed.
Node 2 uses Qualifier controls to pull the key.