When you perform multilayer compositing in Composite, you typically create a new composition, then add a Reaction tool to start building from scratch. However, you can also connect Reaction to any composition or tool output in an existing dependency graph in the Schematic view. This automatically sets the background for the composition.
To add Reaction to a new composition:
If you dropped Reaction on a Player or an empty Layer Editor view, the composition drop gate appears. You can link Reaction to the composition Output node, or add it to the current composition without connecting it to anything.
To add Reaction to the current composition: