Whether you want to quickly perform straightforward 2D compositing or create 3D effects, the first thing you'll do is set your composition background to define the rendering plane, then you'll create layers in Reaction.
You do not have to connect an image generator to set the background; you can work on an empty background. You'll see that when you build a composition using the Layer Editor; the background appears automatically when you add a Reaction tool to your composition.
