There is no abstract art. You must always start with something. Afterward you can remove all traces of reality.
A particle physicist works with atoms, bakers and bankers each work with their own form of dough, and compositors work with selections—many different types of selections, even thousands, each derived one at a time.
If compositing were simply a question of taking pristine, perfect foreground source A and overlaying it onto perfectly matching background plate B, there would be no compositor in the effects process; an editor could accomplish the job before lunchtime. Instead, ...