I'm fixing a hole where the rain gets in And stops my mind from wandering Where it will go.
John Lennon and Paul McCartney
A particle physicist works with atoms, bakers and bankers work with their own types of dough, and compositors work with selectionsmany different types of selections, each of them derived uniquely.
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, compositors break sequences of images apart and reassemble them, sometimes painstakingly, and usually including the element of motion. Often, it is one element, one frame, or one area of a shot that needs special attention. By the clever use of selections, a compositor can save fellow artists and camera operators, taking control of whatever part of the source footage is necessary.
In this chapter, we'll look at the foundation techniques that define how a layer merges with those behind it. Then throughout Section II, "Effects Compositing Essentials," and in particular in Chapters 6 and 7, we will focus on particular ways to refine selections, creating high-contrast mattes, and pulling keys from blue-screen footage.
The Many Ways to Create Selections