One of Photoshop’s greatest powers lies in its ability to preserve complex transparency. It’s possible to have several different levels of transparency within a Photoshop document, which leads to greater flexibility in compositing multiple layers together. For example, in an image set to 8-bit mode, Photoshop supports 256 levels of transparency. Switch to 16-bit mode, and that number jumps dramatically to 65,536 levels. By employing masks, both in layers and embedded into the saved files as alpha channels, this transparency data can travel seamlessly into the nonlinear editing (NLE) or motion-graphics environment.
Before we begin exploring transparency in depth, it is necessary to establish some essential terminology:
Foreground element: A foreground element is any layer that contains objects to be composited over the background layer or plate.
Matte: The matte determines the opacity of the foreground elements and defines the edges of the foreground element. It’s important to keep your background in mind when developing independent foreground elements (and their mattes, masks, or alpha channels) inside of Photoshop. Know what you intend to key and where you intend to key it.