All images from Inspired 3D Lighting and Compositing by David Parrish, series edited by Kyle Clark and Michael Ford. Reprinted with permission.

This is the fourth in a number of adaptations from the new Inspired series published by Premier Press. Comprised of four titles and edited by Kyle Clark and Michael Ford, these books are designed to provide animators and curious moviegoers with tips and tricks from Hollywood veterans. The following is excerpted from Lighting and Compositing.

Basic Compositing Operations
Compositing at its most basic level is the combination of images. In this book, those images are referred to as layers. The way those layers are combined is important in creating the final look of the image. Compositing software packages have many functions for combining layers, but a solid understanding of the basic ones goes a long way in helping the compositor make the most of the input images. The following examples provide a brief explanation along with a visual representation of the most common layering processes found in compositing software packages. (The mathematics behind each function is described in the Appendix.) For the purposes of consistency, each example in this section utilizes the same simple, computer graphics elements. The elements are a building block, which will be the A input for each example (see Figure 19), and a beach ball, which will be the B input (see Figure 20).

[Figures 19 & 20] A building block element (left) and a beach ball element (right).

Over and Under
To begin with, examine one of the most commonly used functions in any compositor’s toolbox: the over function. The most basic compositing script can be described as A over B. In the simplest terms, this means putting one element (called A) over another element (called B). The key to understanding how the over function works lies in the alpha channel and the concept of pre-multiplied images. The alpha channel defines the portion of the image that is maintained during the over-processing. The black portion of the alpha is what gets thrown away, whereas the white portion is what gets composited. This is true, however, only if the image is pre-multiplied, which means multiplying the alpha channel values times the color channel values. Where the alpha is black (a value of zero), this multiplication yields a result of zero. Where the alpha is white (a value of one), this multiplication yields the exact same color value seen in the red, green, and blue channels. Some software packages automatically pre-multiply an image when the over function is applied, and others do not. Without pre-multiplied images, the over function will not work as expected, so it is vital to determine whether the software performs this function automatically.