Compositing. 1. Theory

Associating a pixel´s color with its opacity is the basis for a compositing function that is simple, elegant, and general. However, there are more reasons than mere prettiness to store pixels this way. One of the most important anti-aliening tools in computer graphics comes from a generalization of the simple act of storing a pixel into a frame buffer. Several people simultaneously discovered the usefulness of this operation, so it goes by several names: matting, image compositing, alpha blending, overlaying, or lerping. It was most completely codified in a previous paper by Porter and Duff (1984), where they call it the over operator. The author shows a new way to derive the over operator and describes some implementation details that he has found useful.<>