In computer graphics, alpha compositing is the process of combining one image with a background to create the appearance of partial or full transparency. It is often useful to render picture elements (pixels) in separate passes or layers and then combine the resulting 2D images into a single, final image called the composite. Compositing is used extensively in the film when combining computer-rendered image elements with live footage. Alpha blending is also used in 2D computer graphics to put rasterized foreground elements over a background.
In order to combine the picture elements of the images correctly, it is necessary to keep an associated matte for each element in addition to its color. This matte layer contains the coverage information—the shape of the geometry being drawn—making it possible to distinguish between parts of the image where something was drawn and parts that are empty.
Although the most basic operation of combining two images is to put one over the other, there are many operations, or blend modes, that are used.
To store matte information, the concept of an alpha channel was introduced by Alvy Ray Smith in the late 1970s and fully developed in a 1984 paper by Thomas Porter and Tom Duff. In a 2D picture element (pixel), which stores a color for each pixel, additional data is stored in the alpha channel with a value ranging from 0 to 1. A value of 0 means that the pixel is transparent and does not provide any coverage information; i.e. there is no occlusion at the image pixel window because the geometry did not overlap this pixel. A value of 1 means that the pixel is fully occluding because the geometry completely overlaps the pixel window.