GLOSSARY
Alpha channel shown in black, on the right
Alpha is a channel of data, found in 32-bit bitmap files, that assigns transparency to the pixels in the image. A 24-bit, truecolor file contains three channels of color information: red, green, and blue, or RGB. The intensity of these three channels determines the color of each pixel in the image. By adding a fourth, alpha channel, the file can specify the transparency, or opacity of each of the pixels. Transparency is important for compositing operations, such as those in Video Post, where several images are blended together in layers.
Each channel of a truecolor bitmap file is typically 8-bits, providing 256 levels of intensity. Thus, an RGB file is 24-bits with 256 levels each of red, green, and blue. An RGBA file (red, green, blue, alpha) is 32-bits, and the extra 8 bits of alpha provide 256 levels of transparency. An alpha value of 0 is transparent, an alpha value of 255 is opaque, and values in between are semi-transparent.