Alpha Channel: Video is often stored/communicated as a 32 bit value associated with each pixel even though only 24 bits are used for color. Given that PC busses are usually some multiple of 16 bits wide, sending 32 bits saves the complexity of dealing with pixels split across multiple buss words as would occur if 24 bit values were sent on a 32 bit bus. The 32 bit value is typically divided into four eight bit values (0-255). Three of these represent the intensity of the colors Red, Green, and Blue. A fourth -- the Alpha Channel value -- may be present. It is used to represent "transparency" -- the extent to which the "background" is allowed to bleed through. Although the process is described as background blending, it varies continuously from fully transparent to fully opaque -- from a image that is nothing but background to one that is nothing but foreground. It can be used to simulate viewing through glass, smoke or other transparent media, for masking, or for anti-aliasing at object boundaries.
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Copyright 1994-2002 by Donald Kenney.