From PanoTools.org Wiki
Revision as of 20:15, 28 October 2005 by Erik Krause
An Alpha channel is an additional image channel just like the red, green or blue channel, too. It can contain any information in form of a grayscale image but commenly it is used to store a selection or a mask.
- Black alpha channel pixels usually stand for masked, invisible or unselected pixels in the main image.
- White alpha channel pixels stand for unmasked, visible or selected pixels.
- Gray alpha channel pixels stand for more or less translucent pixels according to the brightness of gray.
More information on Wikipedia: