From PanoTools.org Wiki
Revision as of 20:15, 28 October 2005 by Erik Krause (Short explanation, wikipedia link)
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: