In computer graphics
or video graphics
, an alpha channel
is a separate layer of image data that adjusts what parts of the main image are visible. Also known as a mask channel
For example, to overlay a cg-rendered dinosaur into a scene with actors, such as in Jurassic Park, the computer image of the dinosaur has an alpha channel. In the alpha channel, 1's are put where the dinosaur is drawn, and 0's everywhere the dinosaur isn't. When compositing, the photography is allowed to show through only where the dinosaur isn't.
More elaborate alpha channels use fractional alpha, allowing translucent effects (like score boards over sporting events on television), or to soften the otherwise harsh edges and stair-stepping that is seen in computer-generated images.