Using Black Alpha Matte and White Alpha Matte keys

Typically, an image that contains a premultiplied alpha channel also contains either a black or white background. If you are superimposing the image, you likely need to key out the background.

To key out a black background, use the Black Alpha Matte; to key out a white background, use the White Alpha Matte.

Superimposing and Compositing > Using keys to superimpose and create composites > Using Black Alpha Matte and White Alpha Matte keys