mask

Video masking is a technique that allows you to focus an effect on some parts of an image, while leaving other parts uneffected. It's particularly effective when used in the alpha channel of a video, causing some parts of the image to be transparent while other parts remain opaque, allowing for one image to be composited on top of another. The "mask" is a shape, often simply a 1-plane black-and-white or grayscale image, that can be used in the alpha plane to designate which parts of an image should be transparent.