Ways to Layer and Isolate Elements in Clips

People have been layering visual elements together since the dawn of the movie industry. Although digital techniques have increased the potential number of layers from two or three to two or three thousand (and even more), the same basic principles are used. The names vary from one industry to the next, but as film and video merge on a single digital horizon, the terms are becoming more interchangeable.

What Are Mattes and How Can You Use Them?

Matte, or hold matte, originated with film and photography. It traditionally refers to any opaque material that, when held in front of a camera lens, prevents certain areas of the film from being exposed during shooting. Then, the camera can be rewound, and a matte of the reverse shape can be used to prevent exposure on the already exposed part of the film while the other portion is exposed. The result is two different images shot at different times combined together in one frame.

The same principle can be used in digital applications. In the case of cameras, the mattes are handmade, physical objects, but on a computer they can be drawn and applied digitally.

In Final Cut Pro, a matte is a customizable, polygonal shape used to make part of a clip transparent. By outlining part of an image with a matte, you can turn everything outside or inside the shape transparent. Final Cut Pro allows you to create four- and eight-point mattes.

Figure. Canvas windows showing a four-point matte outlining the image of a woman and the resulting image on a transparent background.

What Is Keying and How Can You Use It?

Keying refers to the process of turning an area of consistent color or brightness in a video clip transparent to isolate a foreground subject. The actual keying process results in the creation of a matte, which is then automatically applied to composite an image with a background. In television and in movies, keying is used in a variety of ways to composite actors in front of graphics and other video clips for a variety of effects.

For example, a weather reporter on television often stands in front of a weather map. The reporter is actually standing in front of a large blue or green screen, but the color is made transparent (keyed out), and a map is inserted behind the person, instead. Generating a matte signal using a color signal is referred to as chroma keying, while generating a matte signal using a black-and-white signal is called luma keying.

Figure. Diagram showing the resulting composite image of a map and a woman against a green screen.

What Are Masks and How Are They Used?

A mask is an image that is used to create areas of transparency in another image. For example, the luma in one clip can be used to create transparency in another clip. You can also assign the alpha channel of one clip to a completely different clip. (For more information about alpha channels, see Alpha Channels and Key, Matte, and Mask Filters.) Using additional mask filters, you can further modify the resulting regions of transparency—widening, narrowing, or feathering them as needed. Clips used to create masks can be in motion, creating a moving area of transparency.

Figure.  Diagram showing a background of music, an oval mask, and an image of a guitar player composited together.

Alpha Channels and Key, Matte, and Mask Filters

Key, matte, and mask filters all create or modify the alpha channel of the clip to which they’re applied. A clip’s alpha channel defines areas of transparency within that clip.

Key filters generally create new alpha channel information and are useful in situations where the foreground subject is moving or has a complex or changing shape. For more information about keying, see Using Keying to Isolate Foreground Elements.

Matte filters can create alpha channel information, but they can also be used to add to or subtract from alpha channel information that has already been applied to the clip. Matte filters are useful when you want to simply isolate a region of the frame, or when you’re cutting out a static foreground object with a relatively simple shape. For more information about using mattes, see Using Mattes to Add or Modify Alpha Channels.

Masks are most useful when you’re copying an alpha channel (static or in motion) from one clip to another, although masks can also change the quality of a clip’s alpha channel, letting you feather out edges, for example. For more information about using masks, see Using Masks to Replace or Modify Alpha Channels.

For information about enabling or disabling alpha channels in sequence settings, see Confirming Alpha Channel Status.