Alpha Channels

You use alpha channels to determine how much of the front image shows over the back image(s).

If you are looking through a keyhole inside a room, the door is the front, the keyhole is the alpha, and the room is the back. Combined together, these layers form a composite. The following illustration shows the function an alpha channel performs when compositing.

If you render the contents of the alpha channel, you get what is called a matte.

A matte can be completely black and white (called a "hi-con" for high-contrast), or can contain many shades of gray. In video and film, mattes are rarely static, and are rarely pure black and white. Any areas of the front containing transparency (glass, smoke, curly blond hair) or motion blur will appear in the matte as gray areas, allowing the transparent (or blurred) areas to partially show through.

The process of generating a matte (from blue screen, for example) is also known as "pulling" a matte. Getting as close as possible to the perfect matte is perhaps the most important part of compositing. Because you are superimposing layers on layers with the aim of creating a "reality," any imperfections in the matte can become glaringly obvious and ruin the illusion.

If the matte is too tight to the subject, the edges and any fine detail (such as curly hair) will be lost.

Conversely, if the matte is too loose to the subject, there will be color spill, meaning that the blue- or green-screen origins of the material will be apparent. combustion contains tools to manage these issues, such as blur, shrink, and erode. For more information, see Using Matte Controls Operator.

In addition to these concerns, broadcast TV and film have their own matte demands. Video footage may sometimes contain jagged fields (because of rapid movement, for example) and may require blurring or de-interlacing before the matte is pulled. You may also find a lot of noise in one or more of the channels (red, green, and blue), making the matte "sizzle" around the edges and perhaps causing individual pixels to disappear from the matte.

Each type of film stock, in turn, has a distinct grain, which necessitates the use of a common film stock when shooting the different elements of the composite. You can also use degraining before pulling the matte and regrain the entire composite afterwards to match the grain. For more information on degraining and regraining, see Adding and Removing Grain.