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which are opaque (white). Using techniques, such as softening the edge of
mattes, you can refine the results to create realistic composites.
Keying
Keying is the process of isolating a region of an image by selecting pixels of a
particular color and making those pixels transparent, or creating an alpha
channel where those pixels are located. The main purpose of using the keyer
tools in Composite is to generate mattes and remove color spill from a shot.
The two basic approaches to creating an alpha channel are based on:
■ Pixel color The removal of pure green pixels for example.
■ Luminance The removal of pixels that are very light.
You can also use garbage masks to key out undesired elements in an image.
However, garbage masks will be discussed in a later section as creating garbage
masks are a manual operation that involves creating paint objects or the
creation and manipulation of splines (rotoscoping)—see Masking Concepts
on page 538.
Mattes
A matte is an image used to define or control the transparency of another
image. When you pull a key, you generate a matte that defines the transparency
of the front source. There are several types of mattes.
Articulate matte A matte whose shape changes over time and is designed
to accurately follow the contours of the object to which it corresponds.
Complementary matte The matte that results when the primary matte is
inverted.
Difference matte A matte created by subtracting an image in which the
subject is present from an otherwise identical image in which the subject is
not present.
Edge matte A matte that includes only the outlines or borders of an object.
Fixed or static matte A matte that does not change position or shape over
time.
Garbage matte A matte that isolates unwanted elements from the primary
element in an image. Garbage mattes are also referred to as garbage masks.
510 | Chapter 22 Pulling Keys and Creating Mattes