Luminescent Premultiply

Luminescent Premultiply

Luminescent Premultiply is one method you can use to
remove premultiplication on the fl y from source footage,
retaining bright values in edge pixels that are otherwise
clipped. Premultiplication over black causes all semitransparent
pixels to become darker; removing it can cause
them to appear dimmer than they should.
Luminescent Premultiply can be useful in cases where an
element with transparency has been created against a black
background within After Effects, bypassing the opportunity
to remove premultiplication on import.

Track Mattes

Track mattes allow you to use the alpha or luminance
information of one layer as the transparency of another
layer (Figure 3.34). It’s a simple enough concept, yet one
that is absolutely fundamental as a problem-solving tool for
complex composites.
The perceptual difference between an alpha channel and
a track matte isn’t, for the most part, too diffi cult to grasp.
In both cases, you have pixels with a value (in 8-bit color

space) between 0 and 255, whether a grayscale alpha
channel or three channels of color. With color, the three
channels are simply averaged together to make up a single
grayscale alpha. With 16 and even 32 bpc, it’s fi ner increments
in the same range.
To set a track matte, place the layer that contains the transparency
data directly above its target layer in the Timeline
and choose one of the four options from the Track Matte
pop-up menu:
. Alpha Matte: The alpha channel of the track matte
layer is the alpha
. Alpha Inverted Matte: Same but the black areas are
opaque
. Luma Matte: Uses the average brightness of red, green,
and blue as the alpha
. Luma Inverted Matte: Same but the black areas are
opaque
By default, visibility of the track matte layer is disabled
when you activate it from the layer below by choosing one
of these four modes, which is generally desirable. Some
clever uses of track mattes leave them on; for example,
by matting out the bright areas of the image and turning
on the matte, setting it to Add mode, you could naturally
brighten those areas even more.
Track mattes solve a lot of compositing problems. They
also help overcome limitations of After Effects. Chapter 7
describes how you can keep an entire sequence buffered
in RAM for fast access while rotoscoping on a track matte
layer, and how you can add tracking capabilities to masks
in this manner.