Adobe AFTER EFFECTS 5.5 Help Manual page 374

Adobe After Effects Help
Using Help | Contents
About layer maps (PB only)
A layer map is an image in which each pixel's brightness value is used by an effect in a
calculation. Particle Playground uses a layer map to exercise precise control of a particle
property such as opacity. In this way, After Effects doesn't use a layer map as a picture but
as a matrix of numbers. In many cases, you never see the actual layer map in the final
movie—you only see the result of an effect applying the layer map's pixel values to the
corresponding pixels in a destination layer.
Layer maps (left) can change properties of the particles that pass over them, such as scale (right).
Regardless of the color depth of the image that you use as a layer map, After Effects always
uses its red, green, and blue channels as if each were an 8-bit grayscale image. If you
create a layer map using colors, the Property Mappers in Particle Playground can extract
the brightness values from each RGB color channel separately (see "Using layer-map RGB
channels to alter multiple properties independently (PB only)" on page 372).
Creating a layer map (PB only)
Layer maps are often created by painting them in an image-editing program such as
Adobe Photoshop, although you can use any program that can save an image compatible
with After Effects. The key to creating a good layer map is to remember that the brightness
value of each pixel influences an effect property. Here are some hints and tips for creating
layer maps:
If you want a layer map to match the shape of an image that already exists, simply use
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that image. For best results, create a layer map that has the same dimensions as the
layer containing the image.
You can create a layer map by precomposing a white solid layer, a black solid layer, and
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a mask on the top layer that determines which areas are white and black. Increasing the
mask's Mask Feather property softens the transition between black and white values.
In Photoshop, an easy way to create a layer map is to create a layer with a black or white
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background, draw a selection, and fill the selection with the opposite color. Blurring the
entire layer softens the transition between black and white values.
You can set layer map values more precisely by painting shades of gray within a range
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from 0 (black) to 255 (white). This is the tonal range of an 8-bit channel. To simplify
painting or drawing, see if your painting or drawing program provides or allows you to
create a palette of 256 gray shades.
Note: While images created for use as displacement maps (in other effects or programs)
often map tones to values on a scale from –127 to +127, Particle Playground interprets
tones of gray as values on a scale from 0.0 (black) to 1.0 (white). If you're using images
created as displacement maps, use the Min and Max properties to modify the range of
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