Adobe AFTER EFFECTS CS3 PROFESSIONAL User Manual page 510

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-bpc 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.
A layer map is often used as a stationary layer of the same dimensions as its destination layer. The value of each pixel
in the layer map applies only to one specific pixel at the corresponding position in the destination layer. However,
when you animate a layer, the appearance of the pixels in the destination layer changes depending on which layer-
map pixel corresponds to it at a particular point in time. Layer maps are often animated so that the layer map effect
appears to sweep through the destination layer.
Layer maps are often created by painting 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 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 a mask on the top layer
that determines which areas are white and black. Increasing the mask's feather 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 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 from 0 (black) to 255 (white).
This is the tonal range of an 8-bit channel. To simplify painting or drawing, see if your image-editing 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 controls to modify the range of tones produced
by the layer map.
• The alpha channel in a layer map modifies the value before it's applied to the destination layer. Areas where the
alpha channel is completely off (transparent areas of a layer map) don't affect on particle values. Areas where the
alpha channel has a partial value (semitransparent areas of a layer map) partially affect the particle value. For
example, if a layer-map pixel has a value of 10 and the layer-map alpha channel has a value of 127 (50%), the layer-
map pixel is affected by 50%, and its true value is 5. When you use the Persistent and Ephemeral Property
Mappers, the actual value applied to a particle is also affected by the range set for the Min and Max controls.
• If you want to change any of the layer map's layer properties (Masks, Effects, or Transform), change them,
precompose the layer, and then use the resulting composition as the layer map. Otherwise, Particle Playground
ignores any property settings.
• The contrast between adjacent pixel values determines how smoothly the values change across the surface of the
layer map. To create smooth changes, paint using a soft or anti-aliased brush, or apply gradients. To create abrupt
changes, avoid intermediate shades, using just a few widely spaced shades, such as 50% gray, black, and white.
• You can adjust overall edge contrast with blur or sharpen filters, if your painting or drawing program provides them.
Note: If you want to edit individual pixels, open the layer map in the program you used to create it and make the change.
AFTER EFFECTS CS3 504
User Guide