Adobe After Effects CS3 - Manual (Page 511)

AFTER EFFECTS CS3
User Guide
506
• You set the initial velocity of a particle and then use a layer map to affect the X Speed value. However, you find
that the difference between the fastest and slowest particles is too great. By raising the Min value and lowering the
Max value for the layer map channel that is mapped to the X Speed value, you narrow the resulting range of
particle speeds.
• You use a layer map to affect the Scale property of particles and find that the smallest particles aren’t small enough
while the largest resulting particles are too large. In this case, the entire output range needs to be shifted down;
lower both the Min and Max values.
• You have a layer map that modifies particles in the opposite direction from the one you want. Swap the Min and
Max values, which has the same result as inverting the layer map.
Note: The alpha channel of the layer map is used as the selection map for the Persistent and Ephemeral Property
Mappers.
Operator controls for the Ephemeral Property Mapper
When you use the Ephemeral Property Mapper controls, Particle Playground replaces the value of a particle’s
property with the value represented by the layer map pixel at the particle’s current location. You can also amplify,
attenuate, or limit the resulting values by specifying a mathematical operator and then using both the value of a
particle’s property and its corresponding layer map pixel value.
Set Replaces the value of a particle property by the value of the corresponding layer map pixel. For example, to
simply replace the value of a particle property with the brightness value of the corresponding pixel on the layer map,
use Set. This is the most predictable operator and is the default.
Add Uses the sum of the value of a particle property and the value of the corresponding layer map pixel.
Difference Uses the absolute value of the difference of the value of a particle property and the brightness value of the
corresponding pixel on the layer map. Because it takes the absolute value of the difference, the resulting value is
always positive. This operator is useful when you want to limit values to only positive values. If you’re trying to model
realistic behavior, the Difference operator may not be ideal.
Subtract Starts with the value of a particle property and subtracts the value of the brightness value of the corre-
sponding pixel on the layer map.
Multiply Multiplies the value of a particle property by the brightness value of the corresponding pixel on the layer
map and uses the result.
Min Compares the brightness value of the layer map to the value of the particle property and uses the lower value.
To limit a particle property so that it is less than or equal to a value, use the Min operator and set both the Min and
Max controls to that value. If you use a white solid as a layer map, you need only set the Max control to that value.
Max Compares the brightness value of the layer map to the value of the particle property and uses the higher value.
To amplify existing values of properties, try applying the Add operator with positive values or the Multiply operator
with values above 1.0. To attenuate (tone down) property value changes, try applying the Multiply operator using
values between 0 and 1.0.
Layer maps for Particle Playground
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 see only the result of an effect applying the layer map’s pixel values to the corresponding
pixels in a destination layer.