Adobe AFTER EFFECTS 5.5 Help Manual page 366

Adobe After Effects Help
Using Help | Contents
2 From the Boundary menu, choose None.
About Property Mappers (PB only)
Control specific properties of individual particles using the Property Mapper. You can't
alter a specific particle directly, but you can use a layer map to specify what will happen to
any particle that passes over a specific pixel in the layer. Particle Playground interprets the
brightness of each layer map pixel as a specific value (see "About layer maps (PB only)" on
page 374). The Property Mapper options associate a specific layer map channel (Red,
Green, or Blue) with a specific property, so that as a particle passes over a certain pixel,
Particle Playground uses the brightness value provided by the layer map at that pixel to
modify the specified property for that particle.
For example, if you want to control the size of particles at certain areas within the frame,
create a layer map and associate it with the Scale property so that the particle size
increases as the pixel brightness value increases. Particles passing over lighter areas will be
larger than particles passing over darker areas. If you associate the layer map with the
Angular Velocity property instead, particles passing over lighter areas rotate around their
own centers faster than they would when passing over darker areas of the layer map.
Persistent and Ephemeral Property Mappers (PB only)
A particle property can be modified in either a persistent or an ephemeral way:
A persistent change to a particle property retains the most recent value set by a layer
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map for the remaining lifespan of the particle, unless the particle is modified by
another operator such as Repel, Gravity, or Wall. For example, if you use a layer map to
modify particle size, and you animate the layer map so it exits the frame, the particles
will keep the last size value set by the layer map after it has exited the frame.
An ephemeral change to a particle property causes the property to revert to its original
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value after each frame. For example, if you use a layer map to modify particle size, and
you animate the layer map so that it exits the frame, each particle returns to its original
size value as soon as no layer map pixels correspond to it. Similarly, if you apply an
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