Adobe AFTER EFFECTS 5.5 Help Manual page 364

Adobe After Effects Help
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Rotation At particle creation, the Cannon and Grid set no rotation; the Particle Exploder
takes rotation from the exploded dot, layer, or character. Use Auto-Orient Rotation to
make particles rotate automatically along their respective trajectories; for instance, a
particle can point up as it climbs an arc, and point down as it descends. Rotation isn't
easily visible for a dot particle. It is easier to observe only when you replace the dot
particle with text characters or a layer. After particle creation, use a layer map to set values
for the Angle, Angular Velocity, and Torque properties in the Property Mappers.
Note: The appearance and rotation of a layer map particle changes in relation to the
mapped layer's original appearance and rotation. For instance, if you replace the default
dot particles with a movie of a spinning wheel, the spinning-wheel particles appear to
spin when no particle rotation is applied.
To rotate particles automatically along their trajectories:
1 In the Effect Controls window under Particle Playground, click Options.
2 Select Auto-Orient Rotation to turn it on, and click OK.
Using gravity to pull particles (PB only)
Use Gravity to pull existing particles in a direction you specify. As in the physical universe,
particles accelerate in the direction of gravity.
Gravity set to zero (left) does not affect particle direction. Increased gravity (right) pulls particles
down.
You can adjust the following options for Gravity:
Force Specifies the force of gravity. Positive values increase the force, pulling particles
more strongly. Negative values reduce the force.
Force Random Spread Specifies a range of randomness for the Force. At zero, all
particles fall at the same rate. At a higher value, particles fall at slightly different rates.
Although pure gravity accelerates all objects equally, increasing the Force Spread can
produce more realistic results with subjects such as leaves falling through air, where there
is enough air resistance to vary the leaves' rates of descent.
Direction Specifies the angle along which gravity pulls. The default is 180 degrees, which
simulates the real world by pulling particles toward the bottom of the frame.
Affects Specifies a subset of the layer's particles to which Gravity applies. See "Using
Affects options to specify particles (PB only)" on page 372.
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