Adobe AFTER EFFECTS 5.5 Help Manual page 365

Adobe After Effects Help
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Repelling and attracting particles (PB only)
Use Repel to control how nearby particles repel or attract each other. This feature
simulates adding a positive or negative magnetic charge to each particle. You can specify
which particles, layers, or characters are the repelling force and which are repelled.
Note: If you want to repel an entire layer of particles away from a specific area, use Wall
(see "Containing particles with a wall (PB only)" on page 365) or Gradient Force in Property
Mappers (see "About Property Mappers (PB only)" on page 366).
You can adjust the following options for Repel:
Force Specifies the repel force. Greater values repel particles with more force; negative
values result in particle attraction.
Force Radius Specifies the radius (measured in pixels) within which particles are repelled.
Another particle must be within this radius to be repelled.
Repeller Specifies which particles act as the repellers or attractors to another subset you
specify using the Affects option. See "Using Affects options to specify particles (PB only)"
on page 372.
Affects Specifies a subset of the layer's particles to which repulsion or attraction applies.
See "Using Affects options to specify particles (PB only)" on page 372.
Containing particles with a wall (PB only)
Use Wall to contain particles, limiting the area within which particles can move. A wall is a
closed mask that you create using a mask tool, such as the pen tool. By creating a wall you
can make particles stay within a specific area. When a particle hits the wall, it bounces off
at a velocity based on the force with which it hit.
Without walls, particles can roam about or exit a frame (left). Walls added to the left and right sides
restrict particle movement to the middle of the frame (right).
To contain particles within an area:
1 Open the Particle Playground layer in the Layer window and create a closed path to use
as a wall.
2 In the Effect Controls window under Particle Playground, expand the Wall property.
3 From the Boundary menu, choose the path you created.
To apply the effect to a subset of particles, specify the Affects options as necessary
(see "Using Affects options to specify particles (PB only)" on page 372).
To turn off the wall:
1 In the Effect Controls window under Particle Playground, expand the Wall property.
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