Adobe 12040118 - After Effects Standard Tutorial page 478

Wall controls
Wall controls contain particles, limiting the area within which particles can move. A wall is a closed mask that you create by using a mask tool,
such as the Pen tool. When a particle hits the wall, it bounces off at a velocity based on the force with which it hit.
Boundary Specifies the mask to use as the wall. You can create a new mask by drawing one on the effect layer.
Affects controls
Many Particle Playground controls include Affects controls. Affects controls specify which particles the encompassing control affects. For example,
the Affects controls within the Particle Exploder controls specify which particles the Particle Exploder affects.
Particles From Specifies the particle generator or combination of particle generators whose particles you want to affect.
Selection Map Specifies the layer map that influences which particles are affected.
Note: The simulation space is not bounded by the dimensions of the layer to which Particle Playground is applied. You may need to use a
selection map that is larger than the Particle Playground layer so that the selection map affects dots that are not visible.
Characters Specifies the characters you want to affect. This control applies only if you are using text characters as the particle type.
Older/Younger Than Specifies the age threshold, in seconds, above or below which you want to affect a particle. Positive values affect older
particles, and negative values affect younger particles. For example, a value of 10 means that as soon as a particle reaches 10 seconds, it
changes to the new value.
Age Feather Specifies the age range in seconds within which the Older/Younger Than value is feathered, or softened. Feathering creates a
gradual, rather than abrupt, change. For example, if you set Older/Younger Than to 10 and Age Feather to 4, about 20% of particles start changing
when they're 8 seconds old, 50% change when they're 10 seconds old (the Older/Younger Than value), and the remainder change by the time
they're 12 seconds old.
Use Particle Playground Property Mapper controls
You can control specific properties of individual particles by using a layer map and either the Persistent Property Mapper or the Ephemeral
Property Mapper. You can't alter a specific particle directly, but you can use a layer map to specify what happens 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. The Property Mapper
associates a specific layer map channel (Red, Green, or Blue) with a specific property, so that as a particle passes over a certain pixel, the
brightness value at that pixel modifies the property.
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 map for the remaining lifespan of the particle, unless
the particle is modified by another control 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 keep the last size value set by the layer map after it exits the frame.
An ephemeral change to a particle property causes the property to revert to its original 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 operator such as Add, each time a particle passes over a different
layer map pixel, the value of the layer map pixel is added to the original value of the particle.
In both the Persistent and Ephemeral Property Mappers, you can control up to three particle properties independently by using a single RGB
image as a layer map. Particle Playground achieves this independent three-component control by extracting brightness values separately from the
red, green, and blue channels in the image. You don't have to use all three channels if you want to modify only one property. To change only one
property or change up to three properties using the same values, use a grayscale image as the layer map because the RGB channels are
identical.
In combination with keyframes or expressions, the Property Mappers provide complete control over individual particle properties in space and time.
Using layer maps, you can change particle properties at any location within a frame. By applying keyframes or expressions to Property Mapper
options and animating a layer map, you can control how particle properties change.
1. For Use Layer As Map, choose a layer map to use as the source for values that modify particle values. The layer map must be part of the
composition.
2. To apply the effect to a subset of particles, specify the Affects controls as necessary.
3. Choose a property for each of the Map Red To, Map Green To, and Map Blue To controls. You don't have to map properties to all of the
color channels. For example, if you want to change scale over an image map, you can map the color red to scale without setting other
properties.
4. Specify the minimum and maximum values you want the layer map to produce for each Map To group. Min is the value to which a black
pixel is mapped, and Max is the value to which a white pixel is mapped. The complete tonal distribution between Min and Max is then scaled
proportionally.
5. If using the Ephemeral Property Mapper, you can apply an operator to the value of a particle property and the value of the corresponding
layer map pixel.
Note: Because particle properties use many kinds of units, such as pixels, degrees, and seconds, you may want to compress or expand the
range of values from the layer map so that all the resulting values are usable in the measurement system of a specific particle property.
First, use the Min and Max controls, which define the range of values to use from the layer map. If further adjustment is necessary and
you're using the Ephemeral Property Mapper, use the Operator control and choose a mathematical operator to amplify, attenuate, or limit
the effect of a layer map.