Adobe After Effects - Manual (Page 577)

Note:
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.
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 replace the value of a particle property
with the brightness value of the corresponding pixel on the layer map, use Set. This operator is the most predictable 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 corresponding 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 in the range 0.0–1.0.
Control layers for Particle Playground
Particle Playground can use a control layer to control a particle property, such as opacity. For general information about creating and using control
layers, see Compound effects and control layers.
Regardless of the color depth of the image that you use as a control layer, Particle Playground always uses its red, green, and blue channels as if
each were an 8-bpc grayscale image. If you create a control layer using colors, the Property Mapper property groups in Particle Playground can
extract the brightness values from each RGB color channel separately.
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