ADOBE AFTER EFFECTS 7.0
User Guide
512
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 corre-
sponding 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 Comparesthebrightnessvalueofthelayermaptothevalueoftheparticlepropertyandusesthehighervalue.
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 between 0 and 1.0.
About layer maps and Particle Playground (Pro only)
A layer map is an image in which each pixel’s brightness value is used by an effect in a calculation. Particle
Playground uses a layer map to exercise precise control of a particle property such as opacity. In this way, After
Effects doesn’t use a layer map as a picture but as a matrix of numbers. In many cases, you never see the actual layer
map in the final movie—you see only the result of an effect applying the layer map’s pixel values to the corresponding
pixels in a destination layer.
Regardless of the color depth of the image that you use as a layer map, After Effects always uses its red, green, and
blue channels as if each were an 8-bit grayscale image. If you create a layer map using colors, the Property Mappers
in Particle Playground can extract the brightness values from each RGB color channel separately. (See “Creating an
RGB layer map for Particle Playground (Pro only)” on page 513.)
A layer map is often used as a stationary layer of the same dimensions as its destination layer. The value of each pixel
in the layer map applies only to one specific pixel at the corresponding position in the destination layer. However,
when you animate a layer, the appearance of the pixels in the destination layer changes depending on which layer-
mappixelcorrespondstoitataparticularpointintime.Layermapsareoftenanimatedsothatthelayermapeffect
appears to sweep through the destination layer.
Creating a layer map for Particle Playground (Pro only)
Layer maps are often created by painting in an image-editing program such as Adobe Photoshop, although you can
use any program that can save an image compatible with After Effects. The key to creating a good layer map is to
remember that the brightness value of each pixel influences an effect property. Here are some hints and tips for
creating layer maps:
• Ifyouwantalayermaptomatchtheshapeofanimagethatalreadyexists,simplyusethatimage.Forbestresults,
create a layer map that has the same dimensions as the layer containing the image.
• You can create a layer map by precomposing a white solid layer, a black solid layer, and a mask on the top layer
thatdetermineswhichareasarewhiteandblack.Increasingthemask’sfeathersoftensthetransitionbetweenblack
and white values.
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