Adobe After Effects CS4 - Manual (Page 66)

60 USING AFTER EFFECTS CS4
Projects and compositions
Last updated 12/21/2009
Precomposing, nesting, and pre-rendering
About precomposing and nesting
If you want to group some layers that are already in a composition, you can precompose those layers. Precomposing
layers places them in a new composition, which replaces the layers in the original composition. The new nested
composition becomes the source for a single layer in the original composition. The new composition appears in the
Project panel and is available for rendering or use in any other composition. You can nest compositions by adding an
existing composition to another composition, just as you would add any other footage item to a composition.
Precomposing a single layer is useful for adding transform properties to a layer and influencing the order in which
elements of a composition are rendered.
Nesting is the inclusion of one composition within another. The nested composition appears as a layer in the
containing composition.
A nested composition is sometimes called a precomposition, which is occasionally abbreviated in casual use to precomp
or pre-comp. When a precomposition is used as the source footage item for a layer, the layer is called a precomposition
layer.
During rendering, the image data and other information can be said to flow from each nested composition into the
composition that contains it. For this reason, nested compositions are sometimes referred to as being upstream of the
compositions that contain them, and the containing compositions are said to be downstream of the nested
compositions that they contain. A set of compositions connected through nesting is called a composition network. You
can navigate within a composition network using the Composition Navigator and Mini-Flowchart. (See “
Opening and
navigating nested compositions ” on page 62.)
Precompositions in After Effects are similar to Smart Objects in Adobe Photoshop.
More Help topics
“ Precompose layers ” on page 61
“ Create layers from footage items or change layer source ” on page 139
“ 3D layer interactions, render order, and collapsed transformations ” on page 184
“ Layer switches and columns in the Timeline panel ” on page 154
“ Parent and child layers ” on page 166
Uses for precomposing and nesting
Precomposing and nesting are useful for managing and organizing complex compositions. By precomposing and
nesting, you can do the following:
Apply complex changes to an entire composition You can create a composition that contains multiple layers, nest the
composition within the overall composition, and animate and apply effects to the nested composition so that all of the
layers change in the same ways over the same time period.
Reuse anything you build You can build an animation in its own composition and then drag that composition into
other compositions as many times as you want.
Update in one step When you make changes to a nested composition, those changes affect every composition in
which it is used, just like changes made to a source footage item affect every composition in which it is used.
Alter the default rendering order of a layer You can specify that After Effects render a transformation (such as
rotation) before rendering effects, so that the effect applies to the rotated footage.