Adobe After Effects - Manual (Page 75)

Precomposing, nesting, and pre-rendering
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About precomposing and nesting
Precompose layers
Opening and navigating nested compositions
Pre-render a nested composition
Render order and collapsing transformations
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 .)
Precompositions in After Effects are similar to Smart Objects in Adobe Photoshop.
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.
Add another set of transform properties to a layer
The layer that represents the composition has its own properties, in addition to the properties of the layers that it contains. This allows you to apply
an additional set of transformations to a layer or set of layers.
For example, you can use nesting to make a planet both rotate and revolve (moving like the Earth, which spins on its own axis and also travels
around the Sun). To create such a system, animate the Rotation property of the planet layer, precompose that layer, modify the Anchor Point
property of the precomposition layer, and then animate the Rotation property of the precomposition layer.
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