Understanding precomposing

Sometimes it's necessary to nest compositions after you have already created a complex project hierarchy. It can be tedious or difficult to use nesting in an existing composition.

Precomposing is an easier way to nest layers within an existing composition. Precomposing moves the layers to a new composition. When you want to change the order in which layer components are rendered, precomposing is a quick way to create intermediate levels of nesting in an existing hierarchy. (See Understanding default rendering order.)

When you precompose, one or more layers that you select move into a new composition. This new composition takes the place of the selected layers--something that does not occur in ordinary nesting. Precomposing also places the new composition in the Project window, available for use in any composition.

After Effects offers two options for working with layer properties and keyframes during precomposing:

Leave All Attributes In (selected composition)

Leaves the selected layer properties and keyframes in the original composition. The frame size of the new composition is the same as that of the selected layer. Select this option when you do not need to change the rendering order, such as when you precompose layers only to simplify or reuse a composition, not to change the rendering order of layer properties. This option is not available when you select more than one layer or a text layer. When you use this option, changes you applied to the properties of the original layer are still applied to that layer in the original composition.

Move All Attributes into the New Composition

Moves the properties and keyframes of one or more selected layers one level further from the main composition in the composition hierarchy. The frame size of the new composition is the same as that of the original composition. Choose this option when you want to change the rendering order in the selected layers, such as when you want to rotate a layer but not its drop shadow. (See Creating animations by nesting compositions.)

To precompose one or more layers:

1. In the Timeline window containing the layers you want to precompose, select the layers.
2. Choose Layer > Pre-compose.
3. Select Leave All Attributes In or Move All Attributes into the New Composition, and then click OK.