Adobe AFTER EFFECTS 5.5 Help Manual page 271

Adobe After Effects Help
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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.
When you precompose only one layer, you can transfer the keyframes, masks, effects, and
transform properties from the original layer to the new layer. You can also choose to keep
the keyframes and properties with the original layer while it moves one rendering step
further from the main composition. When you precompose more than one layer, the
keyframes and properties stay with the original layers.
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 compo-
sition, not to change the rendering order of layer properties. This option is not available
when you select more than one layer.
Note: After you precompose using the Leave All Attributes In option, changes you applied
to the properties of the original layer are still applied to that layer in the original compo-
sition.
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 compo-
sition 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" on page 267.
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 a method for handling layer properties and keyframes (either Leave All
Attributes In or Move All Attributes into the New Composition), and then click OK.
Reducing the project
Use the Reduce Project command to automatically delete any items that are not used,
either directly or indirectly, in selected compositions. For example, this command removes
both unused footage items and all other compositions that are not included within a
selected composition as nested (subordinate) compositions.
It's useful to refer to Flowchart View when you select the compositions you want to
keep. For information about opening Flowchart View, see "Visualizing organization
with Flowchart View" on page 264.
Be aware of two special cases when you use Reduce Project:
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