Adobe After Effects - Manual (Page 118)

Proxy Any file used to temporarily replace a footage item, but most often a lower-resolution or still version of an existing footage item used to
replace the original. Often, storyboard images are used as proxies. You can use a proxy either before you have the final footage or when you
have the actual footage item but you want to speed up previewing or rendering of test movies. You must have a file available to use as a proxy.
Any masks, attributes, expressions, effects, and keyframes that you apply to the layer are retained when you replace its placeholder or proxy with
the final footage item.
In the Project panel, After Effects marks the footage name to indicate whether the actual footage item or its proxy is currently in use:
A filled box indicates that a proxy item is currently in use throughout the project. The name of the proxy appears in bold type at the top of the
Project panel when the footage item is selected.
An empty box indicates that the footage item is in use throughout the project, though a proxy has been assigned.
No box indicates that no proxy is assigned to the footage item.
Proxy items in Project panel
Work with placeholders and missing footage items
For best results, set the placeholder to the same size, duration, and frame rate as the actual footage.
If After Effects cannot find source footage when you open a project, the footage item appears in the Project panel labeled Missing, and the name
of the missing footage appears in italics. Any composition using that item replaces it with a placeholder. You can still work with the missing item in
the project, and any effects you applied to the original footage remain intact. When you replace the placeholder with the source footage, After
Effects places the footage in its correct location in all the compositions that use it.
You can find footage items for which the source items are missing by typing missing in the search field in the Project panel. See Search and
filter in the Timeline, Project, and Effects & Presets panels.
To use a placeholder, choose File > Import > Placeholder.
To replace the selected footage item with a placeholder, choose File > Replace Footage > Placeholder.
To replace a placeholder with the actual footage item, select the placeholder you want to replace in the Project panel, choose File > Replace
Footage > File, and locate the actual footage.
Work with proxies for footage items
When you use a proxy, After Effects replaces the actual footage with the proxy in all compositions that use the actual footage item. When you
finish working, you can switch back to the actual footage item in the project list. After Effects then replaces the proxy with the actual footage item in
any composition.
When you render your composition as a movie, you may choose to use either all the actual high-resolution footage items or their proxies. You may
want to use the proxies for a rendered movie if, for example, you simply want to test motion using a rough movie that renders quickly.
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