Described as the "Photoshop of dynamic media" by After Effects 7: Visual Quick Pro Guide author, Anthony Bolante, After Effects has become one of the best tools for allowing a motion artist to create amazing effects. But on the rare occasion that some footage must be extended, which actually happens more than you would think, After Effects CS3 has a few ways of looping that footage. This won't only save you time, but now you can keep the hair you would have pulled out in frustration.
Things you need
After Effects CS3Digital footage
Quick Footage Loop
1 Drag the footage onto the After Effects icon. This should open up After Effects with that footage in the Project Panel.
2 Select this footage, and go to "File," "Interpret Footage," "Main." An Interpret Footage Window should appear. There should be four boxes within this window. Your only concern will be with the last one. This is the "Other Options" box.
3 Type in the number 2 with in the Loop Times box. For complete cycles type in whole numbers, and if you want partial cycles, use decimals. Then click "OK."
4 "Double click" on the footage to open it up in another window. Hit the space bar to see it loop twice. You can do this as many times as you wish.
Expression Aided Footage Loop
1 Leaving After Effects open. Create a new composition by going to "Composition," "New," then click "OK."
2 Go to "Layer," "New Solid," and press "OK." Create a Square Mask by clicking on the square icon next to the Pen Tool icon in the Tools panel. Click on your solid in the middle composition panel. Drag across until you are happy with the size of the square.
3 Click on the Solid Layer in the timeline below the Composition window and type "P" to reveal the position property. Click on the Stopwatch Icon next to "Position." This will add a key frame, or a marker that will allow you to animate this solid across time. Move the time indicator 20 frames, and move the square across the screen. Now it is time for an expression.
4 Select the Position Property Name on the Timeline, and hold down Shift + Option + Equal Sign (for Mac) or Shift + Alt + Equal Sign (for PC). This will add an expression box.
5 Type "loop_out("cycle",0)" within the box. Hit your space bar to see the square animation loop.