Expression example: Create a trail of images

This example expression instructs a layer to be at the same position as the next higher layer in the Timeline panel, but delayed by a specified amount of time (in this case, 0.5 seconds). You can set similar expressions for the other geometric properties.

  1. Start with two solid-color layers that are scaled to approximately 30% of the composition size. (See Create or modify a solid-color layer or footage item.)
  2. Animate the position of the first layer. (See Motion paths.)
  3. Select the second layer, press P to reveal the Position property, and Alt-click (Windows) or Option-click (Mac OS) the stopwatch button to the left of the property name.
  4. Enter the following in the expression field: 
      thisComp.layer(thisLayer, -1).position.valueAtTime(time - .5)
  5. Duplicate the last layer five times by selecting it and pressing Ctrl+D (Windows) or Command+D (Mac OS) five times.

All layers follow the same path, and each is delayed 0.5 seconds from the previous.

Dan Ebberts provides more examples and techniques for creating trails of images on his MotionScript website.