Expert: Joseph Silver - 11/14/2006

Question
It mean if our displacement map is stil (not animated) then will it work without precomose ?


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The text above is a follow-up to ...

-----Question-----
Sir,
i want to know why we need to do precompose to Displacement map's layer? what the Technical reson behind this ? why this dosen't work without precompose ?
-----Answer-----
I believe that the reason for this has to do with the ordering of how After Effects processes the Displacement Effect. For example, let's say you use a stillframe as your Displacement map. Typically, you'll want to first add some sort of distortion or transform to the stillframe so that the displacement map animates. When you use this animated/distorted stillframe and apply it to another layer as a displacement map, After Effects first uses the untreated stillframe for the map, then calculates the animation/transform later. Due to the ordering in the way After Effects takes it's directions, the animation/transform does not become useful unless the displacement map layer is first precomposited.

www.designomotion.com

Answer
Yes, a stillframe that is not animated will still work as a displacement map. However, the displacement will be still, not animated - of limited value. In order to create an animating displacement map, you will need to give some animation to the stillframe (by adding position keyframes, for example), then precompose.

www.designomotion.com