Au Hasard!
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or this animation I used the bTrace-addon to create a curve from a set of particles. Then I wrote a small python script that copies the current state of the curve every 5 frames. I animated the visibility of the copies to make them appear only if the original curve has passed this position to create a 3D onion skinning like effect. http://www.local-guru.net/blog/2013/9/23/object-traces