Beautiful effect
i just send you a PM on the RetroBG script ,and, hoping will give you not headaches, i will love also here a option to progressively offset the center of the effect
(and also here a option for a ping pong loop may be cool..oh yes that will create a shift from outward to inward and vice versa but may be a cool effect * )
Anyway look a great StarWars FX
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* anyway using gap is trivial create a ping pong loop
Should be possible also only with gimp, somewhere there is a "reverse layerstack "script
so ,manually should be simple as
1 duplicate the animation
2 reverse layerstack
3 save, and from the original reopen as layers
4 optionally delete the last frame of the original or the first imported
