"Swirl"-Filter keeps original source in background

Hello,

when I add the Swirl-filter into a clip (text, image or film) it seems that this filter doubles the source to make a swirl-effect that will be shown in the foreground, but also shows the original source behind.
SC-Vers. 19.08.16

Maybe a bug?

That is correct. It is built on a GL transition and uses the source for both parts. This is the way it is for now. Take it as-is or submit a change.

Thanks for the information!