Hasta La Motion!

I want to distort an foreground element and merge it over a background. For the distortion i used the IDistort node following sathees example he posted in this thread: http://www.vfxtalk.com/forum/create-…ke-t16095.html.
The image that controls the distortion is a small .png with horizontal lines. As long as i don’t merge the distorted image over the colorbars it works fine. But when i merge it in "over" mode, the foreground isnt’t opaque anymore. I searched in the documentation and internet but couldn’t find a sollution to this. It would be great if someone could help me out.
I added an image to make it a bit clearer.

Thanks in advance.