How to create a frosted glass without a specific source?

[TaiYu]

Hi,
There is a need to create a frosted glass without assign a source to FastBlur{}.
In another word, I need to create a given size area to blur everything under it like a filter.
Dose anyone know how to do it? Thanks so much

[sierdzio]

Create a semi-transparent Rectangle that will sit above the area in question and apply FastBlur{} to it.

[TaiYu]

Can you give me a quick example?
I had try something similar but the blur is not effect lower objects.

[sierdzio]

Well, I've checked my suggestion and it does not work :-(