hi everyone!

i'm trying to create this effect but cant seem to wrap my head around how i might get it done, hoping some of you might be able to answer it for me.

basically i have a horizontal menu bar, it's essentially a colored bar with words and icons on it. then on top of that, i have a light gray bar with the alpha down to about 25%. this creates the illusion of the menu bar being under a transparent overlay sort of thing.

what i want to do is to have a sliding "hole" sort of thing that would follow the user's mouse, allowing parts of the colored bar below to be seen in focus (without the transparent overlay on top) as the user mouses over the menu bar.

i have seen this effect done before, but cant remember where, and i dont know what to call it, so searching for a tutorial hasnt been really successful either.

could anyone help point me in the right direction? i'd be eternally grateful