I am making a virtual screen comped onto a video. The screen is a precomp with elements inside. I would really like to have an blur effect inside the precomp that effects the element it is comped on top of. I am trying to make the effect of a frosted glass behind the elements inside the screen. Is it posible to make a adjustmentlayer give an effect on the pixles ouside the precomp?
That's just not possible due to how AE's internal rendering works You really have to have a duplicate of your main footage in the pre-comp and mask it out to the square. Depending on what you actualyl do, aligning things may require tracking and linking positions across comps with expressions, but that would require more info.
You can add a blur effect to any nested comp (Pre-comp). You can add a mask to an adjustment layer.
If your nested comp contains the shelves and the objects on the shelves, then you can pre-compose the objects on the shelves and add a blur to that pre-comp. You could also create a track matte for all of the frames.
If we saw your timeline and understood the layer structure, and we also saw a flow chart so we knew what was on each layer we could point you to the most efficient and professional approach to the design problem you are having.