Effect question/idea

I was wondering, I've heard something about how a Shader can't yet read the pixels behind the object, or somesuch thing? Would that mean it can't currently do lens effects and stuff?

Cos I was wondering, it'd be cool if you could have a shader which simply used the visible area of the sprite (so any colour except #000000) and used an offset to take whatever the background is and shift it up, down, left right, whatever by a certain number of pixels.

It would be much much quicker than lens effects, and I was thinking you could apply it to little squares and circles to create splash effects.

It's currently not possible as you said and understood... :)

Good Idea