Ray effect on sprite with obstacles

I dont really know how to describe my problem, so here is an example :

So I want to be able to move the blue square, and as the circle is "emitting" from his center, his shape should change taking into account the square.

I really dont know how to handle this, by the way that would be great if i could apply this effect to any sprite, having them "emitting from center", not only on a circle, like this for example :

Any ideas ?