3D Spatial Isovist to View Cones

Hi,

I'm really new with GH and I'm trying to modify the definition by Andrew Heumann (3d Spatial Isovist) but i'm finding some problems. For the moment I obtained a semi-sphere from which I subtract all the objects which block the views. The problem is when I have a semi-sphere that incorporate all the objects of the context and then the semi-sphere will not be cutted. Then I have to use in place of the semi-sphere a cone or a portion of sphere and achieve a result similar to this https://vimeo.com/71553560. Do you have any suggestion?