Viewshed done only within a certain angle

Hi to everyone, I'm a beginner user of grasshopper. I apologize for my poor english.  I was using this definition: 

http://www.grasshopper3d.com/forum/topics/cull-mesh-vertices-error-...

I found some problems in trying to make the analysis only within a certain angle, not 360°. Is it possible to perform the view analysis only within a certain angle to achive a result like the one in this video?

https://vimeo.com/71553560