[postgis-users] Viewshed Isovist

[diplonics]

Hi All,
    Just wondering if this can be done in PostGIS and if not does anyone
know of the simplest method to get it done. I have 2 PostGIS polygon
geometry tables and I want to basically do is an isovist/viewshed/line of
sigth operation. I know an intersection operation can isolate where the
polys are spatially equal but what I want is some sort of set complement
operation where it will also remove all polygon sections without direct line
of sight.
Image 1 is a sample of my two data sets. The purple polys are building
outlines while the yellow poly is an unobstructed viewshed from a line.
http://www.nabble.com/file/p23546020/ViewShed01.png 
Image 2 is a sample of the desired result. Using a PostGIS intersection
operation I can isolate the green sections, but I want to return a geometry
that is the yellow section with the green and red sections removed.
http://www.nabble.com/file/p23546020/ViewShed02.png 

Any advice on this problem, or links to help,  would be greatly appreciated.
Thanks

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