Visibility between polygons

[Conn Copas]

Hi all. I have a visibility analysis problem which involves two polygons, ie,
the extent of a development corridor which may be seen from any point within a
town boundary. Looking at <r.los>, it presumably could be made to do this with
some tweaking. I'd appreciate any comments about what I'm proposing to do; even
better if someone can save me the task.

<R.los> can shade all areas of the corridor which are visible from a given
point in the town only. Therefore, the routine needs to be run repeatedly for
each point in the town, saving the results each time and finally overlaying
these through something like <r.combine>. Man pages for <r.combine> suggest
there is no limit to the number of input maps, but it would be prudent to
anticipate troubles in my case, maybe by performing the overlay at set
intervals. It would also be sensible to apply a dynamically updated mask to
each visibility analysis, so that only currently unseen cells are considered.

My main uncertainty concerns how much I can get away with using scripts, and
how much C programming I need to do. For example, I would like a routine which
will go through every cell in a given polygon and return a co-ordinate list,
which can then be passed to <r.los> item by item. To get that list, is
somewhere like <r.neighbours> the best place to look for code which could be
adapted, or am I missing an easier way of doing it? Last wishful extension:
it would be nice to be able to nominate co-ords interactively with the mouse,
and have visible cells highlighted (presumably by overlay) on the map. Does
this involve a simple grafting of <d.where> onto <r.los>, or am I being naively
optimistic? 

The man pages for <r.los> also refer to <r.pat.place>. We have neither code nor
man pages for that routine. Has it been deleted?

-- 
Conn V Copas
Loughborough University of Technology          tel : +44 509 263171  ext 4164
Computer-Human Interaction Research Centre     fax : +44 509 610815      
Leicestershire  LE11 3TU              e-mail - (Janet):C.V.Copas at uk.ac.lut   
G Britain                                      (Internet):C.V.Copas at lut.ac.uk