[GRASS5] [bug #2061] (grass) r.los needs FP update

[Greg Sepesi]

Paul Kelly wrote:
> 
> On Thu, 14 Aug 2003, Greg Sepesi wrote:
> 
> > > The algorithm in PE&RS is from the US Army CERL so it would be especially
> > > appropriate for it to go into the latest GRASS. I would hope some of the
> > > Vectlib and dglib functions could be used for determining the crossing
> > > points and distances between the cells but I will have to learn about
> > > them first.
> > >
> > > Paul
> >
> > Hi Paul,
> >
> > What is the definition of "crossing points" in the algorithm?
> 
> Hello Greg
> My general idea is that vector network functions could be very useful for
> performing some types of raster processing, if we define the centre of
> every raster cell as a vector node. Each node / cell could then be linked
> to its nearest neighbours by various line segments, either 4-way or 8-way
> connectivity depending on the application. If a raster MASK was in force,
> no nodes or lines would be created in the area covered by the MASK. Vector
> networking functions could then be used to find the approximate shortest
> distance between two cells (along the line segments), taking account of
> the MASK, i.e. moving around the outside of it. I've no idea how accurate
> this would be, e.g. it might be necessary to do 16-way connectivity
> between the cells or something so the distances weren't too long.
> 
> The application I had in mind for this was interpolation around an
> impenetrable barrier, see
> http://grass.itc.it/pipermail/grassuser/2003-March/008637.html
> 
> But regarding the application to the line-of-sight algorithm and the
> crossing points, the idea is that there is 4-way connectivity between all
> the cells. When calculating the visibility of a particular target, a line
> is created between the target cell node and the observer cell node. The
> line-of-sight function for the centre of every cell will have already been
> calculated and stored in a temporary map. This line between the target
> and observer will not cross over cell centres exactly and the LOS function
> needs to be calculated at regular intervals along the line, i.e.
> where it crosses one of the horizontal or vertical line segments that
> connect all the nodes. Interpolation will have to be done to find the LOS
> function value at these crossing points.
> 
> So hopefully the vector network functions could tell me exactly where all
> the crossing points were, and the distance to the nearest cell on either
> side, in order to do linear interpolation.
> 
> Probably even there are some GRASS modules that already do something like
> this internally, but I just thought it would be a nice application of the
> directed graph library to apply it to raster processing; another library
> could even be created that would have functions to create the network
> based on the current region and MASK settings, and to find the distances
> and crossing points and that sort of thing.
> 
> Just a few ideas for the future
> 
> Paul

Maybe the discussion of anti-aliased ray tracing in "Computer Graphics"
by Foley (et al.) would be of interest.

I agree, a library of rendering algorithms would be very useful.  

Greg