DESCRIPTION computes the visibility graph of a vector map containing lines, areas (boundaries) and points. The visibility graph is the graph where the nodes are the end point of the lines, boundaries or simply the points. There is an edge between two nodes if they are 'visible'. Two nodes are visibible if there is no segments in between them, i.e. the edge doesn't intersect any line or boundary in the vector map. This is useful to compute the sortest path in a vector map from any two points. To do this, first you need to compute the visibility graph and from it compute the shortest path using or d.path. IMPORTANT : the algorithm doesn't work well with intersecting lines (that includes overlapping)

If you compute a shortest path after computing the visibility graph you will notice that this path might go through a vertix of a line. If this is not wanted you might to run the map through v.buffer first whith a small value. Example:

v.buffer input=map output=bufferedmap buffer=1 type=point,line,area,boundary

The first argument is the input map. It supports lines, boudaries (so areas) and points. For the algorithm to work lines and boundaries must not be intersecting (that includes overlapping). The result map containing the visibility graph is given in the output map. If you need to add additional points to compute a shortest path between them afterwards you can use the coordinate parameter.

where 25556200,6686400 are the coordinate of the first point and 25556400,6686600 are the coordinates of the second point. Of course you can give as many points as you want. They will be added to the visibility graph and edges from them will be computed. You can always add those points after computing the visibility graph. Simply use the vis parameter. The input will be the original vector map, the vis will be the computed visibility graph and the output the new visibility graph which will be the vis + the new points given with coordinate (edges will be computed as well). input=map vis=vis_map output=new_vis_map coordinate=25556200,6686400,25556400,6686600


A simple example showing how to use the module input=lines output=graph
d.vect graph
d.vect lines col=red


An example on how to use v.buffer with the module
v.buffer input=lines output=buffered_lines buffer=1 input=buffered_lines output=graph
d.vect graph
d.vect lines col=red


An example on how to use the coordinate parameter. This will compute the visibility graph of the vector map lines with the point 2555678,6686343 input=lines output=graph coordinate=2555678,6686343
d.vect graph d.vect lines col=red


An example on how to use the coordinate parameter with the vis parameter. Here the vector map graph is computed then a new visibility graph is computed from it with the point 2555678,6686343 extra input=lines output=graph
d.vect graph
d.vect lines col=red input=lines vis=graph output=new_graph coordinate=2555678,6686343
d.vect new_graph
d.vect lines col=red


An example for connections of points (Spearfish): input=archsites output=graph
g.region vect=archsites
d.mon x0
d.vect graph
d.vect archsites col=red


In some cases when 3 points or nodes are collinear, some wrong edges are added. This happens only really rarly and shouldn't be a big problem. When two points have the exact same x coordinate and are visible, some wrong edges are added.




Maximilian Maldacker
Mentor: Wolf Bergenheim

Last changed: $Date$