[GRASS-dev] Re: [GRASS GIS] #399: Added "backlink" functionality to r.walk, r.cost & r.drain

[GRASS GIS]

#399: Added "backlink" functionality to r.walk, r.cost & r.drain
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  Reporter:  cnielsen     |       Owner:  grass-dev at lists.osgeo.org
      Type:  enhancement  |      Status:  new                      
  Priority:  major        |   Milestone:  6.4.0                    
 Component:  Raster       |     Version:  unspecified              
Resolution:               |    Keywords:                           
  Platform:  All          |         Cpu:  All                      
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Comment (by cnielsen):

 Thanks for the testing and the how-to Dylan. I should emphasize that the
 changes I made only record the directions taken by the original cost
 accumulation algorithm. These means that I have not (well only a slight
 change to fix a mathematical error) changed how r.cost or r.walk
 calculates paths, only how those same paths are recorded and then turned
 into paths using r.drain.

 The clearest way I can put it is to think of the knight's move. With the
 old modules, r.drain had no way to tell if the cost surface was created
 with or without knight's move. Instead it just followed a water drop
 "drain" through the map only looking at 8 neighbours. Now it uses the
 direction surface to remember what the cost accumulation algorithm did and
 recreates that path.

 Therefore the cardinal directions are necessary (and precisely sufficient)
 since they record either eight directions, or for knight's move sixteen
 directions, based on the original moves... does that make sense?

 Also patches for grass_trunk aka grass70 are ready for testing (they work
 for me).

 -Colin

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Ticket URL: <http://trac.osgeo.org/grass/ticket/399#comment:21>
GRASS GIS <http://grass.osgeo.org>