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  Help implementing path finding.  (Read 1247 times)
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Offline syrinth

Senior Newbie





« Posted 2011-06-29 00:36:30 »

Hello all, I've looked at the sample pages listed in the pathfinding for beginners thread, and yet I'm still having problems implementing my pathfinding. The following code returns a path, and I'm sure the total value is the shortest path.

Unfortunately, what it does is it uses all the horizontal distance first, and then travels diagonally. This creates a very unnatural form of movement. I was hoping someone could help me?

I am using the Chebyshev distance equation listed at http://theory.stanford.edu/~amitp/GameProgramming/Heuristics.html#S7 to calculate the heuristic, I've checked to ensure that the G values are all being added properly and both forms of movement cost the same... What am I doing wrong?

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public void pathFind(Node target){
      this.target = target;
      open.add(node);
      target.setG(0);
      Node current = node;
      Node min = null;

      while(!open.isEmpty()){
         for(int i=0; i<current.getNeighbours().size(); i++){
            Node temp = current.getNeighbours().get(i);
            if(!open.contains(temp) && !closed.contains(temp)){
               open.add(temp);
               temp.setParent(current);
               temp.setG(current.getG() + current.getMovement());
               calculateF(temp);
            }
            else if(open.contains(temp) && !closed.contains(temp)){
               if(temp.getG() < current.getG()+current.getMovement()){
                  temp.setParent(current);
                  temp.setG(current.getG() + current.getMovement());
                  calculateF(temp);
               }
            }
         }
         open.remove(current);
         closed.add(current);
         min = open.get(0);
         for(int i=0; i<open.size(); i++){
            if(open.get(i).getF() < min.getF())
               min = open.get(i);
         }
         current = min;
         
         if(closed.contains(target)){
            deconstruct();
            break;
         }
      }
      System.out.println("FINISHED");
   }
   
   
   public void calculateF(Node aNode){
      int move = aNode.getStraight();
     
      aNode.setH(move * Math.max(Math.abs(aNode.getCentreX()-target.getCentreX()), Math.abs(aNode.getCentreY()-target.getCentreY())));
      aNode.setF(aNode.getG() + aNode.getH());
   }
Offline Gudradain
« Reply #1 - Posted 2011-06-29 04:04:49 »

can't understand question without drawing
Offline Jono
« Reply #2 - Posted 2011-06-29 06:56:11 »

Because the horizontal and diagonal moves both reduce the Chebyshev distance by the same amount (1), it just comes down to the ordering of the neighbours. I'd guess that your getNeighbours() method returns the horizontal move first and in case of ties your open list returns the earlier node.

One work-around is to add a penalty value in addition to f and g. Say, for each consecutive horizontal move add a 0.001 penalty. Just make sure that the penalty values for an entire path never add up to more than the cost of one move.
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