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Separating axis theorem


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#1 razaron   Members   -  Reputation: 101

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Posted 23 January 2012 - 10:04 AM

I'm having trouble implementing the separating axis theorem. If I do the calculation in the code on paper it works, however if I run the code it doesn't.

The aabb part is for (later) optimization.

Here are the relevant bits of code:
Projection Class,
public class Projection {
protected double min;
protected double max;

public Projection(double min, double max){
  min=this.min;
  max=this.max;
}

public boolean overlap(Projection p2){
  boolean temp=true;

  if(min>p2.max || max<p2.min)
   temp=false;

  return temp;
}
}

Collision Detection part of the polygon (convex) class,
		public boolean collides(Polygon poly){
		boolean check = true;
		double temp=0;
		Projection p1;
		Projection p2;
		Point2D.Double axis;
		
		for(int i=0;i<edges;i++){
			if(check){
				double min=0,max=0;
				double dx=edge[i].v2.pos.x - edge[i].v1.pos.x;
				double dy=edge[i].v2.pos.y - edge[i].v1.pos.y;
				
				double x=(dx)/Math.sqrt(Math.pow(dx, 2) + Math.pow(dy, 2));
				double y=(dy)/Math.sqrt(Math.pow(dx, 2) + Math.pow(dy, 2));
				axis = new Point2D.Double(-x, y);
				
				min = max = (axis.x*vertex[0].pos.x)+(axis.y*vertex[0].pos.y);
				
				//project this shape
				for(int j=0;j<vertices;j++){
					temp= (axis.x*vertex[j].pos.x)+(axis.y*vertex[j].pos.y);
					if(temp<min)
						min=temp;
					if(temp>max)
						max=temp;
				}
				p1 = new Projection(min,max);
				
				
				min = max = (axis.x*poly.vertex[0].pos.x)+(axis.y*poly.vertex[0].pos.y);
				//project other shape
				for(int j=0;j<poly.vertices;j++){
					temp= (axis.x*poly.vertex[j].pos.x)+(axis.y*poly.vertex[j].pos.y);
					if(temp<min)
						min=temp;
					if(temp>max)
						max=temp;
				}
				p2 = new Projection(min,max);
				
				if(!p1.overlap(p2))
					check=false;
			}
		}
		
		return check;
	}

The implementation in the main class (done every frame),
				polygon.elementAt(j).aabbCollision=false;
				polygon.elementAt(j).polyCollision=false;
				for(int l=0;l<polygon.capacity();l++){
					if(l!=j){
						if(polygon.elementAt(j).box.compareAABB(polygon.elementAt(l).box)){
							polygon.elementAt(j).aabbCollision=true;
						}
						if(polygon.elementAt(j).collides(polygon.elementAt(l))){
							if(polygon.elementAt(l).collides(polygon.elementAt(j))){
								polygon.elementAt(j).polyCollision=true;
							}
						}
					}
				}


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#2 Infernal-rk   Members   -  Reputation: 135

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Posted 26 January 2012 - 05:23 PM

public Projection(double min, double max){
min
=this.min;
max
=this.max;
}


seems backwards. shouldn't it be: this.min = min; no?

#3 Pontus Palmenäs   Members   -  Reputation: 100

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Posted 27 January 2012 - 07:58 AM

public Projection(double min, double max){
min=this.min;
max=this.max;
}


seems backwards. shouldn't it be: this.min = min; no?


That's where the debugger comes in handy :)

#4 razaron   Members   -  Reputation: 101

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Posted 27 January 2012 - 03:10 PM

public Projection(double min, double max){
min=this.min;
max=this.max;
}


seems backwards. shouldn't it be: this.min = min; no?

You Sir, are a gentleman and a scholar.




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