Moving Platforms in Box2d in a Specified Path

Hi,

In one of the previous post we moved a platform from a point A to point B and then back to point A and so on. In this post we would expand on it and move it in specific path which has more than 2 points which we find in many platformers like mario etc.

It is similar to way we moved a platform in last post just in this case we would have more than 2 points in the path.

Path:

We would define a basic class which will hold the information of the path which the platform will travel over a period of time. There are many ways to do it. This is just one of them.

public class Path{
   ArrayList<Vector2>  positions;
   ArrayList<Float> times;
   Vector2 velocity;
   int currentPointIndex;
   int nextPointIndex;
   int direction=1;
   static final float CHECK_RADIUS=1f;
   public Path(int count){
      positions=new ArrayList<Vector2>();
      times=new ArrayList<Float>();
      velocity=new Vector2();

   }
   public void AddPoint(Vector2 pos,float time){
      positions.add(pos);
      times.add(time);
    }

   public void Reset(){
      currentPointIndex=0;
      nextPointIndex=GetNextPoint();
      SetNextPointVelocity();
   }

   public Vector2 GetCurrentPoint(){
        return positions.get(currentPointIndex);
   }
   public boolean UpdatePath(Vector2 bodyPosition){
      return ReachedNextPoint(bodyPosition);
   } 
       boolean ReachedNextPoint(Vector2 bodyPosition){
               Vector2 nextPointPosition=positions.get(nextPointIndex);
               float d=nextPointPosition.dst2(bodyPosition);
               if(d<CHECK_RADIUS){
                      currentPointIndex=nextPointIndex;
                      nextPointIndex=GetNextPoint();
                      SetNextPointVelocity();
                      return true;
               }
               return false;
        } 

        int GetNextPoint(){
               int nextPoint=currentPointIndex+direction;
               if(nextPoint==positions.size()){
                      nextPoint=0;
               }else if(nextPoint==-1){
                      nextPoint=positions.size()-1;
               }
               return nextPoint;
        }
       void SetNextPointVelocity(){
               Vector2 nextPosition=positions.get(nextPointIndex);
               Vector2 currentPosition=positions.get(currentPointIndex);
               float dx=nextPosition.x-currentPosition.x;
               float dy=nextPosition.y-currentPosition.y;
               float time=times.get(nextPointIndex);
               velocity.set(dx/time,dy/time);
        }

       Vector2 GetVelocity(){
               return velocity;
        }
}

The above can be used as a direction for making a kinematic body move through a specified path. We have to check in the update function of the object class (which is moving in the specified) that if  UpdatePath(bodyPosition) returns true then just set a new velocity to the body by calling GetVelocity().

MovingObject

 ...
 Path path;
 public void Update(float dt){
       .....
       if(path.UpdatePath(bodyPosition)){
                SetBodyVelocity(path.GetVelocity());
       }
 }

We are passing position in update function of path so that it can be used with all types of moving objects and not specific to box objects.

Source Code:

We have attached a sample source code for the above – here.

For setting the example in eclipse please look for instructions in this post.

Thank you for the patience. If you have queries/suggestions please post them in comments section.