与另一个球碰撞后更改球的方向

Question

I need to change Ball Direction after collision with another ball or with a edge of the window. I managed to do something like that:

y += yMove;
x += xMove;

//if the ball moves to the right edge of the window, turn around. 
if(x > width - size)
{   
    x = width - size;
    xMove *= -1;
    if (xMove > 0) {
       xSpeed = xMove + (Math.random() * (1));
    }
    if (xMove <= 0) {
       xSpeed = xMove - (Math.random() * (1));
    }
    if (yMove > 0) {
       ySpeed = yMove + (Math.random() * (1));
    }
    if (yMove <= 0) {
       ySpeed = yMove - (Math.random() * (1));
    }
}

And same for another edges.

I'm trying to use same method for changing direction of balls after they collide with each other, but it's just not working / it's weird. Can anyone help me?

Answer 1

When balls collide, make vector connecting ball centers ( N ) and normalize it ( uN )

Components of velocities parallel to N (normal) are exchanged (due to impulse law)

Components of velocities perpendicular to N (tangential) remain the same

To get components in given local system, use scalar and cross product:

 V1t = dot(V1, uN)  
 V2t = dot(V2, uN)  

 V1n = cross(V1, uN)
 V2n = cross(V2, uN)

after collision

V1t' = V2t
V2t' = V1t

V1n' = V1n
V2n' = V2n

To return into global system (I did not checked signs thoroughly):

V1x = V1t` * uN.X + V2n` * uN.Y 
V1y = -V1t` * uN.Y + V2n` * uN.X 

(This is essentially dot and cross products again, but I expanded expressions to show different bases)

Note that this approach is like to ball-edge collision, when N is normal to the border and you reverse only one component of velocity vector.

Answer 2

For your BouncingBall class, you can have a method like flipDirection() , but you can have a finer directional control by splitting it into 2 methods which filps the direction of the ball vertically and horizontally.

class BouncingBall{
    public void horizontalFlip(){
        moveX *= -1;
    }

    public void verticalFlip(){
        moveY *= -1;
    }

    //To have move control over each direction, you can have a method for each direction.
    public void moveNorth(){
        moveY = Math.abs(moveY) * -1;
    }

    public void moveSouth(){
        moveY = Math.abs(moveY);
    }

    public void moveWest(){
        moveX = Math.abs(moveX) * -1;
    }

    public void mpveEast(){
        moveX = Math.abs(moveX);
    }
}

Depending on how you want the ball to bounce off. In a simple bounce off, the balls can bounce towards 4 possible directions:

• North West
• North East
• South West
• South East

The direction of the ball to bounce off will be relative to the position of the ball it is colliding with and you do not want 2 collided balls which move in the same direction to switch direction just because they collided. Hence you need to check the positions of the 2 balls, and flipDirection() becomes insufficinet to achieve that.

if(b1.intersects(b2)){
    if(b1.getX() < b2.getX()){ // b1 above b2
        b1.moveNorth();
        b2.moveSouth();
    }
    else{
        b1.moveSouth();
        b2.moveNorth();
    }
   if(b1.getY() < b2.getY()){ // b1 at left side of b2
        b1.moveWest();
        b2.moveEast();
    }
    else{
        b1.moveEast();
        b2.moveWest();
    }
}

For example, to change direction when hitting the walls on the left and right:

if(ball.getPosX() <= 0 || ball.getPosX() >= PNL_WIDTH-Ball.SIZE)
    ball.horizontalReverse();

Same logic for verticalReverse.