Collision between two shape

I was working on an animation on processing.
But, I have a question about the code below:

So, you can see on the output, the ball is going everywhere on the window. I want to make a barrier for the ball. The ball can pass from the middle but if it hit the line it goes somewhere with the speed of (int)random(1, 3).

How to make that?
Here is a picture of what I would like to achieve:

Answer

For your toy model, the easiest way to handle collisions is considering a collision zone. So,

  1. Establish the collision zone limits x1, x2, y1, y2. When the center of the ball enters this zone, collision rules apply.
  2. Consider elastic collisions: your ball only collides with an horizontal line, so only the vertical component of the velocity can be affected, and it never looses speed.
  3. Consider to test your toy model with a good bunch of initial conditions, so you can discover any artifacts due to the smallness of your model, the values you are using, and so on. (I have modified the initial position of the ball for a random initial position, run the model several times to see how it behaves under different initial conditions).
int dirX = (int)random(1, 3);
int dirY = (int)random(1, 3);
//int x = 20, y = 20;
int middle = 20;

int x = (int)random(20, 180);
int y = (int)random(20, 80);
int x1, x2, y1, y2;

void setup() {
  size(200,200);
  // collision zone limits
  x1 = (width/2 - middle) + 20/2 - 1; // 89
  x2 = (width/2 + middle) - 20/2 + 1; // 111
  y1 = height/2 - 20/2 + 1; // 91
  y2 = height/2 + 20/2 - 1; // 109
}

void draw() {
  background(255);
  ellipse(x, y, 20, 20);
  line(0, height/ 2, width/2 - middle, height/2); // (0, 100, 80, 100)
  line(width/2 + middle, height/2, width, height/2); // (120, 100, 200, 100)
  
  x += dirX;
  y += dirY;

  if (x > width || x < 0)
    dirX *= -1;
  if (y > height || y < 0)
    dirY *= -1;

  if ((x < x1 && y > y1 && y < height/2 && dirY > 0) || // upper left and going down
      (x < x1 && y > height/2 && y < y2 && dirY < 0) || // lower left and going up
      (x > x2 && y > y1 && y < height/2 && dirY > 0) || // upper right and going down
      (x > x2 && y > height/2 && y < y2 && dirY < 0))   // lower right and going up
    dirY *= -1;
}

For a more complex model, you could consider more sophisticated situations as, for example, the backwards collision due to the ends of the lines, the collision of the contour of the ball with the lines, etc.

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