Collision (Pong).

Move the mouse up and down to move the paddle.

// Global variables for the ball
float ball_x;
float ball_y;
float ball_dir = 1;
float ball_size = 5;  // Radius
float dy = 0;  // Direction

// Global variables for the paddle
int paddle_width = 5;
int paddle_height = 20;

int dist_wall = 15;

void setup()
{
  size(200, 200);
  rectMode(RADIUS);
  ellipseMode(RADIUS);
  noStroke();
  smooth();
  ball_y = height/2;
  ball_x = 1;
}

void draw() 
{
  background(51);
  
  ball_x += ball_dir * 1.0;
  ball_y += dy;
  if(ball_x > width+ball_size) {
    ball_x = -width/2 - ball_size;
    ball_y = random(0, height);
    dy = 0;
  }
  
  // Constrain paddle to screen
  float paddle_y = constrain(mouseY, paddle_height, height-paddle_height);

  // Test to see if the ball is touching the paddle
  float py = width-dist_wall-paddle_width-ball_size;
  if(ball_x == py 
     && ball_y > paddle_y - paddle_height - ball_size 
     && ball_y < paddle_y + paddle_height + ball_size) {
    ball_dir *= -1;
    if(mouseY != pmouseY) {
      dy = (mouseY-pmouseY)/2.0;
      if(dy >  5) { dy =  5; }
      if(dy < -5) { dy = -5; }
    }
  } 
  
  // If ball hits paddle or back wall, reverse direction
  if(ball_x < ball_size && ball_dir == -1) {
    ball_dir *= -1;
  }
  
  // If the ball is touching top or bottom edge, reverse direction
  if(ball_y > height-ball_size) {
    dy = dy * -1;
  }
  if(ball_y < ball_size) {
    dy = dy * -1;
  }

  // Draw ball
  fill(255);
  ellipse(ball_x, ball_y, ball_size, ball_size);
  
  // Draw the paddle
  fill(153);
  rect(width-dist_wall, paddle_y, paddle_width, paddle_height);  
}