if(ball.x + ball.radius > right)
     {
      ball.x = right - ball.radius;
      vx *= -1;
     }
     else if(ball.x - ball.radius < left)
     {
      ball.x = left + ball.radius;
      vx *= -1;
     }
     if(ball.y + ball.radius > bottom)
     {
      ball.y = bottom - ball.radius;
      vy *= -1;
     }
     else if(ball.y - ball.radius < top)
     {
      ball.y = top + ball.radius;
      vy *= -1;
     }
    }
   }
  }