Detect collision between corners of objects

Question

I’m writing a game based on Breakout, but I can’t think of a way to detect the collision between the corner of the ball area and the paddle, so I can reverse the horizontal direction of the ball.

In my class Ball, I have a method that takes care of the movement of the ball, and when it detects collision, it just reverses the vertical direction(axis y).

import java.awt.Graphics2D;
import java.awt.Rectangle;

public class Ball {

    private int x = 0;
    private int y = 15;
    private final int DIAMETER = 30;
    private int xSpeed = 1;
    private int ySpeed = 1;

    private Board board;

    public Ball(Board board) {
        this.board = board;
        y = board.paddle.getTopY() - DIAMETER;
        x = board.getPreferredSize().width / 2 - DIAMETER / 2;
    }

    public void move() {

        if (x > board.getWidth() - DIAMETER || x < 0) {
            xSpeed = -xSpeed;
        }

        if (y < 15) {
            ySpeed = -ySpeed;
        }

        if (y > board.getHeight() - DIAMETER) {
            board.gameOver();
        }

        //entre nesta if quando uma colisão é detectada
        if (collision()) {
            y = board.paddle.getTopY() - DIAMETER;
        }

        x += xSpeed;
        y += ySpeed;
    }

    public void setSpeed(int speed) {
        this.xSpeed = speed;
        this.ySpeed = speed;
    }

    public void paint(Graphics2D g2) {
        g2.fillOval(x, y, DIAMETER, DIAMETER);
    }

    public boolean collision() {
        //detecta colisão entre a area da bola e o paddle
        return board.paddle.getBounds().intersects(this.getBounds());
    }

    public Rectangle getBounds() {
        return new Rectangle(x, y, DIAMETER, DIAMETER);
    }
}

The result so far:

Only when the area of the ball collides with the corner of the paddle, it continues the trajectory of the x axis, and to simulate a more real physics, I would like to be able to detect when the collision occurs at the ends of the objects and reverse the trajectory of the x axis of the ball. How do I detect it?

As there are 4 different large classes, not to mess up the question, I put a fully executable and complete example of the code in gist.

Answer 1

In the case of this game, what you want to do is very simple.

When your collision happens, the ball is necessarily touching the top of the staff (paddle). Thus, the axis value y doesn’t matter. You need to compare the axis value x relatively.

By relative I mean you need to check how much the value in x ball is offset from the value in x staff center. Do so (pseudo-code):

centro_bastao = bastao.x + (bastao.width / 2);
posRelativa = Math.abs(bola.x - centro_bastao) / (bastao.width / 2);

The value of posRelativa will be between 0 and 1 so that 0 means the ball is right in the center and 1 the ball is exactly at one end. If you want to know which end (that is, the side), remove the abs of the above code and check the sign (negative will be on the left and positive on the right). Then make the decision according to the value of this variable.

P.S.: Ideally you should move the ball using a vector speed (that is, a vector with the direction and magnitude of the speed of motion). Thus, it would be easier to "reflect" the vector simply rotating it according to the value of posRelativa and then reversing it (0 does not rotate, and the inverse is exactly the force to be applied back up). Escapes the scope of this site to teach a class but I suggest reading about it on the Internet. It’s an important thing know to develop games. This my other answer has something in that sense and can be useful too.