Java 2d rotation in direction mouse point

Question

So far I have a java app where I draw a circle(player) and then draw a green rectangle on top(gun barrel). I have it so when the player moves, the barrel follows with it. I

Answer 1

Using AffineTransform, sorry, only way I know how :P

public class RotatePane extends javax.swing.JPanel {

    private BufferedImage img;
    private Point mousePoint;

    /**
     * Creates new form RotatePane
     */
    public RotatePane() {

        try {
            img = ImageIO.read(getClass().getResource("/MT02.png"));
        } catch (IOException ex) {
            ex.printStackTrace();
        }

        addMouseMotionListener(new MouseAdapter() {

            @Override
            public void mouseMoved(MouseEvent e) {

                mousePoint = e.getPoint();

                repaint();

            }

        });

    }

    @Override
    public Dimension getPreferredSize() {

        return new Dimension(img.getWidth(), img.getHeight());

    }

    @Override
    protected void paintComponent(Graphics g) {

        super.paintComponent(g);

        Graphics2D g2d = (Graphics2D) g.create();

        double rotation = 0f;

        int width = getWidth() - 1;
        int height = getHeight() - 1;

        if (mousePoint != null) {

            int x = width / 2;
            int y = height / 2;

            int deltaX = mousePoint.x - x;
            int deltaY = mousePoint.y - y;

            rotation = -Math.atan2(deltaX, deltaY);

            rotation = Math.toDegrees(rotation) + 180;

        }

        int x  = (width - img.getWidth()) / 2;
        int y  = (height - img.getHeight()) / 2;

        g2d.rotate(Math.toRadians(rotation), width / 2, height / 2);
        g2d.drawImage(img, x, y, this);

        x = width / 2;
        y = height / 2;
        g2d.setStroke(new BasicStroke(3));
        g2d.setColor(Color.RED);
        g2d.drawLine(x, y, x, y - height / 4);
        g2d.dispose();

    }
}

Will produce this effect

The red line (point out from the center) will want to follow the cursor.

Answer 2

Using the Graphics2D rotation method is indeed the easiest way. Here's a simple implementation:

int centerX = width / 2;
int centerY = height / 2;
double angle = Math.atan2(centerY - mouseY, centerX - mouseX) - Math.PI / 2;

((Graphics2D)g).rotate(angle, centerX, centerY);

g.fillRect(...); // draw your rectangle

If you want to remove the rotation when you're done so you can continue drawing normally, use:

Graphics2D g2d = (Graphics2D)g;
AffineTransform transform = g2d.getTransform();

g2d.rotate(angle, centerX, centerY);

g2d.fillRect(...); // draw your rectangle

g2d.setTransform(transform);

It's a good idea to just use Graphics2D anyway for anti-aliasing, etc.