Java 2D Rotation and Global Coordinates?

Question

I am making a simple game where there's an object (a square with a rectangular barrel) that looks towards the mouse. The rotation part works pretty well, but I want the object to move when a button is pressed along the y axis, but directly upwards, not relative to its rotation. Now, when a button is pressed, the object moves forward in the direction it is facing. How is it possible to move it directly up the y axis, regardless of what direction the object is in? Here's the code:

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Image;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionListener;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class RotationTestMain extends JFrame implements MouseMotionListener, KeyListener{

    public static final long serialVersionUID = 666L;

    public static final int SIZE = 50;

    double xComponent;
    double yComponent;

    Image dbImage;
    Graphics dbg;

    int x;
    int y;
    int centerX;
    int centerY;

    JPanel area;

    public RotationTestMain(){
        area = new JPanel();
        area.addMouseMotionListener(this);
        addKeyListener(this);

        x = 487;
        y = 359;

        centerX = x + SIZE/2;
        centerY = y + SIZE/2;

        add(area);

        setSize(1024,768);
        setLocationRelativeTo(null);
        setVisible(true);
        setDefaultCloseOperation(EXIT_ON_CLOSE);
    }

    @Override
    public void paint(Graphics g){
        dbImage = createImage(getWidth(), getHeight());
        dbg = dbImage.getGraphics();
        Graphics2D g2d = (Graphics2D) dbg;
        draw(g2d);
        g.drawImage(dbImage, 0, 0, this);
    }

    public void draw(Graphics2D g2d){
        g2d.setColor(Color.BLACK);
        g2d.rotate(calculateRotation(), centerX, centerY);
        g2d.fillRect(x, y, SIZE, SIZE);
        g2d.fillRect(centerX-5,y-20,10,25);
        repaint();
    }

    public double calculateRotation(){
        double rot;
        if(yComponent >= 0){
            rot = 3 * Math.PI - Math.atan(xComponent/yComponent);
        }else {
            rot = 2 * Math.PI - Math.atan(xComponent/yComponent);
        }
        return rot;
    }

    public static void main(String[] args) {
        new RotationTestMain();
    }

    @Override
    public void mouseDragged(MouseEvent m) {
        centerX = x + SIZE/2;
        centerY = y + SIZE/2;
        xComponent = m.getX() - centerX;
        yComponent = m.getY() - centerY;
    }

    @Override
    public void mouseMoved(MouseEvent m) {
        centerX = x + SIZE/2;
        centerY = y + SIZE/2;
        xComponent = m.getX() - centerX;
        yComponent = m.getY() - centerY;
    }

    @Override
    public void keyPressed(KeyEvent arg0) {
        //TODO FIX THE MOVING PROBLEM
        this.y -= 3;
    }

    @Override
    public void keyReleased(KeyEvent arg0) {
    }

    @Override
    public void keyTyped(KeyEvent arg0) {
    }
}

Thanks so much for the help!!

score 1 · Answer 1 · answered Apr 06 '13 at 20:47

1

I think the problem is in this line g2d.rotate(calculateRotation(), centerX, centerY);

you are not rotating the object but the whole g2d. Therefore the x,y axis are also rotated, as they are part of the g2d.

So when you do this.y -= 3; You are indeed moving along the y axis, but the y axis has been rotated.

I recommend to find a way to rotate the object instead of the whole g2d.

answered Apr 06 '13 at 20:47

Xiaomeng Ye

11
1

1

Like using an [AffineTransofmration](http://docs.oracle.com/javase/tutorial/2d/advanced/transforming.html) for instance? – MadProgrammer Apr 06 '13 at 21:20

Java 2D Rotation and Global Coordinates?

1 Answers1