Question

I am trying to draw a rectangle in JPanel that would translate and then rotate itself to mimic the movement of a car. I have been able to make the rectangle translate and rotate, however it rotates around the origin of (0,0). I’m very pleased that I was able to have the rectangle move and rotate as I am very new to Java GUI, but I can not seem to get how to have the rectangle rotate around itself, because I experimented more with it, and when I initialized the rectangle and rotate it 45 degrees it’s position was changed, which I would assume is the transform matrix that is appended from the rotate method.

I checked through the site on how would I solve this, however I only found how to rotate a rectangle and not on how to rotate and move like the movement of a simulated car. I would presume it is concerning about its transform matrix, but I’m only speculating. So my question is how would I be able to have the rectangle be able to rotate and move around itself and not against a point in JPanel.

Here’s the code that I have come up so far:

public class Draw extends JPanel implements ActionListener {


private int x = 100;
private int y = 100;
private double theta = Math.PI;

Rectangle rec = new Rectangle(x,y,25,25);

Timer timer = new Timer(25,this);

Draw(){
    setBackground(Color.black);
    timer.start();
}

public void paintComponent(Graphics g){
    super.paintComponent(g);
    Graphics2D g2d = (Graphics2D)g;     
    g2d.setColor(Color.white);
    rec.x = 100;
    rec.y = 100;
    g2d.rotate(theta);
    g2d.draw(rec);
    g2d.fill(rec);

}

public void actionPerformed(ActionEvent e) {
    x = (int) (x + (Math.cos(theta))*1);
    y = (int) (y + (Math.sin(theta))*1);
    theta = theta - (5*Math.PI/180);
    repaint();
}

Answer 1

One of two approaches are commonly used:

• Rotate the graphics context around the center (x, y) of the Shape, as shown here.

  rotate(double theta, double x, double y)
  

• Translate to the origin, rotate and translate back, as shown here.

  g2d.translate(this.getWidth() / 2, this.getHeight() / 2);
  g2d.rotate(theta);
  g2d.translate(-image.getWidth(null) / 2, -image.getHeight(null) / 2);
  

Note the apparent reverse order of concatenation in the second example.

Addendum: Looking more closely at your example, the following change rotates the Rectangle around the panel’s center.

g2d.rotate(theta, getWidth() / 2, getHeight() / 2);

Also, use the @Override annotation, and give your panel a reasonable preferred size:

@Override
public Dimension getPreferredSize() {
    return new Dimension(640, 480);
}