Help really needed in rotating a bufferedImage

Question

Its so silly, but i've been trying to get this for more than a week now and i cant get it to work.
I have a gravity simulator and i want to rotate an image of a small ship. Just to mention it.. i have one thread that does the physics. I want to do two things: move and rotate (i always get the move method working, but not the rotate one)
I have a class Nave (ship in spanish) and Graficador (the jpanel that paints everything). I want to say that ive tried really ALOT of things (yes, ive read google, the forums... i just like to ask for help when i really need it).
So: here's what i've tried:
-Ive tried to have a general affineTransform in Nave, then, when i call for example rotate, i just modify the AffineTransform and then use it to paint in the Jpanel. Something like this:
In Nave
public void rotarDer(){
        this.at.rotate(Math.PI/2,this.info.getPosX()+5,this.info.getPosY()+5);
    }And in the Jpanel:
if(this.nave.at!=null){
            g2.drawImage(this.nave.imagen,this.nave.at,null);
        }Ive also tried to use drawRenderedImage with the AffineTransform, but the result are the same: the coordinate system changes (i think because of the AffineTransform) so the gravity takes the ship to the right instead of making it fall down. Ive tried to "store" the affineTransform before any change, and then reapply it after drawing the image, but doesnt seem to work either.
Also, with this approach, ive tried to create a "buffer" in Nave, then paint the image in the buffer with the affineTransform, and finally paint this buffer in the jpanel, but it gets messed up anyway.
Ive tried many approaches, but i think i will just mention this one, and see if with your help i get this one to work... i just need to make the coordinates "right" after drawing the image in the jpanel so the gravity works well...

Answer 1

It wasn't clear how you were trying to use AffineTransform other than rotation in a
gravitational field. To avoid altering the coordinate system orientation try making a
transform for each rendering of an image by moving the image to the location where you
want to show it and then rotatng the image about the specific point in/on the image that
will give the desired final location/orientation. This sounds vague because there are
multiple ways to see/do this. Here's an example of one way:
import java.awt.*;
import java.awt.event.*;
import java.awt.geom.AffineTransform;
import java.awt.image.BufferedImage;
import javax.swing.*;
public class RotationExample extends JPanel {
    BufferedImage image;
    Point[] points;
    RotationExample() {
        makeImage();
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D)g;
        g2.setRenderingHint(RenderingHints.KEY_INTERPOLATION,
                            RenderingHints.VALUE_INTERPOLATION_BICUBIC);
        if(points == null) initPoints();
        markPointGrid(g2);
        // Center image over points[0] and rotate 45 degrees.
        int cx = image.getWidth()/2;
        int cy = image.getHeight()/2;
        double theta = Math.PI/4;
        double x = points[0].x - cx;
        double y = points[0].y - cy;
        AffineTransform at = AffineTransform.getTranslateInstance(x, y);
        at.rotate(theta, cx, cy);
        g2.drawRenderedImage(image, at);
        // Center image over points[1] and rotate 135 degrees.
        x = points[1].x - cx;
        y = points[1].y - cy;
        theta = Math.PI*3/4;
        at.setToTranslation(x, y);
        at.rotate(theta, cx, cy);
        g2.drawRenderedImage(image, at);
        // Move tail over points[2] and rotate 180 degrees.
        x = points[2].x;
        y = points[2].y - cy;
        theta = Math.PI;
        at.setToTranslation(x, y);
        at.rotate(theta, 0, cy);
        g2.drawRenderedImage(image, at);
        // Mark points.
        g2.setPaint(Color.cyan);
        for(int j = 0; j < points.length; j++)
            g2.fillOval(points[j].x-2, points[j].y-2, 4, 4);
    private void markPointGrid(Graphics2D g2) {
        int w = getWidth(), h = getHeight();
        g2.setPaint(new Color(220, 230, 240));
        for(int j = 0; j < points.length; j++) {
            g2.drawLine(points[j].x, 0, points[j].x, h);
            g2.drawLine(0, points[j].y, w, points[j].y);
    private void makeImage() {
        int w = 75, h = 45, type = BufferedImage.TYPE_INT_RGB;
        image = new BufferedImage(w, h, type);
        Graphics2D g2 = image.createGraphics();
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        g2.setStroke(new BasicStroke(4f));
        g2.setBackground(getBackground());
        g2.clearRect(0,0,w,h);
        g2.setPaint(Color.red);
        g2.drawLine(w, h/2, w-30, 0);
        g2.drawLine(w, h/2, w-30, h);
        g2.setPaint(Color.blue);
        g2.drawLine(0, h/2, w, h/2);
        g2.dispose();
    private void initPoints() {
        int w = getWidth();
        int h = getHeight();
        points = new Point[3];
        for(int j = 0; j < points.length; j++) {
            points[j] = new Point((j+1)*w/4, (j+1)*h/4);
    public static void main(String[] args) {
        JFrame f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().add(new RotationExample());
        f.setSize(400,400);
        f.setLocation(200,200);
        f.setVisible(true);
}