Finding AffineTransform origin on screen

Question

Let's say I have an AffineTransform (transform) and call a bunch of it's methods. Lets say I rotate and translate it. Then I transform a graphics object (g2d) with it:

g2d.transform(transform);

I want to find the coordinate on my screen where my new (0, 0) is. So if I drew a rectangle at those coordinates with an untransformed g2d and one with my transformed g2d they would overlap. So how can I get this point, do I have to do some math, does AffineTransform or Graphics2D have a built in way (I couldn't find one)?

Answer 1

You could...

Create a copy of the original Graphics context (Graphics#create) (which is always a good idea) then apply the transform to the copy (this will leave the original unaffected, just don't forget to dispose of the copy when you're done)

import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.FontMetrics;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.geom.AffineTransform;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.imageio.ImageIO;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class Test {

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

    public Test() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                    ex.printStackTrace();
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public class TestPane extends JPanel {

        private BufferedImage img;

        public TestPane() {
            try {
                img = ImageIO.read(your source image here...);
            } catch (IOException ex) {
                ex.printStackTrace();
            }
        }

        @Override
        public Dimension getPreferredSize() {
            return img == null ? new Dimension(200, 200) : new Dimension(img.getWidth(), img.getHeight());
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g.create();
            int x = (getWidth() - img.getWidth()) / 2;
            int y = (getHeight() - img.getHeight()) / 2;
            AffineTransform at = AffineTransform.getTranslateInstance(x, y);
            at.rotate(Math.toRadians(180), img.getWidth() / 2, img.getHeight() / 2);
            g2d.setTransform(at);
            g2d.drawImage(img, 0, 0, this);
            g2d.dispose();

            // I don't like touching the original ;)
            g2d = (Graphics2D) g.create();
            FontMetrics fm = g2d.getFontMetrics();
            g2d.drawString("Top Left", 0, 0 + fm.getAscent());

            String txt = "Bottom right";
            g2d.drawString(txt, getWidth() - fm.stringWidth(txt) - 1, ((getHeight() - fm.getHeight()) - 1) + fm.getAscent());
            g2d.dispose();
        }

    }

}

You could...

Reverse the transformation, always tricky if it's a complex transformation...

You could...

Get a reference to the original transformation first, apply the new transform, paint what ever you needed painted and then reapply the original transformation

You could...

Use AffineTransform#setToIdentity to "reset" the transform back to it's original state...

    AffineTransform at = g2d.getTransform();
    at.translate(x, y);
    at.rotate(Math.toRadians(180), img.getWidth() / 2, img.getHeight() / 2);
    // Some drawing...

    at.setToIdentity();
    g2d.setTransform(at);
    // Back to basics