AffineTransform: scaling a Shape from its center

Question

I'm trying to scale a rectangle from its center using AffineTransform. I'm sure the solution is obvious but I cannot make it work ! Here is what I've tested so far...

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import java.awt.geom.AffineTransform;

import javax.swing.JOptionPane;
import javax.swing.JPanel;

public class Test extends JPanel {
    Test()
        {
        super(null);
        setOpaque(true);
        setBackground(Color.WHITE);
        setPreferredSize(new Dimension(200,200));
        }
    @Override
    protected void paintComponent(Graphics g1) {
        super.paintComponent(g1);
        Rectangle r= new Rectangle(5,5,getWidth()-10,getHeight()-10);
        double cx= r.getCenterX();
        double cy= r.getCenterY();
        Graphics2D g=(Graphics2D)g1;
        g.setColor(Color.BLACK);
        AffineTransform old= g.getTransform();
        for(double zoom=0.9; zoom>=0.5; zoom-=0.1)
            {
            AffineTransform tr2= new AffineTransform(old);
            tr2.translate(-cx, -cy);
            tr2.scale(zoom, zoom);
            tr2.translate(cx/zoom,cy/zoom);
            g.setTransform(tr2);
            g.draw(r);
            g.setTransform(old);
            }
        }


    public static void main(String[] args) {
        JOptionPane.showMessageDialog(null, new Test());
        }
    }

But it doesn't work.... Any suggestion ?

Answer 1

I see what you mean when you're dealing with rectangles. The reason is because the initial calculation for the translation didn't take into account the size of the container object.

Use this instead:

tr2.translate(
    (this.getWidth()/2) - (r.getWidth()*(zoom))/2,
    (this.getHeight()/2) - (r.getHeight()*(zoom))/2
);
tr2.scale(zoom,zoom);
g.setTransform(tr2);

What this is doing is translating the rectangle to the center of the panel before scaling it. In my tests it works just fine.