AffineTransform: scaling a Shape from its center.

Question

Hi all, 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

Assuming scaling fixes the location of the top lefthand corner of the rectangle (which I think is right but it's been a long time since I've done graphics in Java), you need to translate the rectangle in the direction opposite to the scaling.

tr2.translate(
    r.getWidth()*(1-zoom)/2,
    r.getHeight()*(1-zoom)/2
);
tr2.scale(zoom,zoom);
g.setTransform(tr2);

So you move the rectangle left and up half of the change in width and height.

Answer 2

This involves a process called conjugating a transform.

If S is the scaling you want to do, and T is the transformation that takes point (0,0) to the point that is to be the center of your scaling, then the transform that does the job is

T(S(inverse(T)))

Answer 3

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.

Answer 4

(Later) Here is a solution working without any prior knowledge of the Dimension of the panel.

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Shape;
import java.awt.geom.AffineTransform;
import java.awt.geom.Ellipse2D;

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

public class Test extends JPanel
    {
    private static final long serialVersionUID = 1L;
    private Test()
        {
        super(null);
        setOpaque(true);
        setBackground(Color.WHITE);
        setPreferredSize(new Dimension(600,600));
        }

    @Override
    protected void paintComponent(Graphics g1) {
        super.paintComponent(g1);
        Shape r= new Ellipse2D.Double(5,380,400,200);
        double cx= r.getBounds2D().getCenterX();
        double cy= r.getBounds2D().getCenterY();
        Graphics2D g=(Graphics2D)g1;
        g.setColor(Color.BLACK);
        AffineTransform old= g.getTransform();
        g.drawLine((int)cx-10, (int)cy, (int)cx+10, (int)cy);
        g.drawLine((int)cx, (int)cy-10, (int)cx, (int)cy+10);
        for(double zoom=1; zoom>=0.1; zoom-=0.1)
                {


                AffineTransform tr2 =AffineTransform.getTranslateInstance(-cx, -cy);
                AffineTransform  tr= AffineTransform.getScaleInstance(zoom,zoom);
                tr.concatenate(tr2); tr2=tr;
                tr =AffineTransform.getTranslateInstance(cx, cy);
                tr.concatenate(tr2); tr2=tr;

                tr= new AffineTransform(old);
                tr.concatenate(tr2);  tr2=tr;

                g.setTransform(tr2);



                g.draw(r);
                g.setTransform(old);
                }
        }


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