Re: Zoom relative to mouse position

From:

Amir Kouchekinia <amir_nospam@pyrus_nospam.us>

Newsgroups:

comp.lang.java.programmer

Date:

Wed, 21 May 2008 22:17:40 -0700

Message-ID:

<Vl7Zj.104$ZE5.83@nlpi061.nbdc.sbc.com>

I've finally figured this out. Here's my code if you are interested:

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.MouseWheelEvent;
import java.awt.event.MouseWheelListener;
import java.awt.geom.AffineTransform;
import java.awt.geom.NoninvertibleTransformException;
import java.awt.geom.Point2D;
import java.awt.geom.Rectangle2D;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;

@SuppressWarnings("serial")
public class ZoomDemo extends JPanel implements Runnable {

   AffineTransform tx = new AffineTransform();

   Rectangle2D.Double rect1 = new Rectangle2D.Double(100, 100, 30, 60);
   Rectangle2D.Double rect2 = new Rectangle2D.Double(150, 250, 60, 40);

   public ZoomDemo() {
     this.addMouseWheelListener(new ZoomHandler());
   }

   @Override
   public void paint(Graphics g) {
     super.paint(g);
     Graphics2D g2 = (Graphics2D) g;
     g2.setColor(Color.RED);
     g2.draw(tx.createTransformedShape(rect1));
     g2.setColor(Color.BLUE);
     g2.draw(tx.createTransformedShape(rect2));
   }

   private class ZoomHandler implements MouseWheelListener {

     double scale = 1.0;

     public void mouseWheelMoved(MouseWheelEvent e) {
       if (e.getScrollType() == MouseWheelEvent.WHEEL_UNIT_SCROLL) {

         Point2D p1 = e.getPoint();
         Point2D p2 = null;
         try {
           p2 = tx.inverseTransform(p1, null);
         } catch (NoninvertibleTransformException ex) {
           // should not get here
           ex.printStackTrace();
           return;
         }

         scale -= (0.1 * e.getWheelRotation());
         scale = Math.max(0.1, scale);

         tx.setToIdentity();
         tx.translate(p1.getX(), p1.getY());
         tx.scale(scale, scale);
         tx.translate(-p2.getX(), -p2.getY());

         ZoomDemo.this.revalidate();
         ZoomDemo.this.repaint();
       }
     }
   }

   public void run() {
     JFrame f = new JFrame("Zoom Demo");
     f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
     f.getContentPane().add(this);
     f.setSize(600, 600);
     f.setLocationRelativeTo(null);
     f.setVisible(true);
   }

   public static void main(String[] args) {
     SwingUtilities.invokeLater(new ZoomDemo());
   }
}

Amir Kouchekinia wrote:

Hi,

I am trying to figure out how to zoom relative to the mouse pointer
position. Below, please find my sample code.

When I initially position the mouse pointer on one corner of the red
rectangle and use the mouse wheel, I am able to zoom in and out as
expected. As soon as I move the pointer position, let's say to another
corner of the red rectangle, and try to zoom in or out, my position
relative to the rectangle shifts around.

I think I may be missing a transform or two. In another attempt, in the
mouseWheelMoved method, I inverted tx from the previous iteration and
transformed the mouse position p with the inverted transform before
recalculating the new transform; but the behavior got more erratic.

Any help is greatly appreciated. What am I missing here?

Thanks.

Here is my code:

import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.event.MouseWheelEvent;
import java.awt.event.MouseWheelListener;
import java.awt.geom.AffineTransform;
import java.awt.geom.Path2D;
import java.awt.geom.Rectangle2D;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;

@SuppressWarnings("serial")
public class ZoomDemo extends JPanel {

    AffineTransform tx = new AffineTransform();

    Rectangle2D.Double rect = new Rectangle2D.Double(100, 100, 20, 30);

    public ZoomDemo() {
      this.addMouseWheelListener(new ZoomHandler());
    }

    @Override
    public void paint(Graphics g) {
      super.paint(g);
      Graphics2D g2 = (Graphics2D) g;

      Path2D.Double path;
      g2.setColor(Color.RED);
      path = new Path2D.Double(rect, tx);
      g2.draw(path);
    }

    private class ZoomHandler implements MouseWheelListener {

      Point oldPoint = null;

      double scale = 1.0;

      public void mouseWheelMoved(MouseWheelEvent e) {
        if (e.getScrollType() == MouseWheelEvent.WHEEL_UNIT_SCROLL) {

          scale += (.1 * e.getWheelRotation());
          scale = Math.max(0.1, scale);
          Point p = e.getPoint();

          tx = AffineTransform.getTranslateInstance(p.getX(), p.getY());
          tx.scale(scale, scale);
          tx.translate(-p.getX(), -p.getY());

          ZoomDemo.this.revalidate();
          ZoomDemo.this.repaint();
        }
      }
    }

    public static void main(String[] args) {

      JFrame f = new JFrame("ZoomDemo");
      ZoomDemo zoomDemo = new ZoomDemo();
      JScrollPane sp = new JScrollPane(zoomDemo);
      f.getContentPane().add(sp);
      f.setSize(500, 500);
      f.setLocationRelativeTo(null);
      f.setVisible(true);
    }
}