Zooming into the center of an image

Question

Hi, I have this code that allows the user to zoom into an image that they have selected. However, when the image is zoomed into using the slider, it goes towards the top left corner of the image rather than the center. How can I modify my code to zoom into the center of the image instead?
import java.awt.*;
import java.awt.geom.AffineTransform;
import java.awt.image.BufferedImage;
import java.io.*;
import javax.imageio.ImageIO;
import javax.swing.*;
import javax.swing.event.*;
public class MapScale extends JPanel
     BufferedImage image;
     double scale = 1.0;
     public MapScale(BufferedImage image)
          this.image = image;
     protected void paintComponent(Graphics g)
          super.paintComponent(g);
          Graphics2D g2 = (Graphics2D)g;
          g2.setRenderingHint(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_BICUBIC);
          double x = (getWidth() - scale*image.getWidth())/2;
          double y = (getHeight() - scale*image.getHeight())/2;
          AffineTransform at = AffineTransform.getTranslateInstance(x,y);
          at.scale(scale, scale);
          g2.drawRenderedImage(image, at);
     public Dimension getPreferredSize()
          int w = (int)(scale*image.getWidth());
          int h = (int)(scale*image.getHeight());
          return new Dimension(w, h);
     private JSlider getSlider()
          int min = 1, max = 36;
          final JSlider slider = new JSlider(min, max, 16);
          slider.setMajorTickSpacing(5);
          slider.setMinorTickSpacing(1);
          slider.setPaintTicks(true);
          slider.setSnapToTicks(true);
          slider.setPaintLabels(true);
          slider.addChangeListener(new ChangeListener()
               public void stateChanged(ChangeEvent e)
                    int value = slider.getValue();
                    scale = (value+4)/20.0;
                    revalidate();
                    repaint();
          return slider;
     public static void main(String[] args) throws IOException
          JFileChooser selectImage = new JFileChooser();
          if (selectImage.showOpenDialog(null) == JFileChooser.APPROVE_OPTION)
               String path = selectImage.getSelectedFile().getAbsolutePath();
               BufferedImage image = ImageIO.read(new File(path));
               MapScale test = new MapScale(image);
               JFrame f = new JFrame();
               f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
               f.getContentPane().add(new JScrollPane(test));
               f.getContentPane().add(test.getSlider(), "Last");
               f.setSize(400,400);
               f.setLocation(200,200);
               f.setVisible(true);
}

Answer 1

KGCeltsGev wrote:
Hi, I have this code that allows the user to zoom into an image that they have selected. However, when the image is zoomed into using the slider, it goes towards the top left corner of the image rather than the center. How can I modify my code to zoom into the center of the image instead?Since you use a JScrollPane and change the client size when you zoom in/out you need to scroll to the center every time you zoom in/out.
This is done by using scrollRectToVisible.
slider.addChangeListener(new ChangeListener() {
    public void stateChanged(ChangeEvent e) {
        int value = slider.getValue();
        scale = (value+4)/20.0;
        revalidate();     
        Dimension d = getPreferredSize();
        Dimension extentSize = ((JViewport) getParent()).getExtentSize();
        scrollRectToVisible(new Rectangle((d.width-extentSize.width)/2, (d.height-extentSize.height)/2, extentSize.width, extentSize.height));
});