Here is the completed, fully functional Code. I still don't understand why the hack is necessary, but at least it now works as expected:
import javax.swing.*;
import java.awt.*;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseWheelEvent;
import java.awt.image.BufferedImage;
import java.util.Random;
/**
* Zoom-able scroll panel
*/
public class ZoomScrollPanel {
// the size of our image
private final static int IMAGE_SIZE = 600;
// create an image to display
private BufferedImage getImage() {
BufferedImage image = new BufferedImage(IMAGE_SIZE, IMAGE_SIZE, BufferedImage.TYPE_INT_RGB);
Graphics g = image.getGraphics();
// draw the small pixel first
Random rand = new Random();
for (int x = 0; x < IMAGE_SIZE; x += 10) {
for (int y = 0; y < IMAGE_SIZE; y += 10) {
g.setColor(new Color(rand.nextInt(255),rand.nextInt(255),rand.nextInt(255)));
g.fillRect(x, y, 10, 10);
}
}
// draw the larger transparent pixel second
for (int x = 0; x < IMAGE_SIZE; x += 100) {
for (int y = 0; y < IMAGE_SIZE; y += 100) {
g.setColor(new Color(rand.nextInt(255),rand.nextInt(255),rand.nextInt(255), 180));
g.fillRect(x, y, 100, 100);
}
}
return image;
}
// the image panel that resizes according to zoom level
private class ImagePanel extends JPanel {
private final BufferedImage image = getImage();
@Override
public void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2 = (Graphics2D)g.create();
g2.scale(scale, scale);
g2.drawImage(image, 0, 0, null);
g2.dispose();
}
@Override
public Dimension getPreferredSize() {
return new Dimension((int)Math.round(IMAGE_SIZE * scale), (int)Math.round(IMAGE_SIZE * scale));
}
}
// the current zoom level (100 means the image is shown in original size)
private double zoom = 100;
// the current scale (scale = zoom/100)
private double scale = 1;
// the last seen scale
private double lastScale = 1;
// true if currently executing setViewPosition
private boolean blocked = false;
public void alignViewPort(Point mousePosition) {
// if the scale didn't change there is nothing we should do
if (scale != lastScale) {
// compute the factor by that the image zoom has changed
double scaleChange = scale / lastScale;
// compute the scaled mouse position
Point scaledMousePosition = new Point(
(int)Math.round(mousePosition.x * scaleChange),
(int)Math.round(mousePosition.y * scaleChange)
);
// retrieve the current viewport position
Point viewportPosition = scrollPane.getViewport().getViewPosition();
// compute the new viewport position
Point newViewportPosition = new Point(
viewportPosition.x + scaledMousePosition.x - mousePosition.x,
viewportPosition.y + scaledMousePosition.y - mousePosition.y
);
// update the viewport position
blocked = true;
scrollPane.getViewport().setViewPosition(newViewportPosition);
blocked = false;
// remember the last scale
lastScale = scale;
}
}
// reference to the scroll pane container
private final JScrollPane scrollPane;
// constructor
public ZoomScrollPanel() {
// initialize the frame
JFrame frame = new JFrame();
frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
frame.setSize(600, 600);
// initialize the components
final ImagePanel imagePanel = new ImagePanel();
final JPanel centerPanel = new JPanel();
centerPanel.setLayout(new GridBagLayout());
centerPanel.add(imagePanel);
scrollPane = new JScrollPane();
scrollPane.setViewport(new JViewport() {
private boolean inCall = false;
@Override
public void setViewPosition(Point pos) {
if (!inCall || !blocked) {
inCall = true;
super.setViewPosition(pos);
inCall = false;
}
}
});
scrollPane.getViewport().add(centerPanel);
scrollPane.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
scrollPane.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_ALWAYS);
frame.add(scrollPane);
// add mouse wheel listener
imagePanel.addMouseWheelListener(new MouseAdapter() {
@Override
public void mouseWheelMoved(MouseWheelEvent e) {
super.mouseWheelMoved(e);
// check the rotation of the mousewheel
int rotation = e.getWheelRotation();
boolean zoomed = false;
if (rotation > 0) {
// only zoom out until no scrollbars are visible
if (scrollPane.getHeight() < imagePanel.getPreferredSize().getHeight() ||
scrollPane.getWidth() < imagePanel.getPreferredSize().getWidth()) {
zoom = zoom / 1.3;
zoomed = true;
}
} else {
// zoom in until maximum zoom size is reached
double newCurrentZoom = zoom * 1.3;
if (newCurrentZoom < 1000) { // 1000 ~ 10 times zoom
zoom = newCurrentZoom;
zoomed = true;
}
}
// check if a zoom happened
if (zoomed) {
// compute the scale
scale = (float) (zoom / 100f);
// align our viewport
alignViewPort(e.getPoint());
// invalidate and repaint to update components
imagePanel.revalidate();
scrollPane.repaint();
}
}
});
// display our frame
frame.setVisible(true);
}
// the main method
public static void main(String[] args) {
new ZoomScrollPanel();
}
}