Need help to drag a rectangle to define a zoom level

Question

Can some help me figure this out. I am trying to draw some graphics on a JPanel, then drag a rectangle to define a new zoom extent. I am having problems understanding how to do this with the AffineTransform. If you run the following code, you will see what I am trying to do....
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import java.awt.Shape;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseMotionListener;
import java.awt.geom.AffineTransform;
import java.awt.image.BufferedImage;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JToolBar;
public class TestCase extends JPanel implements MouseListener, MouseMotionListener {
  private Integer _startX = 0; // start x for dragging of rectangle
  private Integer _startY = 0; // start y for dragging of rectangle
  private Integer _lastX = 0; // end x for dragging of rectangle
  private Integer _lastY = 0; // end y for dragging of rectangle
  private boolean _isDragging = false;
  private Rectangle _rect = new Rectangle();
  private Rectangle[] _rects = null;
  private Color[] _colors = null;
  protected BufferedImage _imageBuffer = null;
  protected AffineTransform _xform = null;
  public TestCase() {
    _xform = new AffineTransform();
    _rects = new Rectangle[6];
    _rects[0] = new Rectangle(0, 0, 100, 100);
    _rects[1] = new Rectangle(100, 100, 100, 100);
    _rects[2] = new Rectangle(200, 200, 100, 100);
    _rects[3] = new Rectangle(300, 300, 100, 100);
    _rects[4] = new Rectangle(200, 0, 50, 50);
    _rects[5] = new Rectangle(250, 50, 50, 50);
    _colors = new Color[6];
    _colors[0] = Color.RED;
    _colors[1] = Color.WHITE;
    _colors[2] = Color.BLUE;
    _colors[3] = Color.GREEN;
    _colors[4] = Color.YELLOW;
    _colors[5] = Color.CYAN;
    setBackground(Color.LIGHT_GRAY);
    addMouseListener(this);
    addMouseMotionListener(this);
  // The next three methods enable user to drag a retangle
  // First, set the start points of the rectangle
  public void mousePressed(MouseEvent e) {
    _startX = new Integer(e.getX());
    _startY = new Integer(e.getY());
    _lastX = new Integer(e.getX());
    _lastY = new Integer(e.getY());
    _isDragging = true;
  // Second, drag the rectangle
  public void mouseDragged(MouseEvent e) {
    _lastX = new Integer(e.getX());
    _lastY = new Integer(e.getY());
    repaint();
  // Third, complete the rectangle, then zoom
  public void mouseReleased(MouseEvent event) {
    _lastX = new Integer(event.getX());
    _lastY = new Integer(event.getY());
    _lastX = new Integer(event.getX());
    _lastY = new Integer(event.getY());
    Rectangle rect = new Rectangle(_startX, _startY, _lastX - _startX, _lastY - _startY);
    Double widPanel = new Integer(this.getWidth()).doubleValue() / _xform.getScaleX();
    Double hgtPanel = new Integer(getHeight()).doubleValue() / _xform.getScaleY();
    Double widRect = rect.getWidth();
    Double hgtRect = rect.getHeight();
    Double scalex = widPanel.doubleValue() / widRect;
    Double scaley = hgtPanel.doubleValue() / hgtRect;
    Double translatex = _startX.doubleValue() * scalex;
    Double translatey = _startY.doubleValue() * scaley;
    _xform.setToScale(scalex, scaley);
    _xform.setToTranslation(translatex, translatey);
    _isDragging = false;
    repaint();
  protected void paintComponent(Graphics g) {
    super.paintComponent(g);
    Graphics2D g2 = (Graphics2D) g;
    if(_isDragging){
      g2.drawImage(_imageBuffer, 0, 0, this);
      g2.setPaint(Color.RED);
      g2.drawRect(_startX, _startY, _lastX - _startX, _lastY - _startY);
    else{
      paintPanel(g2);
  protected void paintPanel(Graphics2D g) {
    Double width = this.getSize().getWidth();
    Double height = this.getSize().getHeight();
    Dimension size = this.getSize();
    _imageBuffer = new BufferedImage(Math.round(width.floatValue()), Math.round(height.floatValue()), BufferedImage.TYPE_INT_RGB);
    Graphics2D graphics = (Graphics2D) _imageBuffer.getGraphics();
    graphics.setColor(Color.LIGHT_GRAY);
    graphics.fillRect(0, 0, this.getWidth(), this.getHeight());
    for(int i = 0; i <= _rects.length - 1; i++){
      Rectangle rect = _rects;
Shape shape = _xform.createTransformedShape(rect);
graphics.setColor(_colors[i]);
graphics.fill(shape);
g.drawImage(_imageBuffer, 0, 0, this);
protected void zoomToExtent() {
_xform.setToIdentity();
repaint();
public void mouseClicked(MouseEvent event) {
public void mouseEntered(MouseEvent event) {
public void mouseExited(MouseEvent event) {
public void mouseMoved(MouseEvent event) {
public static void main(String[] args) {
final TestCase test = new TestCase();
JButton btnFullExtent = new JButton("Reset");
btnFullExtent.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent event) {
test.zoomToExtent();
JToolBar toolBar = new JToolBar();
toolBar.add(btnFullExtent);
final JFrame f = new JFrame();
f.getContentPane().setLayout(new BorderLayout());
f.getContentPane().add(toolBar, BorderLayout.NORTH);
f.getContentPane().add(new TestCase(), BorderLayout.CENTER);
f.setSize(600, 400);
f.setVisible(true);

Answer 1

I modified you code a little bit, just so I could improve my understanding. I tried to swap out the class level scale and translate variables, with an AffineTransform object, but wasn't able to get it working? Do you know why this won't work?
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.Rectangle;
import java.awt.Shape;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseMotionListener;
import java.awt.geom.Rectangle2D;
import java.util.Random;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JToolBar;
public class TestCase extends JPanel implements MouseListener, MouseMotionListener {
  private Point start;
  private Point end;
  private boolean dragging = false;
  private double zoom_x = 1.0;
  private double zoom_y = 1.0;
  private double translateX;
  private double translateY;
  private Rectangle[] _rects = null;
  private Color[] _colors = null;
  public TestCase() {
    setBackground(Color.LIGHT_GRAY);
    _rects = new Rectangle[6];
    _rects[0] = new Rectangle(0, 0, 100, 100);
    _rects[1] = new Rectangle(100, 100, 100, 100);
    _rects[2] = new Rectangle(200, 200, 100, 100);
    _rects[3] = new Rectangle(300, 300, 100, 100);
    _rects[4] = new Rectangle(200, 0, 50, 50);
    _rects[5] = new Rectangle(250, 50, 50, 50);
    _colors = new Color[6];
    _colors[0] = Color.RED;
    _colors[1] = Color.WHITE;
    _colors[2] = Color.BLUE;
    _colors[3] = Color.GREEN;
    _colors[4] = Color.YELLOW;
    _colors[5] = Color.CYAN;
    addMouseListener(this);
    addMouseMotionListener(this);
  public void mousePressed(MouseEvent e) {
    start = e.getPoint();
    dragging = true;
  public void mouseDragged(MouseEvent e) {
    end = e.getPoint();
    repaint();
  public void mouseReleased(MouseEvent e) {
    end = e.getPoint();
    double visibleWidth = getWidth();
    double visibleHeight = getHeight();
    double selectedWidth = Math.abs(end.getX() - start.getX());
    double selectedHeight = Math.abs(end.getY() - start.getY());
    double zoomFactorX = visibleWidth / selectedWidth;
    double zoomFactorY = visibleHeight / selectedHeight;
    translateX -= start.x / zoom_x;
    translateY -= start.y / zoom_y;
    zoom_x *= zoomFactorX;
    zoom_y *= zoomFactorY;
    dragging = false;
    repaint();
  protected void paintComponent(Graphics g) {
    super.paintComponent(g);
    Graphics2D g2 = (Graphics2D) g;
    paintShapes((Graphics2D) g2.create());
    if(dragging){
      g2.setPaint(Color.RED);
      g2.drawRect(start.x, start.y, end.x - start.x, end.y - start.y);
  private void paintShapes(Graphics2D g2) {
    System.out.println("translateX=" + translateX + ", translateY=" + translateY + ", zoom_x=" + zoom_x + ", zoom_y=" + zoom_y);
    g2.scale(zoom_x, zoom_y);
    g2.translate(translateX, translateY);
    for(int i = 0; i <= _rects.length - 1; i++){
      Rectangle rect = _rects;
g2.setColor(_colors[i]);
g2.fill(rect);
public void reset() {
zoom_x = 1.0;
zoom_y = 1.0;
translateX = 0;
translateY = 0;
repaint();
public void mouseClicked(MouseEvent event) {
public void mouseEntered(MouseEvent event) {
public void mouseExited(MouseEvent event) {
public void mouseMoved(MouseEvent event) {
public static void main(String[] args) {
final TestCase test = new TestCase();
JButton btnFullExtent = new JButton("Reset");
btnFullExtent.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent event) {
test.reset();
JToolBar toolBar = new JToolBar();
toolBar.add(btnFullExtent);
final JFrame f = new JFrame();
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
f.getContentPane().setLayout(new BorderLayout());
f.getContentPane().add(toolBar, BorderLayout.NORTH);
f.getContentPane().add(test, BorderLayout.CENTER);
f.setSize(600, 400);
f.setLocationRelativeTo(null);
f.setVisible(true);