Zooming a plot in a JScrollPane

Question

Hi,
I am stuck! I'd appreciate it if you can get me unstuck ...
Below a trimmed-down .java file with a "main" that shows my problem. I create a JFrame. In the contentPane of this JFrame I put a JScrollPane, and in that JScrollPane I put a JPanel. I then show() the JFrame and set the preferredSize of the JPanel equal to it's actual getSize(). This way the scrollbars are just not required. If the user makes the JFrame a little bit smaller with the mouse, then the scrollbars appear. Try it!
In the JPanel I plot an artificial profile in a white rectangle, i.e. a simple cross-plot. To convert from userspace to physical space I use a combination of 4 affineTransforms, all according to 'the book'. All works lovely.
Now I want to zoom by dragging a box in the plot. Note that I already have the code that shows the zoom-box! Once I let go of the mouse (mouseReleased) I want to zoom on that box, and that's where it goes wrong. With some jickerypoke I can actually do the zoom (not in the code below), but ... the scrollBars do not appear, and the whole idea of course if that they do appear, so that the user can scroll up/down sideways without having to change the scaling.
( In essence the question is this: if I plot outside of the (user) min/max (for instance change the line "line.lineTo((float)x, (float)y[i]);" to "line.lineTo((float)x[i], (float)(y[i]+500));"), why does the JScrollPane not decide to pop-up the scrollBars so that the user can scroll to the bit outside of the (user) min/max? )
I'd be grateful for some pointers as to how to fix this, as I am going around in circles.
Cheers - Michel - London
import java.awt.*;
import java.awt.geom.*;
import java.awt.event.*;
import javax.swing.*;
public class PanelModelTest2 extends JPanel implements MouseListener, MouseMotionListener {
//Constants
private final double MARGIN_TOP = 50.0;
private final double MARGIN_BOTTOM = 10.0;
private final double MARGIN_LEFT = 50.0;
private final double MARGIN_RIGHT = 10.0;
//Private parts
private Model model_;
private Point2D mouseDownPosition_ = null;
private Point2D mouseDragPosition_ = null;
private boolean flMouseInPanel_;
private AffineTransform affineTransform_;
public PanelModelTest2(Model model) {
model_ = model;
addMouseListener(this);
addMouseMotionListener(this);
public void paintComponent(Graphics g) {
//Always call this method on the parents first. (Why???)
super.paintComponent(g);
//Cast g onto g2d to expose the extra methods!
Graphics2D g2d = (Graphics2D)g;
//Determine the userspace
double xMin = model_.getXMin();
double xMax = model_.getXMax();
double xRng = xMax - xMin;
double yMin = model_.getYMin();
double yMax = model_.getYMax();
double yRng = yMax - yMin;
//Determine the physical space (width and height is all you need)
Dimension panelDimension = getSize();
double xRngPhy = panelDimension.getWidth();
double yRngPhy = panelDimension.getHeight();
//Instantiate the affineTransform_, then compose in reverse order!
affineTransform_ = new AffineTransform(); //Default: the Identity transform
//==>Step 4: Move axes "Margin' away from the origin
affineTransform_.translate(MARGIN_LEFT, MARGIN_TOP);
//==>Step 3: Map from panelspace to axesspace
affineTransform_.scale((xRngPhy - MARGIN_LEFT - MARGIN_RIGHT)/xRngPhy, (yRngPhy - MARGIN_TOP - MARGIN_BOTTOM)/yRngPhy);
//==>Step 2: Map from userspace to panelspace
affineTransform_.scale(xRngPhy/xRng, yRngPhy/yRng);
//==>Step 1: Map (xMin,yMin) onto (0,0)
affineTransform_.translate(-xMin, -yMin);
//1. Make, then draw the axes rectangle
Rectangle2D whiteAxes = new Rectangle2D.Double(xMin, yMin, xRng, yRng);
g2d.setColor(Color.WHITE);
g2d.fill(affineTransform_.createTransformedShape(whiteAxes));
g2d.setColor(Color.BLACK); //Reset color to black!
//2. Plot the axes
//Still outstanding!!
//3. Get, then plot the data
double[] y = model_.getY();
double[] x = model_.getX();
GeneralPath line = new GeneralPath();
for (int i=0; i<y.length; i++) {
if (i==0) line.moveTo((float)x[i], (float)y[i]);
line.lineTo((float)x[i], (float)y[i]);
g2d.clip(affineTransform_.createTransformedShape(whiteAxes));
g2d.draw(affineTransform_.createTransformedShape(line));
//Zoom box
if (flMouseInPanel_) {
if (mouseDownPosition_ != null) {
Rectangle2D zoomBox = new Rectangle2D.Double(mouseDownPosition_.getX(), mouseDownPosition_.getY(), mouseDragPosition_.getX() - mouseDownPosition_.getX(), mouseDragPosition_.getY() - mouseDownPosition_.getY());
g2d.draw(zoomBox);
public void mouseEntered(MouseEvent evt) {
flMouseInPanel_ = true;
public void mouseExited(MouseEvent evt) {
flMouseInPanel_ = false;
public void mouseMoved(MouseEvent evt) {
public void mouseReleased(MouseEvent evt) {
//Here code to zoom!!!???
mouseDownPosition_ = null; //This ensures that the
mouseDragPosition_ = null; //zoombox is no longer plotted!
repaint();
public void mousePressed(MouseEvent evt) {
mouseDownPosition_ = evt.getPoint();
public void mouseClicked(MouseEvent evt) {
public void mouseDragged(MouseEvent evt) {
mouseDragPosition_ = evt.getPoint();
repaint();
public static void main(String[] args) {
Model model = new Model();
MainJFrame mainJFrame = new MainJFrame(model);
class Model {
//Private parts ...
private double yMin_;
private double yMax_;
private double xMin_;
private double xMax_;
private double[] y_ = new double[20];
private double[] x_ = new double[20];
public Model() {
//Make fake profile
for (int i=0; i<20; i++) {
y_[i]=50*i+1300;
x_[i]=Math.sin(i/3);
//Set corresponding mins and maxs
xMin_ = -1;
xMax_ = 1;
yMin_ = 1300;
yMax_ = 2300;
public double getXMin() {return xMin_;}
public double getXMax() {return xMax_;}
public double getYMin() {return yMin_;}
public double getYMax() {return yMax_;}
public double[] getX() {return x_;}
public double[] getY() {return y_;}
class MainJFrame extends JFrame {
//Constants
public final static int WINDOWS_START_BAR_HEIGHT = 28;
//Private parts ...
private Model model_;
private PanelModelTest2 pnlModel_;
public MainJFrame(Model model) {
//Set the private parts associated with the input arguments ...
model_ = model;
//Deal with using clicking the 'x' in the top right corner
addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {dispose(); System.exit(0);}
//Panel goes 'in' JScrollPane, which goes 'in' JFrame's contentPane
pnlModel_ = new PanelModelTest2(model_);
JScrollPane scroller = new JScrollPane(pnlModel_);
getContentPane().add(scroller, BorderLayout.CENTER);
//Pack, set to full screen, show and set preferredSize of panel to it's getSize
Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
pack();
setBounds(0, 0, (int)screenSize.getWidth(), (int)screenSize.getHeight() - WINDOWS_START_BAR_HEIGHT);
show();
pnlModel_.setPreferredSize(pnlModel_.getSize()); //Now, if you make window wee bit
} //smaller, scrollbars appear
public PanelModelTest2 getPnlModel() {return pnlModel_;}

Answer 1

Oops, I forgot the tags. Here it is again ...
import java.awt.*;
import java.awt.geom.*;
import java.awt.event.*;
import javax.swing.*;
public class PanelModelTest2 extends JPanel implements MouseListener, MouseMotionListener {
    //Constants
    private final double MARGIN_TOP = 50.0;
    private final double MARGIN_BOTTOM = 10.0;
    private final double MARGIN_LEFT = 50.0;
    private final double MARGIN_RIGHT = 10.0;
    //Private parts
    private Model model_;
    private Point2D mouseDownPosition_ = null;
    private Point2D mouseDragPosition_ = null;
    private boolean flMouseInPanel_;
    private AffineTransform affineTransform_;
    public PanelModelTest2(Model model) {
        model_ = model;
        addMouseListener(this);
        addMouseMotionListener(this);
    public void paintComponent(Graphics g) {
        //Always call this method on the parents first. (Why???)
        super.paintComponent(g);
        //Cast g onto g2d to expose the extra methods!
        Graphics2D g2d = (Graphics2D)g;
        //Determine the userspace
        double xMin = model_.getXMin();
        double xMax = model_.getXMax();
        double xRng = xMax - xMin;
        double yMin = model_.getYMin();
        double yMax = model_.getYMax();
        double yRng = yMax - yMin;
        //Determine the physical space (width and height is all you need)
        Dimension panelDimension = getSize();
        double xRngPhy = panelDimension.getWidth();
        double yRngPhy = panelDimension.getHeight();
        //Instantiate the affineTransform_, then compose in reverse order!
        affineTransform_ = new AffineTransform(); //Default: the Identity transform
        //==>Step 4: Move axes "Margin' away from the origin
        affineTransform_.translate(MARGIN_LEFT, MARGIN_TOP);
        //==>Step 3: Map from panelspace to axesspace
        affineTransform_.scale((xRngPhy - MARGIN_LEFT - MARGIN_RIGHT)/xRngPhy, (yRngPhy - MARGIN_TOP - MARGIN_BOTTOM)/yRngPhy);
        //==>Step 2: Map from userspace to panelspace
        affineTransform_.scale(xRngPhy/xRng, yRngPhy/yRng);
        //==>Step 1: Map (xMin,yMin) onto (0,0)
        affineTransform_.translate(-xMin, -yMin);
        //1. Make, then draw the axes rectangle     
        Rectangle2D whiteAxes = new Rectangle2D.Double(xMin, yMin, xRng, yRng);
        g2d.setColor(Color.WHITE);
        g2d.fill(affineTransform_.createTransformedShape(whiteAxes));
        g2d.setColor(Color.BLACK); //Reset color to black!
        //2. Plot the axes
        //Still outstanding!!
        //3. Get, then plot the data
        double[] y = model_.getY();
        double[] x = model_.getX();
        GeneralPath line = new GeneralPath();
        for (int i=0; i<y.length; i++) {
            if (i==0) line.moveTo((float)x, (float)y[i]);
line.lineTo((float)x[i], (float)y[i]);
g2d.clip(affineTransform_.createTransformedShape(whiteAxes));
g2d.draw(affineTransform_.createTransformedShape(line));
//Zoom box
if (flMouseInPanel_) {
if (mouseDownPosition_ != null) {
Rectangle2D zoomBox = new Rectangle2D.Double(mouseDownPosition_.getX(), mouseDownPosition_.getY(), mouseDragPosition_.getX() - mouseDownPosition_.getX(), mouseDragPosition_.getY() - mouseDownPosition_.getY());
g2d.draw(zoomBox);
public void mouseEntered(MouseEvent evt) {
flMouseInPanel_ = true;
public void mouseExited(MouseEvent evt) {
flMouseInPanel_ = false;
public void mouseMoved(MouseEvent evt) {
public void mouseReleased(MouseEvent evt) {
//Here code to zoom!!!???
mouseDownPosition_ = null; //This ensures that the
mouseDragPosition_ = null; //zoombox is no longer plotted!
repaint();
public void mousePressed(MouseEvent evt) {
mouseDownPosition_ = evt.getPoint();
public void mouseClicked(MouseEvent evt) {
public void mouseDragged(MouseEvent evt) {
mouseDragPosition_ = evt.getPoint();
repaint();
public static void main(String[] args) {
Model model = new Model();
MainJFrame mainJFrame = new MainJFrame(model);
class Model {
//Private parts ...
private double yMin_;
private double yMax_;
private double xMin_;
private double xMax_;
private double[] y_ = new double[20];
private double[] x_ = new double[20];
public Model() {
//Make fake profile
for (int i=0; i<20; i++) {
y_[i]=50*i+1300;
x_[i]=Math.sin(i/3);
//Set corresponding mins and maxs
xMin_ = -1;
xMax_ = 1;
yMin_ = 1300;
yMax_ = 2300;
public double getXMin() {return xMin_;}
public double getXMax() {return xMax_;}
public double getYMin() {return yMin_;}
public double getYMax() {return yMax_;}
public double[] getX() {return x_;}
public double[] getY() {return y_;}
class MainJFrame extends JFrame {
//Constants
public final static int WINDOWS_START_BAR_HEIGHT = 28;
//Private parts ...
private Model model_;
private PanelModelTest2 pnlModel_;
public MainJFrame(Model model) {
//Set the private parts associated with the input arguments ...
model_ = model;
//Deal with using clicking the 'x' in the top right corner
addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {dispose(); System.exit(0);}
//Panel goes 'in' JScrollPane, which goes 'in' JFrame's contentPane
pnlModel_ = new PanelModelTest2(model_);
JScrollPane scroller = new JScrollPane(pnlModel_);
getContentPane().add(scroller, BorderLayout.CENTER);
//Pack, set to full screen, show and set preferredSize of panel to it's getSize
Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
pack();
setBounds(0, 0, (int)screenSize.getWidth(), (int)screenSize.getHeight() - WINDOWS_START_BAR_HEIGHT);
show();
pnlModel_.setPreferredSize(pnlModel_.getSize()); //Now, if you make window wee bit
} //smaller, scrollbars appear
public PanelModelTest2 getPnlModel() {return pnlModel_;}