line Drawing in java

Question

There are number of problems with this code

public class LineEx extends JFrame implements MouseMotionListener,MouseListener{
    int x1,y1,x2,y2;
    public LineEx(){
        JLabel image=new JLabel("");
        JFileChooser chooser=new JFileChooser();

        chooser.setCurrentDirectory(new File("."));
        int r=chooser.showOpenDialog(new JFrame());
        File file=chooser.getSelectedFile();
        if(r==JFileChooser.APPROVE_OPTION){
            try {
                BufferedImage bf;
                bf = ImageIO.read(file);
                ImageIcon icon=new ImageIcon(bf);
                image.setIcon(icon);
                image.setHorizontalAlignment(JLabel.CENTER);

            } catch (IOException ex) {
                Logger.getLogger(LineEx.class.getName()).log(Level.SEVERE, null, ex);
            }
        }
        JScrollPane jsp=new JScrollPane(image);
        getContentPane().add(jsp);
        image.addMouseListener(this);
        image.addMouseMotionListener(this);
        this.pack();
    }
    public static void main(String ar[]){
        LineEx line=new LineEx();
        line.setVisible(true);
        line.setDefaultCloseOperation(EXIT_ON_CLOSE);
    }

    public void mouseClicked(MouseEvent e) {

    }

    public void mousePressed(MouseEvent e) {
        x1=e.getX();
        y1=e.getY();
    }

    public void mouseReleased(MouseEvent e) {
        JOptionPane.showMessageDialog(rootPane, "X1="+x1+"  Y1="+y1);
    }

    public void mouseEntered(MouseEvent e) {
    }

    public void mouseExited(MouseEvent e) {
    }
    @Override
    public void paint(Graphics g){
        super.paintComponents(g);
        Graphics2D gd=(Graphics2D)g;
        gd.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
        Line2D line=new Line2D.Double(x1,y1,x2,y2);
        gd.draw(line);
    }

    public void mouseDragged(MouseEvent e) {
        x2=e.getX();
        y2=e.getY();
        repaint();
    }

    public void mouseMoved(MouseEvent e) {
    }
}

1. MouseEvents are not getting exact co-ordinates that means whenever i draw a line it is not on its position. What is the reason behind this?
2. I want to move line along the image when scrollbar goes up and down, how can i do that?

Answer 1

1. Don't extend anything (especially not JFrame).
2. Keep a reference to the original BufferedImage that was loaded.
3. When a mouse event occurs (on the label, as mentioned by others), paint the line to a copy of the image, which is then used for the label.
4. Put the label in a panel with GridBagLayout with no constraint, so it is centered.
5. Drop the panel into a scroll pane that is then added to some constraint (e.g. BorderLayout.CENTER) of a parent component.

Note that you might also add the line objects to an expandable collection such as an ArrayList or DefaultListModel, then display them in a JList to the WEST of the image scroll pane. This would make it easier to manage (and potentially delete) groups of lines.

Answer 2

1. The source could be insets of JLabel's border.
2. You have the visible rect. Add visible rect x and y to your basic coordinates.

Answer 3

• MouseEvents are not getting exact co-ordinates that means whenever i draw a line it is not on its position. What is the reason behind this?
• I want to move line along the image when scrollbar goes up and down, how can i do that?

You are getting the correct coordinates from the JLabel but paints on the JFrame. And the frame coordinates begins at the top left point and "includes" the window title/border.

Override the paintComponent method on the JLabel and it you will get the correct insets and coordinates.

---

Example:

class ImageComponent extends JComponent 
        implements MouseListener, MouseMotionListener {
    private final BufferedImage img;
    private Point p1, p2;

    public ImageComponent(URL url) throws IOException {
        img = ImageIO.read(url);
        setPreferredSize(new Dimension(img.getWidth(), img.getHeight()));
        addMouseListener(this);
        addMouseMotionListener(this);
    }
    @Override protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawImage(img, 0, 0, img.getWidth(), img.getHeight(), this);
        if (p1 != null && p2 != null)
            g.drawLine(p1.x, p1.y, p2.x, p2.y);
    }
    @Override public void mousePressed(MouseEvent e) {
        p1 = e.getPoint();
    }
    @Override public void mouseDragged(MouseEvent e) {
        mouseReleased(e);
    }
    @Override public void mouseReleased(MouseEvent e) {
        p2 = e.getPoint();
        repaint();
    }
    @Override public void mouseMoved(MouseEvent e) {}
    @Override public void mouseClicked(MouseEvent e) {}
    @Override public void mouseEntered(MouseEvent e) {}
    @Override public void mouseExited(MouseEvent e) {}
}

Test code (generates this screenshot):

public static void main(String[] args) throws Exception {

    final URL lenna =
        new URL("http://upload.wikimedia.org/wikipedia/en/2/24/Lenna.png");

    final ImageComponent image = new ImageComponent(lenna);

    JFrame frame = new JFrame("Test");
    frame.add(new JScrollPane(image));

    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setSize(400, 300);
    frame.setVisible(true);
}