Applet flickering

[samson]

I have a simple applet using frames and canvas, when I change the image inside the applet the screen flicker, How can this be avoided ?

[goldhouse]

try using JApplet instead of applet assuming you are using double bufferised strategy

[marco]

if it is a .gif image it will make it flicker

[hardwired]

Code:

//  <applet code="AppletImageTest" width="400" height="400"></applet>
import java.applet.Applet;
import java.awt.*;
import java.awt.event.*;
import java.awt.image.BufferedImage;
import java.io.*;
import java.net.*;
import javax.imageio.ImageIO;

public class AppletImageTest extends Applet implements ActionListener {
    AppletImageCanvas canvas = new AppletImageCanvas();
    FileDialog fileDialog;

    public void init() {
        ScrollPane scrollPane = new ScrollPane();
        scrollPane.add(canvas);
        setLayout(new BorderLayout());
        add(scrollPane);
        add(getUIPanel(), "Last");
    }

    public void actionPerformed(ActionEvent e) {
        showDialog();
        String folder = fileDialog.getDirectory();
        String fileName = fileDialog.getFile();
        if(folder != null && fileName != null) {
            File file = new File(folder, fileName);
            System.out.println("file = " + file.getPath());
            BufferedImage image = loadImage(file);
            canvas.setImage(image);
            canvas.getParent().validate();
        }
    }

    private void showDialog() {
        fileDialog = new FileDialog(new Frame());
        // Filter does not work in windows os - see api.
        fileDialog.setFilenameFilter(new ImageFileFilter());
        fileDialog.pack();
        fileDialog.setVisible(true);
    }

    private BufferedImage loadImage(File file) {
        BufferedImage image = null;
        try {
            URL url = file.toURL();
            image = ImageIO.read(url);
        } catch(MalformedURLException e) {
            System.out.println("URL error: " + e.getMessage());
        } catch(IOException e) {
            System.out.println("IO error: " + e.getMessage());
        }
        return image;
    }

    private Panel getUIPanel() {
        Button button = new Button("open");
        button.addActionListener(this);
        Panel panel = new Panel();
        panel.add(button);
        return panel;
    }

    public static void main(String[] args) {
        Applet applet = new AppletImageTest();
        Frame f = new Frame();
        f.addWindowListener(closer);
        f.add(applet);
        f.setSize(400,400);
        f.setLocation(200,200);
        applet.init();
        f.setVisible(true);
    }

    private static WindowListener closer = new WindowAdapter() {
        public void windowClosing(WindowEvent e) {
            System.exit(0);
        }
    };
}

class AppletImageCanvas extends Canvas {
    BufferedImage image;

    public void paint(Graphics g) {
        if(image != null) {
            int x = (getWidth() - image.getWidth())/2;
            int y = (getHeight() - image.getHeight())/2;
            g.drawImage(image, x, y, this);
        }
    }

    public Dimension getPreferredSize() {
        if(image == null)
            return new Dimension(100,100);
        return new Dimension(image.getWidth(), image.getHeight());
    }

    public void setImage(BufferedImage image) {
        this.image = image;
        invalidate();
        repaint();
    }
}

class ImageFileFilter implements FilenameFilter {
    final static String BMP  = "bmp";
    final static String GIF  = "gif";
    final static String JPG  = "jpg";
    final static String JPEG = "jpeg";
    final static String PNG  = "png";
 
    public boolean accept(File dir, String name) {
        String ext = getExtension(name);
        return ext.equals(BMP)  || ext.equals(GIF) || ext.equals(JPG) ||
               ext.equals(JPEG) || ext.equals(PNG);
    }

    private String getExtension(String fileName) {
        int dot = fileName.lastIndexOf(".");
        if(dot > -1 && dot < fileName.length()-1)
            return fileName.substring(dot+1).toLowerCase();
        return null;
    }
}

Edit: add these two lines

Code:

            canvas.getParent().validate();
        invalidate();

to validate scrollPane after setting image.