How to draw a JPanel in an offscreen image

Question

I am still working on painting a JPanel on an offline image
I found the following rules :
- JPanel.setBounds shall be called
- the JPanel does not redraw an offline image, on should explicitly override paint()
to paint the children components
- the Children components do not pain, except if setBounds is called for each of them
Still with these rules I do not master component placement on the screen. Something important is missing. Does somebody know what is the reason for using setBounds ?
sample code :
private static final int width=512;
private static final int height=512;
offScreenJPanel p;
FlowLayout l=new FlowLayout();
JButton b=new JButton("Click");
JLabel t=new JLabel("Hello");
p=new offScreenJPanel();
p.setLayout(l);
p.setPreferredSize(new Dimension(width,height));
p.setMinimumSize(new Dimension(width,height));
p.setMaximumSize(new Dimension(width,height));
p.setBounds(0,0,width,height);
b.setPreferredSize(new Dimension(40,20));
t.setPreferredSize(new Dimension(60,20));
p.add(t);
p.add(b);
image = new java.awt.image.BufferedImage(width, height,
java.awt.image.BufferedImage.
TYPE_INT_RGB);
Graphics2D g= image.createGraphics();
// later on
p.paint(g);
paint method of offScreenPanel :
public class offScreenJPanel extends JPanel {
public void paint(Graphics g) {
super.paint(g);
Component[] components = getComponents();
for (int i = 0; i < components.length; i++) {
JComponent comp=(JComponent) components;
comp.setBounds(0,0,512,512);
components[i].paint(g);

Answer 1

Unfortunately using pack doesn't work, or I didn't use it the right way.
I made a test case, eliminated anything not related to the problem (Java3D, applet ...). In the
test case if you go to the line marked "// CHANGE HERE" and uncomment the jf.show(), you have
an image generated in c:\tmp under the name image1.png with the window contents okay.
If you replace show by pack you get a black image. It seems there still something.
simplified sample code :[b]
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;
import com.sun.j3d.utils.applet.*;
import com.sun.j3d.utils.image.*;
import com.sun.j3d.utils.universe.*;
import java.io.*;
import java.awt.image.*;
import javax.imageio.*;
public class test {
private static final int width=512;
private static final int height=512;
public test() {
JPanel p;
BorderLayout lay=new BorderLayout();
java.awt.image.BufferedImage image;
// add a JPanel with a label and a button
p=new JPanel(lay);
p.setPreferredSize(new Dimension(width,height));
JLabel t=new JLabel("Hello");
t.setPreferredSize(new Dimension(60,20));
p.add(t,BorderLayout.NORTH);
p.setDebugGraphicsOptions(DebugGraphics.LOG_OPTION );
// show the panel for debug
JFrame jf=new JFrame();
jf.setSize(new Dimension(width,height));
jf.getContentPane().add(p);
[b]
// CHANGE HERE->
jf.pack();
//jf.show();
// create an off screen image
image = new java.awt.image.BufferedImage(width, height,
java.awt.image.BufferedImage.TYPE_INT_RGB);
// paint JPanel on off screen image
Graphics2D g= image.createGraphics();
g.setClip(jf.getBounds());
System.err.println("BEFORE PAINT");
jf.paint(g);
System.err.println("AFTER PAINT");
// write the offscreen image on disk for debug purposes
File outputFile = new File("c:\\tmp\\image1.png");
try {
ImageIO.write(image, "PNG", outputFile);
} catch (Exception e) {
System.err.println(e.getMessage());
g.dispose();
jf.dispose();
public static void main(String[] args) {
test t=new test();
}