Create an image from a non-visible AWT Component?

Question

I'm trying to create an image (screen-shot) of a non-visible AWT component. I can't use the Robot classes' screen capture functionality because the component is not visible on the screen. Trying to use the following code:

BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
Graphics2D g = image.createGraphics();
component.paintAll(g);

Works sometimes, but does not work if the component contains things such as a text box or button, or some sort of OpenGL / 3D component (these things are left out of the image!). How can I take a proper screenshot of the whole thing?

Answer 1

(disclamer: woops.. this doesn't seem to work for AWT )-:

I can't believe no one has suggested SwingUtilities.paintComponent or CellRendererPane.paintComponent which are made for this very purpose. From the documentation of the former:

Paints a component to the specified Graphics. This method is primarily useful to render Components that don't exist as part of the visible containment hierarchy, but are used for rendering.

---

Here is an example method that paints a non-visible component onto an image:

import java.awt.*;
import java.awt.image.BufferedImage;

import javax.swing.*;

public class ComponentPainter {

    public static BufferedImage paintComponent(Component c) {

        // Set it to it's preferred size. (optional)
        c.setSize(c.getPreferredSize());
        layoutComponent(c);

        BufferedImage img = new BufferedImage(c.getWidth(), c.getHeight(),
                BufferedImage.TYPE_INT_RGB);

        CellRendererPane crp = new CellRendererPane();
        crp.add(c);
        crp.paintComponent(img.createGraphics(), c, crp, c.getBounds());    
        return img;
    }

    // from the example of user489041
    public static void layoutComponent(Component c) {
        synchronized (c.getTreeLock()) {
            c.doLayout();
            if (c instanceof Container)
                for (Component child : ((Container) c).getComponents())
                    layoutComponent(child);
        }
    }
}

Here is a snippet of code that tests the above class:

JPanel p = new JPanel();
p.add(new JButton("Button 1"));
p.add(new JButton("Button 2"));
p.add(new JCheckBox("A checkbox"));

JPanel inner = new JPanel();
inner.setBorder(BorderFactory.createTitledBorder("A border"));
inner.add(new JLabel("Some label"));
p.add(inner);

BufferedImage img = ComponentPainter.paintComponent(p);

ImageIO.write(img, "png", new File("test.png"));

And here is the resulting image:

                      

Answer 2

Component has a method paintAll(Graphics) (as you already have found). That method will paint itself on the passed graphics. But we have to pre-configure the graphics before we call the paint method. That's what I found about the AWT Component rendering at java.sun.com:

When AWT invokes this method, the Graphics object parameter is pre-configured with the appropriate state for drawing on this particular component:

• The Graphics object's color is set to the component's foreground property.
• The Graphics object's font is set to the component's font property.
• The Graphics object's translation is set such that the coordinate (0,0) represents the upper left corner of the component.
• The Graphics object's clip rectangle is set to the area of the component that is in need of repainting.

So, this is our resulting method:

public static BufferedImage componentToImage(Component component, Rectangle region)
{
    BufferedImage img = new BufferedImage(component.getWidth(), component.getHeight(), BufferedImage.TYPE_INT_ARGB_PRE);
    Graphics g = img.getGraphics();
    g.setColor(component.getForeground());
    g.setFont(component.getFont());
    component.paintAll(g);
    g.dispose();
    if (region == null)
    {
        return img;
    }
    return img.getSubimage(region.x, region.y, region.width, region.height);
}

This is also the better way instead of using Robot for the visible components.

---

EDIT:

A long time ago I used the code I posted here above, and it worked, but now not. So I searched further. I have a tested, working way. It is dirty, but works. The Idea of it is making a JDialog, put it somewhere out of the Screen bounds, set it visible, and then draw it on the graphics.

Here is the code:

public static BufferedImage componentToImageWithSwing(Component component, Rectangle region) {
    BufferedImage img = new BufferedImage(component.getWidth(), component.getHeight(), BufferedImage.TYPE_INT_RGB);
    Graphics g = img.createGraphics();

    // Real render
    if (component.getPreferredSize().height == 0 && component.getPreferredSize().width == 0)
    {
        component.setPreferredSize(component.getSize());
    }

    JDialog f = new JDialog();
    JPanel p = new JPanel();
    p.add(component);
    f.add(p);
    f.pack();
    f.setLocation(-f.getWidth() - 10, -f.getHeight() -10);
    f.setVisible(true);
    p.paintAll(g);
    f.dispose();
    // ---

    g.dispose();
    if (region == null) {
        return img;
    }
    return img.getSubimage(region.x, region.y, region.width, region.height);
}

So, this will work also on Windows and Mac. The other answer was to draw it on a virtual screen. But this doesn't need it.