Can I create a BufferedImage from a JPanel without rendering in a JFrame?

Question

Is it possible to create a BufferedImage from a JPanel without first rendering it in a JFrame? I've searched everywhere I can think of and cannot find an answer. Can anyone help?

Here is some sample code. If I don't un-comment the JFrame code, my BufferedImage is blank.

    test(){
//      JFrame frame = new JFrame();
        JPanel panel = new JPanel();
        Dimension dim = new Dimension(50,50);
        panel.setMinimumSize(dim);
        panel.setMaximumSize(dim);
        panel.setPreferredSize(dim);
        JLabel label = new JLabel("hello");
        panel.add(label);
//      frame.add(panel);
//      frame.pack();
        BufferedImage bi = getScreenShot(panel);

        //...code that saves bi to a jpg
    }

    private BufferedImage getScreenShot(JPanel panel){
        BufferedImage bi = new BufferedImage(panel.getWidth(), panel.getHeight(), BufferedImage.TYPE_INT_ARGB);
        panel.paint(bi.getGraphics());
        return bi;
    }

Possible duplicate of http://stackoverflow.com/questions/12477522/jframe-to-image-without-showing-the-jframe — Guillaume Polet, Sep 18 '12 at 16:04
The other post is still adding the component to a JFrame to render it. I am calling this section of code in a loop, creating an image and saving it to a JPG. The loop executes upward of 2000 times causing the me to run out of heap space. — Brooke, Sep 18 '12 at 16:07
I tried print and it's still blank. To create a BufferedImage, I must pass the width and height of the object into the constructor. Before adding the panel to the frame, both of those values are 0. Even if I set them to an arbitrary size, say 700, the BufferedImage is still blank. — Brooke, Sep 18 '12 at 16:16

score 8 · Accepted Answer · edited May 23 '17 at 12:00

8

See this answer to Swing: Obtain Image of JFrame as well as Why does the JTable header not appear in the image? for tips on painting components that have not yet been rendered. I expect the fix to your problem is shown in the label of LabelRenderTest.java.

JLabel textLabel = new JLabel(title);
textLabel.setSize(textLabel.getPreferredSize());

Update

Dimension dim = new Dimension(50,50);
panel.setSize(dim);  // very important!
panel.setMinimumSize(dim);
panel.setMaximumSize(dim);
panel.setPreferredSize(dim);
// ...

Or here is the complete source. The size of the label also needs to be set.

import java.awt.Color;
import java.awt.Dimension;
import java.awt.image.BufferedImage;

import javax.swing.*;

public class RenderTest {

    RenderTest() {
        JPanel panel = new JPanel();
        panel.setBackground(Color.RED);
        Dimension dim = new Dimension(50,50);
        panel.setSize(dim);
        panel.setMinimumSize(dim);
        panel.setMaximumSize(dim);
        panel.setPreferredSize(dim);
        JLabel label = new JLabel("hello");
        label.setSize(label.getPreferredSize());
        panel.add(label);

        BufferedImage bi = getScreenShot(panel);
        JOptionPane.showMessageDialog(null, new JLabel(new ImageIcon(bi)));
    }

    private BufferedImage getScreenShot(JPanel panel){
        BufferedImage bi = new BufferedImage(
            panel.getWidth(), panel.getHeight(), BufferedImage.TYPE_INT_ARGB);
        panel.paint(bi.getGraphics());
        return bi;
    }

    public static void main(String[] args) {
        new RenderTest();
    }
}

edited May 23 '17 at 12:00

Community

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answered Sep 19 '12 at 00:25

Andrew Thompson

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The concern of the OP was that he wanted to do this without using frames, because he ran out of memory when he used frames. – lbalazscs Sep 19 '12 at 08:57
1

@lbalazscs That does not change the fact that a component does not need to be added to *anything* prior to being rendered successfully. Using the one line tweak shown above (& making an SSCCE) - the panel appeared. – Andrew Thompson Sep 19 '12 at 09:18
Andrew, I could not get it to work without adding to a frame and calling pack (I had an image, but did not see the "hello" label on it). I also discovered the missing panel.setSize. I would be interested in your SSCCE. – lbalazscs Sep 19 '12 at 09:31
@lbalazscs I'd forgotten the label, I just set the BG of the panel red and went with it. See SSCCE above. Note there is no frame, no call to `pack()` and everything is rendered before anything appears on-screen. Abracadabra! – Andrew Thompson Sep 19 '12 at 09:47
Andrew, you are right, no frame is necessary +1 and I'll edit my post. (in my test I forgot the label.setSize(label.getPreferredSize()) and that's why it worked only if I called pack() on a frame. – lbalazscs Sep 19 '12 at 10:23
That did work, but there is one major problem. It's not recognizing any of my layout managers. It starts drawing every single item at 0,0. Is there a way to do this and use layout managers? – Brooke Sep 19 '12 at 18:22
*"one major problem."* One major Q&A at a time. Mark this one answered, ask a new question. I think I know the answer, but post an SSCCE of your best attempt, so I can test my theory. – Andrew Thompson Sep 19 '12 at 18:26
Ok. I'll have to use your example with a few modifications (adding a layout manager and a second label), because I'm not allowed to post company source code. – Brooke Sep 19 '12 at 18:30
for those that are interested, here is a link to the new question: http://stackoverflow.com/questions/12500952/when-creating-a-bufferedimage-from-a-jpanel-w-o-a-jframe-can-i-also-use-a-lay – Brooke Sep 19 '12 at 18:57

lbalazscs · Answer 2 · 2012-09-19T10:28:31.403

4

EDIT2: Basically Andrew Thompson is right in his answer, a frame is not necessary. At the same time it can be practical to have one, because a call to pack() will make the layout managers work. I deleted the first part of my original answer, left is the memory-related part. Note that calling dispose on the Graphics is still needed even without a frame.

About your running out of heap space when using frames: this should not happen. Probably you only need to call dispose() on the frames when you are done with them. If that does not help, I would suggest asking it in a separate question. From the dispose docs:

Releases all of the native screen resources used by this Window, its subcomponents, and all of its owned children. That is, the resources for these Components will be destroyed, any memory they consume will be returned to the OS, and they will be marked as undisplayable. (...) Note: When the last displayable window within the Java virtual machine (VM) is disposed of, the VM may terminate.

EDIT: some more thoughts:

reusing the same JFrame object should also work. Add panel, call pack, create image, remove panel, repeat
don't forget to call dispose() on the Graphics objects created by you as well
In worst case you can restart the JVM from a script from time to time

edited Sep 19 '12 at 10:28

answered Sep 18 '12 at 17:35

lbalazscs

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It appears that there may be a memory leak involved in disposing of JFrames, which is why I was hoping to find a way around them. See the following links: http://stackoverflow.com/questions/7376993/does-disposing-a-jframe-cause-memory-leakage and http://stackoverflow.com/questions/6309407/remove-top-level-container-on-runtime/6310284#6310284 – Brooke Sep 18 '12 at 18:15
So, I do need to add my JPanel to some sort of displayable object to create the buffered image? – Brooke Sep 18 '12 at 18:59
Yes. I think reusing the same JFrame object would also work. Add panel, call pack, create image, remove panel, repeat :) – lbalazscs Sep 18 '12 at 19:09
I started using the same JFrame object and it did make it all the way through without running out of heap space. I watched the memory usage as it ran, and it still seemed to be leaking a bit. I'll be interested to see if anyone can answer my next question. Thanks for the help guys! New Question: http://stackoverflow.com/questions/12500952/when-creating-a-bufferedimage-from-a-jpanel-w-o-a-jframe-can-i-also-use-a-lay – Brooke Sep 19 '12 at 19:01

Can I create a BufferedImage from a JPanel without rendering in a JFrame?

2 Answers2

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