Java Swing - flickering Canvas graphics

Question

I have to write a simple Java app which can load pictures, show it in a GUI form, allow the user to apply some transformation, and show the transformed picture. My solution is working fine, but the UI is flickering a bit, because the repaint method called too often (for example when the user scaling the image with a JSlider)

My code looks like this:

public class ImageCanvas extends Canvas
{
    private BufferedImage image;
    // ...

    @Override
    public void paint(Graphics g) 
    {
        Graphics2D g2d = (Graphics2D) g;
        if(image != null)
        {
             // I draw out the image...
        }
    }

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

    public void setRotation(double rotation)
    {
        this.rotation = rotation;
        this.repaint();
    }

    public void setScale(double scaleX, double scaleY) 
    { 
       //set the scaling field, then repaint ....
    }    

    // and so on...
}

And, of course, I have an ImageCanvas control on my main UI, and I simply call the public methods (see for example the "setRotation" method above) which repaint the canvas area. I know it's a simple question, but I don't even find a DoubleBuffered property on the Canvas...

Any help appreciated.

score 7 · Accepted Answer · edited Dec 06 '11 at 21:07

7

Double buffering is built-in for Swing (i.e. JComponent derived) classes.

If you want built-in double-buffering, you should extend JPanel rather than Canvas, and override paintComponent, not paint.

edited Dec 06 '11 at 21:07

Hovercraft Full Of Eels

283,665
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answered Dec 06 '11 at 21:00

Burleigh Bear

3,274
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There are no JCanvas...Should I use JPanel then? – asdfghjkl Dec 06 '11 at 21:03
@asdfghjkl: yes, JPanel and override `paintComponent`, not `paint`. Answer above edited for correction. – Hovercraft Full Of Eels Dec 06 '11 at 21:06
Okay, it works with JPanel. I don't know, if this could be a "best practice" in Swing or is it a hack, but thanks. – asdfghjkl Dec 06 '11 at 21:08
1

@asdfghjkl: this is definitely "best practice" and no hack. – Hovercraft Full Of Eels Dec 06 '11 at 21:32
Why does everyone always extend JPanel instead of JComponent, which feels a lot cleaner to me. – Martijn Courteaux Dec 06 '11 at 22:11
@Martijn: I do it out of preference. JPanel's are opaque by default, a background color can be set once and forgotten via setBackground(). JComponent would work well too though. – Hovercraft Full Of Eels Dec 06 '11 at 22:25

score 1 · Answer 2 · answered Dec 06 '11 at 21:06

1

If you can use JPanel than go for it. Please make sure you are not overriding the JPanel.paint method, override JPanel.paintComponent instead. See this link for details.

answered Dec 06 '11 at 21:06

GETah

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score -1 · Answer 3 · answered Dec 06 '11 at 21:03

-1

Usually graphic lags in these applications can be caused by setting a empty variable at the top of the script, then changing its value, then waiting for the repaint to update it. You could try changing the:

    setRotation(double rotation);

so that it rotates the image in that method.

Just a general thing I happen to see while dealing with graphics.

answered Dec 06 '11 at 21:03

Matt

95
6

makes no sense to me, even after reading more than once: nothing "usually lags", there is neither an "empty variable" nor a "script". What's empty, though, is the code snippet. – kleopatra Dec 07 '11 at 09:25
Well I'm sorry if you didn't post your whole code. Guess you won't get Full help. – Matt Dec 07 '11 at 16:09

Java Swing - flickering Canvas graphics

3 Answers3