Swing timer - time fluctuating

Question

I am using a Swing Timer in my game but when the game is running it appears to have moments when it runs smoothly and moments when it slows down.

Why is the time fluctuating?

And how do I fix it?

import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;

public class Main extends JFrame {

public Main() {
    super("JFrame");

    // you can set the content pane of the frame
    // to your custom class.

    setContentPane(new ImagePanel());

    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

    setSize(800, 400);
    setResizable(false);
    setVisible(true);

}

public static void main(String[] args) {
    // TODO Auto-generated method stub
    new Main();

}

class ImagePanel extends JPanel {

    Timer movementtimer;

    int x, y;

    public ImagePanel() {

        x = 0;
        y = 0;

        movementtimer = new Timer(12, new ActionListener() {
            public void actionPerformed(ActionEvent e) {
                long timstarted = System.currentTimeMillis();
                moveImage();
                repaint();
                long timefinished = System.currentTimeMillis() - timstarted;
                System.out.println(timefinished + " to run");
            };

        });

        movementtimer.start();

    }

    public void moveImage() {

        x++;
        y++;

        if (x > 800) {
            x = 0;
        }
        if (y > 400) {
            y = 0;
        }

    }

    public void paintComponent(Graphics g) {

        super.paintComponent(g);
        g.setColor(Color.RED);
        g.fillRect(0, 0, 800, 400);
        g.setColor(Color.BLUE);
        g.fillRect(x, y, 50, 50);

    }

}

}

Here is an example of my code. In my actual program I am drawing Images and not just a rectangle. There is also a lot of collision detection and other small calculations happening.

Also, here is a link to the Jar file for the game so you can run it and (hopefull) see what I mean. http://dl.dropbox.com/u/8724803/Get%20To%20The%20Chopper%201.3.jar

Thanks

Tom

BTW - I could not understand the logic of your timing calculations. Make `long timstarted;` an attribute of the `ActionListener` class and you might see what I am seeing - consistent 12s & 13s. — Andrew Thompson, Oct 13 '11 at 00:06

Andrew Thompson · Answer 1 · 2011-10-13T13:11:09.393

The Swing Timer is notorious for its inaccuracy. Use something else instead.

On prompting, I've decided to undelete this post. OTOH most of the extra information that makes it worth reinstating comes from trashgod, so I'll merely quote/paraphrase their comments.

I'd argue that it's reasonably accurate but easy to saturate.

And trashgod goes on to add in a separate comment that:

(I) might cite Clock Quality. javax.swing.Timer isn't very accurate, but it has usefully precise resolution on modern platforms.

score 3 · Accepted Answer · edited May 23 '17 at 11:47

Because the rendering is trivial, I find this variation of your example to be very smooth. The render time is well below a half millisecond, so the 12 millisecond period (~83 Hz) is plenty of time to finish a frame, typically taking less that 10% of one core. As the render time grows, the timer thread becomes saturated, and events are coalesced. The effect is magnified on a single core, as rendering competes with garbage collection and external processing demands. Java is not a real-time system, and not all schedulers are created equal.

You'll certainly want to profile your actual code, as suggested here, to see any correlation with fluctuating performance. One alternative approach is to lengthen the period (decrease the frequency) to meet your rendering deadline and use a larger increment in moveImage() to get the same velocity.

import java.awt.Color;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;

public class Main extends JFrame {

    private static final int W = 800;
    private static final int H = 400;

    public Main() {
        super("JFrame");
        this.add(new ImagePanel());
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.pack();
        setSize(W, H);
        this.setLocationRelativeTo(null);
        setVisible(true);
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {

            @Override
            public void run() {
                new Main();
            }
        });
    }

    class ImagePanel extends JPanel {

        Timer movementTimer;
        int x, y;

        public ImagePanel() {
            x = 0;
            y = 0;
            movementTimer = new Timer(12, new ActionListener() {

                @Override
                public void actionPerformed(ActionEvent e) {
                    moveImage();
                    repaint();
                }
            });
            movementTimer.start();
        }

        public void moveImage() {
            x++;
            y++;
            if (x > W) {
                x = 0;
            }
            if (y > H) {
                y = 0;
            }
        }

        @Override
        public void paintComponent(Graphics g) {
            super.paintComponent(g);
            long start = System.nanoTime();
            g.setColor(Color.RED);
            g.fillRect(0, 0, W, H);
            g.setColor(Color.BLUE);
            g.fillRect(x, y, 50, 50);
            double delta = (System.nanoTime() - start) / 1000000d;
            g.drawString(String.format("%1$5.3f", delta), 5, 15);
        }
    }
}

@Andrew Thompson I agreed with your deleted post (trashgod+1) — mKorbel, Oct 13 '11 at 07:59
@mKorbel It was accurate in one sense, in that if you run a `Timer` every second for 86400 iterations it will 'drift' and not be 24 hours. But I think trashgod identified the real problem with the jerky animation - saturation of the core. — Andrew Thompson, Oct 13 '11 at 08:47
@Andrew: If you restore your deleted post (+1, btw), you might cite [Clock Quality](http://www.ntp.org/ntpfaq/NTP-s-sw-clocks-quality.htm). `javax.swing.Timer` isn't very accurate, but it has usefully precise [resolution](http://mindprod.com/jgloss/time.html#ACCURACY) on modern platforms. — trashgod, Oct 13 '11 at 12:55

score 2 · Answer 3 · answered Oct 13 '11 at 00:40

2

In the panel constructor do:

setBackground(Color.RED);

Then you do not need to erase the background, as you are calling super.paintComponent.

In general calculate positions on actual time passed (System.nanoTime()) and do not rely on timer frames. There are a couple of gaming frameworks out there, so maybe it is worth looking at their solution. I liked the sample.

answered Oct 13 '11 at 00:40

Joop Eggen

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Good idea, but this seems better suited to a moving component. – trashgod Oct 13 '11 at 02:36
I cannot do this because in my actual program, the background is constantly moving. – TomRichardson Oct 13 '11 at 10:35

Swing timer - time fluctuating

3 Answers3

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