Question on frame rate

I was using the CalculateFPSExample.java app in order to test the frame rate. As expected with my HD cam it is always about 30 fps.

Then I combined it with the preview display and was surprised, that the frame rate is only about 15 fps now. The same happens, if I do a webcam.open(false) w/o preview

Any idea, why? For me this is a bit too much “Heisenbergsche Unschärfe” (if you know what I mean) 😃

My sample app:

public static void main(String[] args) {
        final Webcam webcam = Webcam.getDefault();
        webcam.setViewSize(WebcamResolution.VGA.getSize());
        webcam.open();

//        Uncommenting this reduces the framerate by 50 %

//        final WebcamPanel panel = new WebcamPanel(webcam);
//        panel.setFPSDisplayed(true);
//        panel.setImageSizeDisplayed(true);
//        panel.setMirrored(true);
//        JFrame window = new JFrame("Test webcam panel");
//        window.setLayout(new FlowLayout());
//        window.add(panel);
//
//        window.setResizable(true);
//        window.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
//        window.pack();
//        window.setVisible(true);

        new Thread(new Runnable() {
            @Override
            public void run() {
                long t1 = 0;
                long t2 = 0;

                int p = 100;
                int r = 10;

                for (int k = 0; k < p; k++) {
                    t1 = System.currentTimeMillis();
                    for (int i = 0; ++i <= r; webcam.getImageBytes()) {
                    }
                    t2 = System.currentTimeMillis();
                    System.out.println("FPS " + k + ": " + (1000 * r / (t2 - t1)));
                }
            }
        }).start();
}

[sarxos]

Hi @neilyoung 😄

Indeed, the quantum uncertainty may be to blame, and it may be actually caused by a different quantum states of electrons in the CPU due to the corresponding wave function:

But in your specific case this is much simpler to explain. You see, when you take a look on how WebcamPanel is implemented, you may notice a Thread running in background which constantly invokes getImage() and renders it on a screen. This happens with maximum FPS rate to have as smooth preview as possible.

Now add next thread to the solution (the one from your example), so we have two threads now. Because getImage() is blocking by default, these two threads are fighting for access time. Since both have exactly the same priority, JVM gives 50% of access time to first one, and 50% of access time to the second one. Do you see connection here? 100% of access time is 30 FPS (max what your camera can give), then 50% is 15 FPS, and when you add one more thread this will drop down to 33%, which is 10 FPS:

FPS thread 2: 5: 9
FPS thread 1: 5: 9
FPS thread 2: 6: 10
FPS thread 1: 6: 10

You may ask: if this is true, why the hell I see 30 PFS displayed in preview? This is because preview displays FPS of camera, not the thread which capture image. If you sum FPS from all threads then you will get ca. 30 FPS.

Solution? Yes, there is and it’s fairy simple. You have to make getImage() non blocking. This can be done by doing webcam.open(true) which will open webcam in a asynchronous (non-blocking) mode. But please note that this will only work for getImage(), not for getImageBytes().

I have to think about a way of how you can make getImageBytes() non blocking and will come back to you later.

[neilyoung]

@sarxos Thanks for your patience. Will try to check that out later