Re: sampling rate problem

"Daniel Pitts" <>
10 Nov 2006 11:20:48 -0800
wanwan wrote:

I have a thread that records mouse positions at 100Hz, but I can't get
the desired sampling rate. The simplified code is:

static int mousepos_x, mousepos_y; // updated by another class
int[][] samples = new int [15000][2];
int count = 0;
public class SampleMouse implements Runnable {
    public void run () {
        while (running) {
           samples[count][0] = mousepos_x;
           samples[count++][1] = mousepos_y;

I also tried in the run method with another algorithm:

long starttime; // (class scope variable) in ns

while (running) {
    if (System.nanoTime() > starttime + count*10*1000000) {
        samples[count][0] = mousepos_x;
        samples[count++][1] = mousepos_y;

I also tried setting the thread priority to max.

With any of the approaches, I get a discrepancy of over 20%

Does anyone have a good way to get the desired sampling rate?

First, this is a threading nightmare.
You need to syncronize so that your reading of mousepos_x and
mousepos_y is atomic. Also, there is no guaranty that your other
thread's data will be accessible to this thread unless you syncronize.

It would ALSO probably be better for you to use the Timer class,
instead of busy-waiting.

The other suggestion I have for you is to instead use temporal event
based updates (calculating the change in time from the last position),
rather than polling every n milliseconds.

The real question I have for you is, why do you want to sample mouse
movement at all? Is it for some sort of macro? Just an excercise?
Something else that could be done using standard libraries instead of
writing your own sampling method?

Well, hope the suggestions help, good luck finding the right way.

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