Question

I’m in the process of making a game (a shmup) and I’ve started to question the accuracy of the timers in ActionScript. Sure, they’re accurate enough when you want to time things on the order of a few seconds or deciseconds, but it seems to perform pretty poorly when you get to finer ranges. This makes it pretty tough to do things like have a spaceship firing a hundred lasers per second.

In the following sample, I tested how long (on average) the intervals were between 1000 timer ticks intended for 30ms. Time and time again, the results are ~35-36ms. Decreasing the time, I found the floor on the timer delay to be ~16-17ms. This gives me a max fps of ~60, which is great visually but also means I can’t possibly fire more than 60 lasers per second :-(. I ran this test a few times at 100 and 1000 loops, but for both the 30ms test and the 1ms test the results didn’t change. I print to a textField at the end because using trace() and launching the swf in debug mode seems to negatively affect the test. So what I’m wondering is:

• Is this test a decent measure of the Timer class’s performance, or are my results questionable?
• Will these results change dramatically on other machines?

I understand this is dependent on the getTimer() method’s accuracy, but the discussions I find on this topic usually center around getTimer()’s accuracy on larger intervals.

package 
{

    import flash.display.Sprite;
    import flash.events.TimerEvent;
    import flash.text.TextField;
    import flash.utils.getTimer;
    import flash.utils.Timer;

public class testTimerClass extends Sprite
{
    private var testTimer:Timer = new Timer(30, 1000);
    private var testTimes:Array = new Array();
    private var testSum:int = 0;
    private var testAvg:Number;
    private var lastTime:int;
    private var thisTime:int;

    public function testTimerClass()
    {
        testTimer.addEventListener(TimerEvent.TIMER, step);
        testTimer.addEventListener(TimerEvent.TIMER_COMPLETE, printResults);
        lastTime = getTimer();
        testTimer.start();
    }

    private function step(event:TimerEvent):void
    {
        thisTime = getTimer();
        testTimes.push(thisTime - lastTime);
        lastTime = thisTime;
    }

    private function printResults(event:TimerEvent):void
    {
        while (testTimes.length > 0)
        {
            testSum += testTimes.pop();
        }
        testAvg = testSum / Number(testTimer.repeatCount);              
        var txtTestResults:TextField = new TextField();
        txtTestResults.text = testAvg.toString();
        this.addChild(txtTestResults);  
    }       
}

}

I guess the best route to take would be to just draw multiple lasers in the same frame with different positions and avoid having more than one Timer object.

edit: I used stage.frameRate to change the render frameRate and ran the test on several framerates but there was no change.

Answer 1

Tinic Uro (Flash Player engineer) has written an interesting blog post about this issue some time ago.