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  Game loop and frames per second  (Read 967 times)
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Offline RonnieDeRakker

Innocent Bystander




Java games rock!


« Posted 2004-07-10 11:40:25 »

Hi y'all I see some strange things happening when I use the following code to create a game loop:

private static final int MAX_CPS = 50;
private static final int MS_PER_FRAME = 1000 / MAX_CPS;

public void run()
{
 while (running)
 {
   long cycleStartTime = System.currentTimeMillis();

   repaint();

   if (System.currentTimeMillis() - lastCPSTime > 1000)
   {
     lastCPSTime = System.currentTimeMillis();
     cps = cyclesThisSecond;
     cyclesThisSecond = 0;
   }
   else
     cyclesThisSecond++;

   long timeSinceStart = (System.currentTimeMillis() - cycleStartTime);
   if (timeSinceStart < MS_PER_FRAME)
   {
     try
     {
       Thread.sleep(MS_PER_FRAME - timeSinceStart);
     }
     catch (InterruptedException e) {}
   }
 }
}

As you can see, it's just a regular gameloop. The variable cps is drawn to screen every second.
However when I set MAX_CPS=50, cps has the value of 32. When MAX_CPS=40, then cps is still 32! When MAX_CPS=30 then cps is 20. When MAX_CPS=65 then cps is 65! When MAX_CPS=100, then cps is still 65!

I can understand there's some overhead in using the Thread.sleep statement, but this behaviour I cannot explanin.

Can somebody shine a light on this?

Thanx!
Ronnie

Offline Abuse

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Medals: 12


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« Reply #1 - Posted 2004-07-10 12:25:43 »

currentTimeMillis has a resolution of ~50ms on Win9x, and ~10ms in WinNT>.

Thread.sleep() doesn't have an outstanding accuracy either.

Without having a closer look at your loop, I can't say for sure if that is the cause though....
I never bother with sleep myself, better to hope for vsync, or tight loop on the timer. =)

:edit:

oh heyup, this is in the J2ME forum isn't it =)

Tight loop on the timer then.
(shame MIDP2.0 gave us a load of substandard 'game API' classes, instead of useful stuff like a method to expose the vblank signal)

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