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Timing problems

Started by Butabee Sep 19, 2010 at 4:47 AM 3 replies 800+ views

Original Post

117

Sep 19, 2010 at 4:47 AM

So I'm trying to limit my physics updating to 60 times a frame and I've accomplished that when vsync is off however when I turn vsync on, things mess up.

What I'm doing is this.

at framestart
I use QueryPerformanceCounter counter to get a start time.
I also have oldtime initialized to 0;

then I do:

if(time - oldTime > frequency / framerate)
updatephysics();
QueryPerformanceCounter(oldTime);

I don't know why but it's slowing down when vsync is turned on.

I think maybe it's because I'm calling the code when a frame is started? Instead of updating seperately from the render code?

GlaiveVehem

Sep 19, 2010 at 10:10 AM

Isnt it supposed to be something like this

if ( ( time-oldTime/frequency ) >= 0.016667 ) { //for 60 FPS   QueryPerformanceCounter(oldTime);   updatephysics();   ...}

rneckelmann

Sep 19, 2010 at 10:21 AM

Great article on timing:

http://gafferongames.com/game-physics/fix-your-timestep/

TomKQT

116

TomKQT

Sep 20, 2010 at 2:07 AM

How exactly does your rendering code work?
If you just create a device with D3DPRESENT_INTERVAL_ONE and then call device->present, it will wait for vsync (the present call will wait). And if you're doing your physics calculations in the same thread, you get physics calculations limited ALSO to 60 FPS, with no additional work required from you. And if you try to limit it by yourself too, it can behave weird ;)

Hodgman

Mod

5,991

Hodgman

Sep 20, 2010 at 2:18 AM

To use a fixed physics time-step, you've got to use an accumulation variable (so any time not used up in one frame is carried over to the next frame), and your updatephysics call has to be in a loop.
e.g. modifying your code --

time= QueryPerformanceCounteroldTime = 0timePerFrame = QueryPerformanceFrequency / framerateaccumulatedTime = 0while(1){  while( accumulatedTime > timePerFrame )  {    accumulatedTime -= timePerFrame    updatephysics()  }  oldTime = time  time = QueryPerformanceCounter  accumulatedTime += time-oldTime}

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