Problems with timeGetTime()

284

Author

November 18, 2012 04:39 AM

Hey GameDev

I am trying to make my game run based off the delta between 2 frames, so that it will run the same speed on every computer.

I am trying to get the delta between 2 frames like this:



static DWORD lastFrameTime;

static DWORD frameDelta;

frameDelta = timeGetTime() - lastFrameTime;

d3dDevice->Clear(0, 0, D3DCLEAR_TARGET, 0x000000, 1, 0);

d3dDevice->BeginScene();

d3dSpriteMachine->Begin(D3DXSPRITE_ALPHABLEND);

Draw(sprite, frameDelta);

d3dSpriteMachine->End();

d3dDevice->EndScene();

d3dDevice->Present(NULL, NULL, NULL, NULL);

Update(mousePosition, frameDelta);

lastFrameTime = timeGetTime();

The problem is that the delta always ends up 0, or at most, 1. I tried stepping through the code and I find that when lastFrameTime is set to timeGetTime() at the end, then on the next frame the time is the same. What I mean, is that it appears to be run through one frame on say 123342342 (this is just an example) and then on the second frame, timeGetTime returns 123342342 again so the delta is set to 0. I'm obviously missing something. Any help? haha

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They just see the big blue 'e' and think "Internet". The thought process usually does not get much deeper than that.

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Hodgman

52,717

November 18, 2012 04:49 AM

You shouldn't be calling [font=courier new,courier,monospace]timeGetTime[/font] twice per frame like that -- the result of that code is that you're not timing your Draw/Update code at all, you're only timing the code that comes between the bottom line and the top line.
You can change it to only call [font=courier new,courier,monospace]timeGetTime[/font] once per frame like this:

static DWORD lastFrameTime = 0;

DWORD frameDelta;

DWORD thisFrameTime = timeGetTime();

if( lastFrameTime == 0 )//first frame!

  frameDelta = 0;

else

  frameDelta = thisFrameTime - lastFrameTime;

lastFrameTime = thisFrameTime;

//DoStuff

P.S. [font=courier new,courier,monospace]timeGetTime[/font] isn't very accurate, so most people use [font=courier new,courier,monospace]QueryPerformanceCounter[/font]/[font=courier new,courier,monospace]QueryPerformanceFrequency[/font] instead, or you can use someone else's ready-made high accuracy timer, such as timer_lib.

. 22 Racing Series .

littletray26

284

Author

November 18, 2012 04:51 AM

You shouldn't be calling [font=courier new,courier,monospace]timeGetTime[/font] twice per frame like that -- the result of that code is that you're not timing your Draw/Update code at all, you're only timing the code that comes between the bottom line and the top line.
You can change it to only call [font=courier new,courier,monospace]timeGetTime[/font] once per frame like this:static DWORD lastFrameTime; static DWORD frameDelta; DWORD thisFrameTime = timeGetTime(); frameDelta = thisFrameTime - lastFrameTime; lastFrameTime = thisFrameTime; //DoStuff

P.S. [font=courier new,courier,monospace]timeGetTime[/font] isn't very accurate, so most people use [font=courier new,courier,monospace]QueryPerformanceCounter[/font]/[font=courier new,courier,monospace]QueryPerformanceFrequency[/font] instead, or you can use someone else's ready-made high accuracy timer, such as timer_lib.

Thank you, but I'm not sure I understand. What exactly is the problem? And why shouldn't I call TimeGetTime twice?

Thanks

The majority of Internet Explorer users don't understand the concept of a browsing application, or that there are options.
They just see the big blue 'e' and think "Internet". The thought process usually does not get much deeper than that.

Worms are the weirdest and nicest creatures, and will one day prove themselves to the world.

I love the word Clicky

Hodgman

52,717

November 18, 2012 04:59 AM

You're measuring the time from the end of frame#1 to the beginning of frame #2.

A = get the time.

...spend a while doing stuff...

B = get the time

...loop for next frame...

C = get the time

time spent doing stuff previously = C-B  (wrong)

...spend a while doing stuff...

etc

You really want to be measuring the time from the start of frame#1 to the start of frame#2.

A = get the time.

...spend a while doing stuff...

...loop for next frame...

B = get the time

time spent doing stuff previously = B-A

...spend a while doing stuff...

etc

. 22 Racing Series .

littletray26

284

Author

November 18, 2012 05:06 AM

You're measuring the time from the end of frame#1 to the beginning of frame #2.
A = get the time. ...spend a while doing stuff... B = get the time ...loop for next frame... C = get the time time spent doing stuff previously = C-B (wrong) ...spend a while doing stuff... etc

You really want to be measuring the time from the start of frame#1 to the start of frame#2.
A = get the time. ...spend a while doing stuff... ...loop for next frame... B = get the time time spent doing stuff previously = B-A ...spend a while doing stuff... etc

I think I understand you now.

Is this better?



static DWORD lastFrameTime;

static DWORD frameDelta;

static DWORD thisFrameTime;

thisFrameTime = timeGetTime();

if (lastFrameTime == 0)

  frameDelta = 0;

else

frameDelta = thisFrameTime - lastFrameTime;

lastFrameTime = thisFrameTime;

d3dDevice->Clear(0, 0, D3DCLEAR_TARGET, 0x000000, 1, 0);

d3dDevice->BeginScene();

d3dSpriteMachine->Begin(D3DXSPRITE_ALPHABLEND);

Draw(sprite, frameDelta);

d3dSpriteMachine->End();

d3dDevice->EndScene();

d3dDevice->Present(NULL, NULL, NULL, NULL);

Update(mousePosition, frameDelta);

The majority of Internet Explorer users don't understand the concept of a browsing application, or that there are options.
They just see the big blue 'e' and think "Internet". The thought process usually does not get much deeper than that.

Worms are the weirdest and nicest creatures, and will one day prove themselves to the world.

I love the word Clicky

yckx

1,298

November 18, 2012 06:49 PM

You should initialize lastFrameTime to 0, otherwise there's no reason to expect it will actually be 0 for the first frame. Also, there is no reason for thisFrameTime and frameDelta to be static. You're refiguring them each loop. The reason lastFrameTime needs to be static is that you need to know it's value from the previous iteration.

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