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Cross-platform HiResTimer

Posted by Kazade, 09 November 2007 · 403 views

It's been a while since I've updated this, I'll make an effort to do so more often.

If anyone reading this has read OpenGL Game Programming or Beginning OpenGL Game Programming, you may remember the game timer class that is used in the source code by Dave Astle.

I've just spent a little while making this work on Linux by reimplementing QueryPerformanceFrequency, and QueryPerformanceTimer as private methods the code for these functions has been adapted from the equivalents in the Wine implementation.

I haven't tried recompiling on Windows yet, but I assume it will work as I haven't changed any of the Windows code. Anyway, here it is, I hope it's useful for someone:


/****************************************************************************
HiResTimer.h

Cross-platform implementation of the HiResTimer originally written
by Dave Astle for the book OpenGL Game Programming.

Author : Luke Benstead

Wrapper for the high-resolution timer. Can't be used (on Windows) if the
hi-res timer doesn't exist.

*****************************************************************************/



#ifndef __TIMER_H_INCLUDED__
#define __TIMER_H_INCLUDED__

#ifdef WIN32
#include <windows.h>
#endif

#ifdef unix
#include <sys/time.h>

/*
This is the definition of LARGE_INTEGER
for unix platforms only, Windows already has
this defined.
*/

typedef union _LARGE_INTEGER
{
struct
{
unsigned long LowPart;
long HighPart;
};
struct
{
unsigned long LowPart;
long HighPart;
} u;
long long QuadPart;
} LARGE_INTEGER;

//This is the number of ticks per second for
//unix platforms
const long TicksPerSecond = 10000000;
#endif

class HiResTimer
{
public:

/*****************************************************************************
Init()

If the hi-res timer is present, the tick rate is stored and the function
returns true. Otherwise, the function returns false, and the timer should
not be used.
*****************************************************************************/

bool Init()
{
if (!QueryPerformanceFrequency(&m_ticksPerSecond))
{
// system doesn't support hi-res timer
return false;
}
else
{
QueryPerformanceCounter(&m_startTime);
return true;
}
} // end Init()

float GetElapsedSeconds(unsigned long elapsedFrames = 1)
{
static LARGE_INTEGER s_lastTime = m_startTime;
LARGE_INTEGER currentTime;

QueryPerformanceCounter(&currentTime);

float seconds = ((float)currentTime.QuadPart - (float)s_lastTime.QuadPart) / (float)m_ticksPerSecond.QuadPart;

// reset the timer
s_lastTime = currentTime;

return seconds;
} // end GetElapsedSeconds()

/***************************************************************************
GetFPS()

Returns the average frames per second over elapsedFrames, which defaults to
one. If this is not called every frame, the client should track the number
of frames itself, and reset the value after this is called.
***************************************************************************/

float GetFPS(unsigned long elapsedFrames = 1)
{
static LARGE_INTEGER s_lastTime = m_startTime;
LARGE_INTEGER currentTime;

QueryPerformanceCounter(&currentTime);

float fps = (float)elapsedFrames * (float)m_ticksPerSecond.QuadPart / ((float)currentTime.QuadPart - (float)s_lastTime.QuadPart);

// reset the timer
s_lastTime = currentTime;

return fps;
} // end GetFPS

/***************************************************************************
LockFPS()

Used to lock the frame rate to a set amount. This will block until enough
time has passed to ensure that the fps won't go over the requested amount.
Note that this can only keep the fps from going above the specified level;
it can still drop below it. It is assumed that if used, this function will
be called every frame. The value returned is the instantaneous fps, which
will be <= targetFPS.
***************************************************************************/

float LockFPS(unsigned char targetFPS)
{
if (targetFPS == 0)
targetFPS = 1;

static LARGE_INTEGER s_lastTime = m_startTime;
LARGE_INTEGER currentTime;
float fps;

// delay to maintain a constant frame rate
do {
QueryPerformanceCounter(&currentTime);
fps = (float)m_ticksPerSecond.QuadPart/((float)(currentTime.QuadPart - s_lastTime.QuadPart));
} while (fps > (float)targetFPS);

// reset the timer
s_lastTime = m_startTime;

return fps;
} // end LockFPS()

private:
#ifdef unix
/*
These next 2 functions are reimplementations of the Windows
functions of the same name for unix. If we are compiling
on Windows these 2 methods will not be compiled and the
Windows built-in ones will be used instead.
*/

bool QueryPerformanceFrequency(LARGE_INTEGER* frequency)
{
frequency->QuadPart = TicksPerSecond;
return true;
}

bool QueryPerformanceCounter(LARGE_INTEGER* currentTime)
{
unsigned long long Ticks1601To1970 = (369 * 365 + 89) * 86400 * (unsigned long long) TicksPerSecond;
struct timeval now;
gettimeofday( &now, 0 );
currentTime->QuadPart = now.tv_sec * TicksPerSecond + Ticks1601To1970;
currentTime->QuadPart += now.tv_usec * 10;

return true;
}
#endif

LARGE_INTEGER m_startTime;
LARGE_INTEGER m_ticksPerSecond;
};

#endif







Looks good. However, are you aware that QueryPerformanceCounter has a lot of issues on Windows? Particularly on dual core CPUs, and variable frequency CPUs (I.e. laptops)?

In my code, I just use timeGetTime(), since it's usually accurate enough. I only fall back to QPC if it's a very short time, and even then, there's a fair bit of validation done to the results from it.
No I didn't realize that, but I can see why there could be problems, thanks for the heads up! [smile]

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