DXUT-计时器(Timer)

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作者: Darren Chen

  

这些日子把DXUT的核心代码都看了一遍,以下是DXUT库的计时器.

  

 这里有两个需要特别注意的函数QueryPerformanceFrequency 和 QueryPerformanceCounter,

前者是用来获取内部计时器的时钟频率(也就是每秒有多少个"滴答"),后者是用来获取计算机的"滴答"数,

两次QueryPerformanceCounter的调用,结果之差/每秒多少个滴答 = 经过了多少秒

(详细请查阅msdn)

以下是Timer代码:

 

//--------------------------------------------------------------------------------------
// Performs timer operations
// Use DXUTGetGlobalTimer() to get the global instance
//--------------------------------------------------------------------------------------
class CDXUTTimer
{
 public:
    CDXUTTimer();
    void            Reset(); // resets the timer
    void            Start(); // starts the timer
    void            Stop();  // stop (or pause) the timer
    void            Advance(); // advance the timer by 0.1 seconds
    double          GetAbsoluteTime(); // get the absolute system time
     double          GetTime(); // get the current time
     float           GetElapsedTime(); // get the time that elapsed between Get*ElapsedTime() calls
     void            GetTimeValues( double* pfTime, double* pfAbsoluteTime, float* pfElapsedTime ); // get all time values at once
     bool            IsStopped(); // returns true if timer stopped
 
 protected:
    LARGE_INTEGER   GetAdjustedCurrentTime();

    bool m_bUsingQPF;
    bool m_bTimerStopped;
    LONGLONG m_llQPFTicksPerSec;

    LONGLONG m_llStopTime;
    LONGLONG m_llLastElapsedTime;
    LONGLONG m_llBaseTime;
};

CDXUTTimer* WINAPI DXUTGetGlobalTimer();

以上是DXUTmisc.h对CDXUTTimer的定义，先看CDXUTTimer()

构造函数CDXUTTimer()将计时器初始化：

 

CDXUTTimer::CDXUTTimer()
{
    m_bTimerStopped = true; // 当前计时器为停止状态
    m_llQPFTicksPerSec = 0; // Query Performance Frequency ticks per second

    m_llStopTime = 0;       // 记录暂停计时器时的时间基数
    m_llLastElapsedTime = 0;
    m_llBaseTime = 0;　　　　// 时间基数

    // Use QueryPerformanceFrequency to get the frequency of the counter
     LARGE_INTEGER qwTicksPerSec = { 0 };
    QueryPerformanceFrequency( &qwTicksPerSec );
    m_llQPFTicksPerSec = qwTicksPerSec.QuadPart; // 查询出每秒多少个ticks
}

 

//-----------------------------------------------------------------------------
 // If stopped, returns time when stopped otherwise returns current time
 //-----------------------------------------------------------------------------
 LARGE_INTEGER CDXUTTimer::GetAdjustedCurrentTime()
{
    LARGE_INTEGER qwTime;
    if( m_llStopTime != 0 )
        qwTime.QuadPart = m_llStopTime; // 如果当前计时器是暂停了的,那么只需要返回暂停时记录的时间
    else
        QueryPerformanceCounter( &qwTime );  // 查询当前的ticks数
    return qwTime;
}

 

void CDXUTTimer::Reset()
{
    LARGE_INTEGER qwTime = GetAdjustedCurrentTime(); // 获取当前的ticks数

    m_llBaseTime = qwTime.QuadPart;     // 把当前的ticks数记录在BaseTime中
    m_llLastElapsedTime = qwTime.QuadPart;
    m_llStopTime = 0;
    m_bTimerStopped = FALSE;
}

 

void CDXUTTimer::Start()
{
    // Get the current time
    LARGE_INTEGER qwTime = { 0 };
    QueryPerformanceCounter( &qwTime );

    if( m_bTimerStopped ) // 如果之前时钟是处于暂停状态,那么要将流逝了的ticks"补"回去
        m_llBaseTime += qwTime.QuadPart - m_llStopTime;
    m_llStopTime = 0;
    m_llLastElapsedTime = qwTime.QuadPart;
    m_bTimerStopped = FALSE;
}

 

void CDXUTTimer::Stop()
{
    if( !m_bTimerStopped )
    {
        LARGE_INTEGER qwTime = { 0 };
        QueryPerformanceCounter( &qwTime );
        m_llStopTime = qwTime.QuadPart; // 记录暂停时的ticks数
        m_llLastElapsedTime = qwTime.QuadPart; // 最新的时间也记录为当前的ticks数
        m_bTimerStopped = TRUE; // 设置时钟为暂停
    }
}

 

公式: 时间(多少秒) = 当前滴答数 / 每秒多少滴答

double CDXUTTimer::GetAbsoluteTime()
{
    LARGE_INTEGER qwTime = { 0 };
    QueryPerformanceCounter( &qwTime );

    double fTime = qwTime.QuadPart / ( double )m_llQPFTicksPerSec;

    return fTime;
}

 

double CDXUTTimer::GetTime()
{
    LARGE_INTEGER qwTime = GetAdjustedCurrentTime();

    double fAppTime = ( double )( qwTime.QuadPart - m_llBaseTime ) / ( double )m_llQPFTicksPerSec;

    return fAppTime;
}

 

void CDXUTTimer::GetTimeValues( double* pfTime, double* pfAbsoluteTime, float* pfElapsedTime )
{
    assert( pfTime && pfAbsoluteTime && pfElapsedTime ); // 输出

    LARGE_INTEGER qwTime = GetAdjustedCurrentTime(); // 获得当前ticks数
      // 经过的时间(多少秒) = ( 当前ticks数 - 上帧记录的ticks数 ) / 每秒的ticks数
    float fElapsedTime = ( float )( ( double )( qwTime.QuadPart - m_llLastElapsedTime ) / ( double )
                                    m_llQPFTicksPerSec );
    m_llLastElapsedTime = qwTime.QuadPart; // 记录当前帧的ticks数,下一帧调用的时候用

    // Clamp the timer to non-negative values to ensure the timer is accurate.
    // fElapsedTime can be outside this range if processor goes into a 
    // power save mode or we somehow get shuffled to another processor.  
    // However, the main thread should call SetThreadAffinityMask to ensure that 
    // we don't get shuffled to another processor.  Other worker threads should NOT call 
    // SetThreadAffinityMask, but use a shared copy of the timer data gathered from 
    // the main thread.
     if( fElapsedTime < 0.0f )
        fElapsedTime = 0.0f;

    *pfAbsoluteTime = qwTime.QuadPart / ( double )m_llQPFTicksPerSec; // 绝对时间
    *pfTime = ( qwTime.QuadPart - m_llBaseTime ) / ( double )m_llQPFTicksPerSec;
    *pfElapsedTime = fElapsedTime;
}

 

贴了那么多代码，很乱的感觉，但是不贴代码，感觉又说不清楚, 耐心看看代码其实还是很简单的。

总结一下：DXUT库的这个计时器最主要的函数就是GetTimeValues，每帧都会调用这个函数，

传入三个参数分别用来获取pfAbsoluteTime "绝对时间",pfTime "程序运行时间", pfElapsedTime "前一帧到这一帧经过的时间"

 

通常游戏都需要显示每秒能运行多少帧(FPS), 公式: FPS = 1 / ElapsedTime.