時(shí)鐘類GE_TIMER可用來(lái)取得游戲已進(jìn)行的時(shí)間，計(jì)算出兩個(gè)時(shí)間點(diǎn)之間的時(shí)間片大小，從而可在某一時(shí)間點(diǎn)處，自動(dòng)更新某些游戲狀態(tài)。此外，還可用來(lái)獲取程序的幀頻FSP(Frame Per Second)大小，檢驗(yàn)3D渲染的速度，即游戲速度。
Windows API函數(shù)timeGetTime用來(lái)取得游戲開始后的時(shí)間，返回的時(shí)間值單位為ms(毫秒)。
The timeGetTime function retrieves the system time, in milliseconds. The system time is the time elapsed since Windows was started.
DWORD timeGetTime(VOID);
Parameters
This function does not take parameters.
Return Values
Returns the system time, in milliseconds.
但是這個(gè)函數(shù)的精度只有10ms左右，如果需要采用更為精確的時(shí)間，可使用小于1ms時(shí)間精度的Windows API函數(shù)QueryPerformanceCounter和QueryPerformanceFrequency，這兩個(gè)函數(shù)直接使用了Windows 內(nèi)核的精度非常高的定時(shí)器。不同的硬件和操作系統(tǒng)，定時(shí)器的頻率稍有不同。
The QueryPerformanceFrequency function retrieves the frequency of the high-resolution performance counter,
if one exists. The frequency cannot change while the system is running.
Syntax
BOOL QueryPerformanceFrequency(LARGE_INTEGER *lpFrequency);
Parameters
lpFrequency
[out] Pointer to a variable that receives the current performance-counter frequency, in counts per second.
If the installed hardware does not support a high-resolution performance counter, this parameter can be zero.
Return Value
If the installed hardware supports a high-resolution performance counter, the return value is nonzero.
If the function fails, the return value is zero. To get extended error information, call GetLastError. For example,
if the installed hardware does not support a high-resolution performance counter, the function fails.
The QueryPerformanceCounter function retrieves the current value of the high-resolution performance counter.
Syntax
BOOL QueryPerformanceCounter(LARGE_INTEGER *lpPerformanceCount);
Parameters
lpPerformanceCount
[out] Pointer to a variable that receives the current performance-counter value, in counts.
Return Value
If the function succeeds, the return value is nonzero.
If the function fails, the return value is zero. To get extended error information, call GetLastError.
Remarks
On a multiprocessor computer, it should not matter which processor is called. However, you can get different results on
different processors due to bugs in the basic input/output system (BIOS) or the hardware abstraction layer (HAL).
To specify processor affinity for a thread, use the SetThreadAffinityMask function.
這兩個(gè)函數(shù)都使用了結(jié)構(gòu)體
LARGE_INTEGER，我們來(lái)看看它的結(jié)構(gòu)：
The LARGE_INTEGER structure is used to represent a 64-bit signed integer value.
Note Your C compiler may support 64-bit integers natively. For example, Microsoft® Visual C++® supports the __int64 sized integer type.
For more information, see the documentation included with your C compiler.
typedef union _LARGE_INTEGER
{
struct { DWORD LowPart; LONG HighPart; };
struct { DWORD LowPart; LONG HighPart; } u;
LONGLONG QuadPart;
} LARGE_INTEGER, *PLARGE_INTEGER;
Members
LowPart
Low-order 32 bits.
HighPart
High-order 32 bits.
u
LowPart
Low-order 32 bits.
HighPart
High-order 32 bits.
QuadPart
Signed 64-bit integer.
Remarks
The LARGE_INTEGER structure is actually a union. If your compiler has built-in support for 64-bit integers,
use the QuadPart member to store the 64-bit integer. Otherwise, use the LowPart and HighPart members to store the 64-bit integer.
看的出來(lái)，它實(shí)際上是1個(gè)聯(lián)合體。
提示：要正確編譯運(yùn)行，需要鏈接winmm.lib。
由于本人水平有限，可能存在錯(cuò)誤，敬請(qǐng)指出。
源碼下載好了，現(xiàn)在看看GE_COMMON.h的定義，主要用來(lái)包含公用的頭文件和宏定義：
/*************************************************************************************
[Include File]
PURPOSE:
Include common header files and common macro.
*************************************************************************************/
#ifndef GAME_ENGINE_COMMON_H
#define GAME_ENGINE_COMMON_H
#define DIRECTINPUT_VERSION 0x0800 // let compile shut up
#include <windows.h>
#include <tchar.h>
#include <string.h>
#include <stdio.h>
#include <d3d9.h>
#include <d3dx9.h>
#include <dinput.h>
#include <dsound.h>
// defines for small numbers
#define EPSILON_E3 (float)(1E-3)
#define EPSILON_E4 (float)(1E-4)
#define EPSILON_E5 (float)(1E-5)
#define EPSILON_E6 (float)(1E-6)
#define Safe_Release(object) if((object) != NULL) { (object)->Release(); (object)=NULL; }
#define FCMP(a, b) (fabs((a) - (b)) < EPSILON_E3 ? 1 : 0)
#endif
由于浮點(diǎn)數(shù)不能直接比較大小，所以定義了1個(gè)宏來(lái)比較浮點(diǎn)數(shù)的大小。
#define FCMP(a, b) (fabs((a) -& nbsp;(b)) < EPSILON_E3 ? 1& nbsp;: 0)
再來(lái)看看GE_TIMER.h的定義：
/*************************************************************************************
[Include File]
PURPOSE:
Encapsulate system time for game.
*************************************************************************************/
#ifndef GAME_ENGINE_TIMER_H
#define GAME_ENGINE_TIMER_H
class GE_TIMER
{
private:
bool _use_large_time; // flag that indicate whether use large time
__int64 _one_second_ticks; // ticks count in one second
__int64 _tick_counts_start; // tick counts at start count time
unsigned long _time_start; // start time for timeGetTime()
int _frame_count; // frame count number
float _fps; // frame per second
float _time1, _time2, _time_slice; // time flag and time slice
public:
GE_TIMER();
~GE_TIMER();
void Init_Game_Time();
float Get_Game_Play_Time();
void Update_FPS();
float Get_FPS() { return _fps; }
};
#endif
并非所有系統(tǒng)都支持內(nèi)核的定時(shí)器讀取，因此要定義一個(gè)_use_large_time來(lái)標(biāo)志是否使用這個(gè)高精度的定時(shí)器，否則將使用timeGetTime函數(shù)進(jìn)行時(shí)間計(jì)算。
我們來(lái)看看構(gòu)造函數(shù)和析構(gòu)函數(shù)的定義：
//------------------------------------------------------------------------------------
// Constructor, initialize game time.
//------------------------------------------------------------------------------------
GE_TIMER::GE_TIMER()
{
Init_Game_Time();
}
//------------------------------------------------------------------------------------
// Destructor, do nothing.
//------------------------------------------------------------------------------------
GE_TIMER::~GE_TIMER()
{
}
看的出來(lái)，構(gòu)造函數(shù)只是調(diào)用了
Init_Game_Time來(lái)初始化游戲時(shí)間，而析構(gòu)函數(shù)什么都不做。
再來(lái)看看 Init_Game_Time的定義：
//------------------------------------------------------------------------------------
// Initialize game time.
//------------------------------------------------------------------------------------
void GE_TIMER::Init_Game_Time()
{
_frame_count = 0;
_fps = 0;
_time1 = _time2 = _time_slice = 0;
if(QueryPerformanceFrequency((LARGE_INTEGER*) &_one_second_ticks))
{
_use_large_time = true;
QueryPerformanceCounter((LARGE_INTEGER*) &_tick_counts_start);
}
else
{
_use_large_time = false;
_time_start = timeGetTime();
}
}
我們使用Get_Game_Play_Time來(lái)取得當(dāng)前的游戲時(shí)間，來(lái)看看它的定義：
//------------------------------------------------------------------------------------
// Get time has escaped since game start.
//------------------------------------------------------------------------------------
float GE_TIMER::Get_Game_Play_Time()
{
__int64 current_tick_counts;
if(_use_large_time)
{
QueryPerformanceCounter((LARGE_INTEGER*) ¤t_tick_counts);
return ((float) (current_tick_counts - _tick_counts_start) / _one_second_ticks) * 1000;
}
return (float)(timeGetTime() - _time_start);
}
分兩種情況進(jìn)行處理，如果使用高精度時(shí)鐘，將計(jì)算開始和結(jié)束時(shí)鐘計(jì)數(shù)之差，除以時(shí)鐘頻率，再乘以1000，即獲得時(shí)間片大小，單位為 ms。
否則直接利用timeGetTime函數(shù)計(jì)算時(shí)間片大小。
更新幀頻通過(guò)
Update_FPS函數(shù)來(lái)進(jìn)行，每5幀更新一次。
//------------------------------------------------------------------------------------
// Update FPS.
//------------------------------------------------------------------------------------
void GE_TIMER::Update_FPS()
{
// increment frame count by one
_frame_count++;
if(_frame_count % 5 == 1)
_time1 = Get_Game_Play_Time() / 1000;
else if(_frame_count % 5 == 0)
{
_time2 = Get_Game_Play_Time() / 1000;
_time_slice = (float) fabs(_time1 - _time2); // calculate time escaped
}
// update fps
if(! FCMP(_time_slice, 0.0))
_fps = 5 / _time_slice;
}
完整的GE_TIMER.cpp實(shí)現(xiàn)如下所示：
/*************************************************************************************
[Implement File]
PURPOSE:
Encapsulate system time for game.
*************************************************************************************/
#include "GE_COMMON.h"
#include "GE_TIMER.h"
//------------------------------------------------------------------------------------
// Constructor, initialize game time.
//------------------------------------------------------------------------------------
GE_TIMER::GE_TIMER()
{
Init_Game_Time();
}
//------------------------------------------------------------------------------------
// Destructor, do nothing.
//------------------------------------------------------------------------------------
GE_TIMER::~GE_TIMER()
{
}
//------------------------------------------------------------------------------------
// Initialize game time.
//------------------------------------------------------------------------------------
void GE_TIMER::Init_Game_Time()
{
_frame_count = 0;
_fps = 0;
_time1 = _time2 = _time_slice = 0;
if(QueryPerformanceFrequency((LARGE_INTEGER*) &_one_second_ticks))
{
_use_large_time = true;
QueryPerformanceCounter((LARGE_INTEGER*) &_tick_counts_start);
}
else
{
_use_large_time = false;
_time_start = timeGetTime();
}
}
//------------------------------------------------------------------------------------
// Get time has escaped since game start.
//------------------------------------------------------------------------------------
float GE_TIMER::Get_Game_Play_Time()
{
__int64 current_tick_counts;
if(_use_large_time)
{
QueryPerformanceCounter((LARGE_INTEGER*) ¤t_tick_counts);
return ((float) (current_tick_counts - _tick_counts_start) / _one_second_ticks) * 1000;
}
return (float)(timeGetTime() - _time_start);
}
//------------------------------------------------------------------------------------
// Update FPS.
//------------------------------------------------------------------------------------
void GE_TIMER::Update_FPS()
{
// increment frame count by one
_frame_count++;
if(_frame_count % 5 == 1)
_time1 = Get_Game_Play_Time() / 1000;
else if(_frame_count % 5 == 0)
{
_time2 = Get_Game_Play_Time() / 1000;
_time_slice = (float) fabs(_time1 - _time2); // calculate time escaped
}
// update fps
if(! FCMP(_time_slice, 0.0))
_fps = 5 / _time_slice;
}