CStepTimer Class Reference

The step timer class. More...

#include <StepTimer.h>

Inheritance diagram for CStepTimer:

Public Member Functions
	CStepTimer ()
	Constructor.
template<typename t >
void	Tick (const t &)
	One tick per frame. More...
const float	GetElapsedSeconds () const
	Get time in seconds since last Tick. More...
const float	GetTotalSeconds () const
	Get current time in seconds. More...
const uint32_t	GetFrameCount () const
	Get total number of frames since start of the program. More...
const uint32_t	GetFramesPerSecond () const
	Get the current framerate. More...
void	SetFixedTimeStep (bool=true)
	Set fixed or variable timestep mode. More...
void	SetTargetElapsedSeconds (double)
	Set desired seconds per frame. More...

Protected Member Functions
void	ResetElapsedTime ()
	Reset elapsed time in fixed timestep mode. More...
template<typename t >
void	FixedTimeStepTick (const t &)
	One time tick in fixed timestep mode. More...
template<typename t >
void	VariableTimeStepTick (const t &)
	One time tick in variable timestep mode. More...

Static Protected Member Functions
static double	TicksToSeconds (uint64_t)
	Convert ticks to seconds. More...
static uint64_t	SecondsToTicks (double)
	Convert seconds to ticks. More...

Protected Attributes
LARGE_INTEGER	m_qpcFrequency = {0}
	Performance counter frequency in QPC units.
LARGE_INTEGER	m_qpcLastTime = {0}
	Performance counter time in QPC units.
uint64_t	m_qpcMaxDelta = 0
	Performance counter maximum time interval in QPC units.
uint64_t	m_qpcSecondCounter = 0
	Performance counter second counter in QPC units.
uint64_t	m_elapsedTicks = 0
	Elapsed ticks in current frame.
uint64_t	m_totalTicks = 0
	Total ticks since program started.
uint64_t	m_leftOverTicks = 0
	For fixed timestep calculations.
uint32_t	m_frameCount = 0
	Total number of frames.
uint32_t	m_framesPerSecond = 0
	Number of frames per second.
uint32_t	m_framesThisSecond = 0
	Number of frames so far in current second.
bool	m_isFixedTimeStep = false
	In fixed timestep mode (versus variable timestep).
uint64_t	m_targetElapsedTicks = 0
	Desired ticks per frame in fixed timestep mode.
uint64_t	m_nTimeDelta = 0
	Local variable for time interval.

Detailed Description

The step timer class allows you to measure frame time and total time. It has a Tick function that you should call once per frame with the code that you want executed once per frame as a parameter. The step timer can be set into fixed timestep mode in which the frame time reported is always fixed. This is important in physics games since locking to the monitor frame rate is bound to give you unequal frame times because monitor frequency is almost always not what you expect. For example, a monitor advertized at 60Hz (which Windows will report is 60Hz) may actually be 59.3Hz.

Member Function Documentation

FixedTimeStepTick()

template<typename t >

void CStepTimer::FixedTimeStepTick ( const t &  update )

protected

Process one time tick, fixed rate. If the app is running very close to the target elapsed time (within 1/4 of a millisecond) just clamp the clock to exactly match the target value. This prevents tiny and irrelevant errors from accumulating over time. Without this clamping, a game that requested a 60 fps fixed update, running with vsync enabled on a 59.94 NTSC display, would eventually accumulate enough tiny errors that it would drop a frame. It is better to just round small deviations down to zero to leave things running smoothly.

Parameters

update

Function to be called once per tick.

GetElapsedSeconds()

const float CStepTimer::GetElapsedSeconds

(

)

const

Get number of seconds that have elapsed since the last time Tick() was called. Ideally this is the last frame time, assuming that Tick() is called one per frame.

Returns

Elapsed seconds since last frame.

GetFrameCount()

const uint32_t CStepTimer::GetFrameCount

(

)

const

Get total number of times Tick() has been called since the game started. Ideally this is the number of frames rendered, assuming that Tick() is called one per frame.

Returns

Frame count.

GetFramesPerSecond()

const uint32_t CStepTimer::GetFramesPerSecond

(

)

const

Get the current frame rate.

Returns

Frame rate in frames per second.

GetTotalSeconds()

const float CStepTimer::GetTotalSeconds

(

)

const

Get number of seconds since the game started.

Returns

Total seconds since game started.

ResetElapsedTime()

void CStepTimer::ResetElapsedTime ( )

protected

After an intentional timing discontinuity (for instance a blocking IO operation), call this to avoid having the fixed timestep logic attempt a set of catch-up Updat()e calls.

SecondsToTicks()

uint64_t CStepTimer::SecondsToTicks ( double  seconds )

staticprotected

Convert seconds into ticks, the unit used by the Windows performance counter.

Parameters

seconds

The number of seconds.

Returns

The corresponding number of ticks.

SetFixedTimeStep()

void CStepTimer::SetFixedTimeStep

(

bool

isFixedTimestep = true

)

Set whether to use fixed or variable timestep mode.

Parameters

isFixedTimestep

true for fixed time step.

SetTargetElapsedSeconds()

void CStepTimer::SetTargetElapsedSeconds

(

double

targetElapsed

)

Set target frame time for fixed time step mode.

Parameters

targetElapsed

Target frame time in seconds.

Tick()

template<typename t >

void CStepTimer::Tick

(

const t &

update

)

Process one time tick, either fixed or variable rate.

Parameters

update

Function to be called once per tick.

TicksToSeconds()

double CStepTimer::TicksToSeconds ( uint64_t  ticks )

staticprotected

Convert ticks, the unit used by the performance counter, into seconds.

Parameters

ticks

The number of ticks.

Returns

The corresponding number of seconds.

VariableTimeStepTick()

template<typename t >

void CStepTimer::VariableTimeStepTick ( const t &  update )

protected

Process one time tick, variable rate.

Parameters

update

Function to be called once per tick.