Tearing-free drawing with GDI

• Download demo  - 118 Kb

Introduction

This article demonstrates Tearing free drawing with low CPU usage and GDI window properties.

Background

Tearing - A tear occurs when a page flip or blt happens at the wrong time. For example, if a page flips while the monitor scan line is in the middle of displaying a surface, as represented by the dashed line in the preceding figure, a tear occurs. The tear can be avoided by timing the flip to occur only after the entire surface has been displayed (as in the lower example of the figure). A tear can also occur when blitting to a surface that is in the process of being displayed.  For more details, please read this page.

Is it possible to draw without tearing, but with GDI window characteristic, and low CPU usage? Let's see.

The main code

Generally, the display refresh rate could be from 60 to 120Hz. or 1/60 down to 1/120 second. DirectX function ::WaitForVerticalBlank is just to wait for the VSYNC interval, but it is by Polling. That is a little ridiculous. Since the windows system is not a real time operating system, that function consume a lot of CPU resource but sometimes it still misses VSYNC. 

Here I use both multimedia Timer and DirectX function to track the vertical sync. The main point is starting a multimedia Timer with 2ms interval. In the multimedia Timer callback handler, use function ::GetScanLine to get the current scan line number, then judge whether it is the time to draw. I use cdx library to simplify using DirectX function ::GetScanLine, so that please link your code with cdxlib.

When initializing

...
    Screen = new CDXScreen();           // Start the instance

    if (Screen==NULL)
    {
        return -2;
    }
    cdx_DD = Screen->GetDD();

    HWND hOWin = GetForegroundWindow();

    SetPriorityClass(
       GetCurrentProcess(),
       HIGH_PRIORITY_CLASS );

    pTheWnd = new COsdWnd;
    if(!pTheWnd->Initialize(hInstance))
        return -3;

    SetForegroundWindow(hOWin);
... 
...
...
StartTimer(2, FALSE, 0);                // Start mmTimer when entry.

...fall into messages dispatcher
StopTimer();                            // Stop mmTimmer when exit.

Function to start multimedia Timer

///////////////////////////////////////////////////////////////////

// start mmTimer

///////////////////////////////////////////////////////////////////

public:
bool StartTimer(UINT period, bool oneShot, UINT resolution)
{
    bool        res = false;
    MMRESULT    result;

    TIMECAPS    tc;

    if (TIMERR_NOERROR == timeGetDevCaps(&tc,sizeof(TIMECAPS)))
    {
        m_timerRes = min(max(tc.wPeriodMin,resolution),tc.wPeriodMax);
        timeBeginPeriod(m_timerRes);
    }
    else return false;

    result = timeSetEvent(
        period,
        m_timerRes,                                                 
        mmTimerProc,
        0,                                                          
        (oneShot ? TIME_ONESHOT : TIME_PERIODIC)
        ); // Kill with Sync?


    if (NULL != result)
    {
        m_timerId = (UINT)result;
        res = true;
    }

    return res;
}

Function to stop multimedia Timer

//////////////////////////////////////////////////////////////////////

// stop mmTimer

//////////////////////////////////////////////////////////////////////

public:
bool StopTimer()
{
    MMRESULT    result;

    result = timeKillEvent(m_timerId);
    if (TIMERR_NOERROR == result)
        m_timerId = 0;

    if (0 != m_timerRes)
    {
        timeEndPeriod(m_timerRes);
        m_timerRes = 0;
    }

    return TIMERR_NOERROR == result;
}

Multimedia Timer Callback handler

///////////////////////////////////////////////////////////////////////

// mmTimer CALLBACK handler

// From the trace data, I think mmTimer Callback is driven 

// by Semaphore with counting.

// And it won't overrun. In other words, the Callback 

// function won't be re-entried.

///////////////////////////////////////////////////////////////////////

void CALLBACK mmTimerProc(UINT id,UINT msg,DWORD dwUser,DWORD dw1,DWORD dw2)
{
    cdx_DD->GetScanLine( &gdwScanLine );

    XYTrace(TraceError, _T("ScanLine = %d "), gdwScanLine );

    if (pTheWnd->m_bOnScreenUp)
    {
        if (!gbUsed && (gdwScanLine > pTheWnd->m_ScreenHalfHeight) )
        {
            pTheWnd->ForceFrame();
            gbUsed = TRUE;
        }

        if (gdwScanLine < gdwScanLineLastTime)
        {
            gbUsed = FALSE;
        }
        gdwScanLineLastTime = gdwScanLine;
    }
    else
    {
        if ( gdwScanLine < gdwScanLineLastTime )
        {
            pTheWnd->ForceFrame();
        }
        gdwScanLineLastTime = gdwScanLine;
    }
}

///////////////////////////////////////////////////////////////////

// Force draw a new frame

///////////////////////////////////////////////////////////////////

public:
void ForceFrame()
{    
    PaintWithMemDC(m_hWnd);
}


////////////////////////////////////////////////////////////////////

// Paint with MemDC

////////////////////////////////////////////////////////////////////

public:
inline
void PaintWithMemDC( HWND hWnd )
{
    if (m_MemDC == NULL) return;

    HDC    hDC = GetDC(hWnd);

    if (! BitBlt(
        hDC,    // handle to destination DC

        0,      // x-coord of destination upper-left corner

        0,     // y-coord of destination upper-left corner

        WIN_WIDTH,  // width of destination rectangle

        WIN_HEIGHT, // height of destination rectangle

        m_MemDC,   // handle to source DC

        m_ScrollCnt, // x-coordinate of source upper-left corner

        0,           // y-coordinate of source upper-left corner

        SRCCOPY      // raster operation code

    ))
    {
        PostMessage(m_hWnd, WM_CLOSE, 0, 0);
    }

    m_ScrollCnt += m_ScrollSpeed;

    if (m_ScrollCnt > m_StrLen + WIN_WIDTH)
    {
        m_ScrollCnt = 0;
        
        StopTimer();        
        
        CreateMemDC(hWnd); // re create the new memDC

        
        SafeStartTimer();
    }

    ReleaseDC(hWnd, hDC);
}

//////////////////////////////////////////////////////////////////////

// On Left Button Up , at the same time when windows position moved.

//////////////////////////////////////////////////////////////////////

private:
void OnLBtnUp( WINDOWPOS* lpwp)
{
    int cy = GetSystemMetrics(SM_CYSCREEN);

    m_ScreenHalfHeight = cy/2;

    if ( (UINT)(lpwp->y + WIN_HEIGHT) < (m_ScreenHalfHeight) )
    {
        m_bOnScreenUp = TRUE;
        gbUsed        = FALSE;
    }
    else
    {
        m_bOnScreenUp = FALSE;
    }
}

About the Demo

To run the demo, you may need Japanese Fonts. The demo effect is very good on Dell 8200 (P4 1.8G, JW2000, Ti200), flicker-free, tearing-free, low CPU usage and with GDI windows properties. Notice that the opaque mode has the lowest CPU usage because the copy bitblt function is the basic function of display card. This VSYNC detection method could be used in DirectX windows program, but it is only an idea by now.

History

• 4 July 2003 - updated download