Falling Snow on Your Desktop! Part II

• Download source - C# - 60 KB
• Download source - VC6 - 33 KB
• Download source - VC8 - 36 KB
• Download executable - 13 KB

Introduction

Since my previous article was published, I received many user inquiries about how to improve the program. This article demonstrates how to create snow flakes that fall on your desktop, by using the draw functions directly in the desktop context. When the application starts, it creates an array of snow flakes and starts the timers for each flake. Manipulation of the RedrawWindow function allows the drawing of flakes behind and over the desktop icons, making the application more impressive.

General Steps

For better randomizing, the start coordinate, timer interval, flake position (over or behind the desktop icons) and size should be pre-defined as shown below:

 srand((unsigned)time(NULL));
 for(int i=0; i<nArray; i++)
 {
    // the snow flake movement timer
    nTimer = rand()*70/RAND_MAX+10;
    arTimers.Add(nTimer);

    // snow flake start position
    nPosX = rand()*rcWorkArea.Width()/RAND_MAX;
    arPositions.Add(nPosX);

    // draw the flake over or behind the icons
    nOver = rand()*100/RAND_MAX;
    if(nOver>50)
        arOverIcons.Add(1);
    else
        arOverIcons.Add(0);

    // draw a big or small flake
    nBig = rand()*100/RAND_MAX;
    if(nBig<50)
        arBigFlakes.Add(1);
    else
        arBigFlakes.Add(0);
 }

Then, create the flake and start the timer:

 ...
 CFlake* pFlake = new CFlake(nPosX, nBig, nOver);
 m_arFlakes.Add(pFlake);

 SetTimer(i+1,nTimer,0);
 ...

When the timer event occurs, move the appropriate flake on the desktop:

 void CMainFrame::OnTimer(UINT nIDEvent)
 {
    if(nIDEvent >= 0 && nIDEvent <= (UINT)m_arFlakes.GetSize())
    {
        CFlake* pFlake = m_arFlakes.GetAt(nIDEvent-1);
        if(pFlake)
            pFlake->Move();
    }
 }

As shown above, class CFlake (Flake in C# code) is responsible for the desktop drawing.
Unfortunately, Windows Vista does not allow the use of the RedrawWindow function with the RDW_NOERASE flag for the desktop Window correctly. So, you cannot draw the flakes behind the icons in this case.

Most of the snow fall algorithm was taken from my previous article, except the drawing functions:

 HDC hDC = GetDC(m_hWndDesktop);
 if(hDC)
 {
    RECT rc;
    rc.left = m_nCurrentX;
    rc.top = m_nCurrentY;
    rc.right = m_nCurrentX+15;
    rc.bottom = m_nCurrentY+15;

    // 15 is for a little drift at the bottom of the desktop
    int nTestHeight = m_nScreenHeight - 15; 
    
    // redraw the desktop window right away
    if(m_nCurrentY<nTestHeight) // snow drift here
        RedrawWindow(m_hWndDesktop, &rc, NULL, RDW_INVALIDATE 
            | RDW_ERASE | RDW_UPDATENOW );

    m_nCurrentY += 3;
    m_nCounter++;

    if(m_nCounter == 15)
    {
        if((rand()*10/RAND_MAX)>5) m_nIncrement = 1;
        else m_nIncrement = -1;

        m_nCounter = 0;
    }

    m_nCurrentX += m_nIncrement;

    if(m_nCurrentY>m_nScreenHeight)
    {
        m_nCurrentY = 0;
        m_nCurrentX = abs(rand()*m_nScreenWidth/RAND_MAX);
        if(abs(rand()*100/RAND_MAX)>50)
            m_bIsBigFlake = TRUE;
        else
            m_bIsBigFlake = FALSE;
    }

    // Store DC settings
    int storedDC = SaveDC(hDC);

    rc.left = m_nCurrentX;
    rc.top = m_nCurrentY;
    rc.right = m_nCurrentX+15;
    rc.bottom = m_nCurrentY+15;

    HPEN pOldPen = (HPEN)SelectObject(hDC, m_hFlakePen);

    if(m_bIsBigFlake) // draw big flake here
    {
        MoveToEx(hDC, m_nCurrentX+7, m_nCurrentY, 0);
        LineTo(hDC, m_nCurrentX+7, m_nCurrentY+15);
        ...
    }
    else // draw small flake here
    {
        ...
    }

    // specify RDW_NOERASE to keep the desktop from drawing the background
    if(!m_bIsVista)
    {
        if(!m_bOverIcons && m_nCurrentY<m_nScreenHeight)
            RedrawWindow(m_hWndDesktop, &rc, NULL, RDW_INVALIDATE 
                | RDW_NOERASE | RDW_UPDATENOW);
    }

    SelectObject(hDC, pOldPen);

    // Restore DC settings to their original values
    RestoreDC(hDC, storedDC);

    // Release the DC
    ReleaseDC(m_hWndDesktop, hDC);
 }

Finally, when the application closes, clean up the desktop in the CFlake destructor (the current flake on screen position), and in the CMainFrame::OnClose function (snow drift at the bottom of the desktop):

 CFlake::~CFlake()
 {
    ...
    RedrawWindow(m_hWndDesktop, &rc, NULL, RDW_INVALIDATE | RDW_ERASE | RDW_UPDATENOW );
 }

 void CMainFrame::OnClose()
 {
    ...
    // redraw the snow drifts area
    if(hWndDesktop)
    {
        CRect rcWorkArea;
        SystemParametersInfo(SPI_GETWORKAREA,0,(LPVOID)&rcWorkArea,0);

        rcWorkArea.top = rcWorkArea.bottom - 16;
        ::RedrawWindow(hWndDesktop, &rcWorkArea, NULL, RDW_INVALIDATE | RDW_ERASE | 
            RDW_UPDATENOW );
    }
 }

Known Problems

• Application does not work properly when the Active Desktop is enabled.
• Drawing the snowflakes behind the icons with this code under Windows Vista is not implemented.

Any improvements, comments or suggestions to this code are welcome.