似乎无法使用多个监视器进行双缓冲绘制

Question

我正在尝试重新制作用C++和WinAPI编写的视窗屏幕保护程序，以便在多台显示器上工作。我找到了提供基础知识的this article。但是当我在我自己的代码中实现它时，我得到了一个奇怪的结果。请看下面的代码：

case WM_PAINT:
{
    PAINTSTRUCT ps = {0};
    HDC hdcE = BeginPaint(hWnd, &ps );

    EnumDisplayMonitors(hdcE, NULL, MyPaintEnumProc, 0);

    EndPaint(hWnd, &ps);
}
break;

BOOL CALLBACK MyPaintEnumProc(
      HMONITOR hMonitor,  // handle to display monitor
      HDC hdc1,     // handle to monitor DC
      LPRECT lprcMonitor, // monitor intersection rectangle
      LPARAM data       // data
      )
{
    MONITORINFO mi = {0};
    mi.cbSize = sizeof(mi);
    if(GetMonitorInfo(hMonitor, &mi))
    {
        //Is it a primary monitor?
        BOOL bPrimary = mi.dwFlags & MONITORINFOF_PRIMARY;

        DoDrawing(bPrimary, hdc1, &mi.rcMonitor);
    }

    return 1;
}

void DoDrawing(BOOL bPrimaryMonitor, HDC hDC, RECT* pRcMonitor)
{
//#define DIRECT_PAINT          //Comment out for double-buffering

    int nMonitorW = abs(pRcMonitor->right - pRcMonitor->left);
    int nMonitorH = abs(pRcMonitor->bottom - pRcMonitor->top);

    HDC hMemDC = ::CreateCompatibleDC(hDC);
    if(hMemDC)
    {
        HBITMAP hMemBmp = ::CreateCompatibleBitmap(hDC, nMonitorW, nMonitorH);
        if(hMemBmp)
        {
            HBITMAP hOldBmp = (HBITMAP)SelectObject(hMemDC, hMemBmp);

            COLORREF clr, clrBorder;
            if(bPrimaryMonitor)
            {
                clr = RGB(0, 128, 0);           //Green
                clrBorder = RGB(255, 0, 0);
            }
            else
            {
                clr = RGB(128, 0, 0);           //Red
                clrBorder = RGB(0, 255, 0);

            }

            RECT rcRect;
#ifndef DIRECT_PAINT
            //With double-buffering
            rcRect.left = 0;
            rcRect.top = 0;
            rcRect.right = nMonitorW;
            rcRect.bottom = nMonitorH;
#else
            rcRect = *pRcMonitor;
#endif

            HBRUSH hBrush = ::CreateSolidBrush(clr);

#ifndef DIRECT_PAINT
            //With double-buffering
            ::FillRect(hMemDC, &rcRect, hBrush);
#else
            ::FillRect(hDC, &rcRect, hBrush);
#endif


#ifndef DIRECT_PAINT
            //With double-buffering
            ::BitBlt(hDC, pRcMonitor->left, pRcMonitor->top, nMonitorW, nMonitorH, hMemDC, 0, 0, SRCCOPY);
#endif

//Debugging output
CString _s;
_s.Format(_T("%s\n")
          _T("%s\n")
          _T("hDC=0x%X\n")
          _T("hMemDC=0x%X\n")
        _T("RcMonitor: L=%d, T=%d, R=%d, B=%d")
          , 
          bPrimaryMonitor ? _T("Primary") : _T("Secondary"),
#ifndef DIRECT_PAINT
          _T("Double-buffering"),
#else
          _T("Direct paint"),
#endif
          hDC,
          hMemDC,
          pRcMonitor->left,
          pRcMonitor->top,
          pRcMonitor->right,
          pRcMonitor->bottom);
::DrawText(hDC, _s, _s.GetLength(), pRcMonitor, DT_NOCLIP | DT_NOPREFIX);


            SelectObject(hMemDC, hOldBmp);
            ::DeleteObject(hMemBmp);
        }

        ::DeleteDC(hMemDC);
    }

}

绘画总是在主监视器上工作。但是当我绘制到辅助显示器时，我只能直接绘制到它的DC。当我使用双缓冲技术(使用DIRECT_PAINT预处理器指令注释掉)时，我只在辅助显示器上看到黑屏，而实际上它应该是红色的。

我在这里附上两个截图。

第一个使用直接绘画的作品：

​

然后使用双缓冲的那个失败了：

​

你知道我在哪里做错了吗？

Answer 1

将WM_PAINT的代码替换为

case WM_PAINT:
   hdc = BeginPaint(hWnd, &ps);
   EnumDisplayMonitors(hdc, NULL, MyPaintEnumProc, 0);
   EndPaint(hWnd, &ps);

使用

case WM_PAINT:
   hdc = GetDC(NULL);
   EnumDisplayMonitors(hdc, NULL, MyPaintEnumProc, 0);
   ReleaseDC(NULL, hdc);

它会起作用的。

请参阅此http://msdn.microsoft.com/en-us/library/windows/desktop/dd162610(v=vs.85).aspx。