使用MSDN库进行多屏幕截图

Question

我正在进行多屏幕截图的工作。由于GetDesktopWindow()只能获得主显示器的句柄，因此我尝试使用EnumDisplayMonitors()来完成这项工作。

在阅读MSDN网站后，我在main()函数中写下了以下代码：

HDC hdc = GetDC(NULL);
EnumDisplayMonitors(hdc, NULL, MyCapScreenEnumProc, 0);
ReleaseDC(NULL, hdc);

对于 "BOOL CALLBACK MyCapScreenEnumProc (HMONITOR hMonitor，HDC hdcMonitor，LPRECT lprcMonitor，LPARAM dwData)" 回调函数，我从 MSDN:Capturing an Image 复制了示例函数 "int CaptureAnImage(HWND hWnd)" 并做出以下修改：

1. 将 HWND 参数声明为函数参数，并使用 GetDesktopWindow() 初始化它
2. 删除了用于拉伸设备上下文的代码
3. 使用 hdcMonitor 参数作为设备上下文
4. 使用 lprcMonitor 参数作为 RECT
5. 添加了生成唯一文件名的代码

以下是完整代码：

BOOL CALLBACK MyCapScreenEnumProc(HMONITOR hMonitor, HDC hdcMonitor, LPRECT lprcMonitor, LPARAM dwData)
{
    HWND hWnd = GetDesktopWindow();
    HDC hdcMemDC = NULL;
    HBITMAP hbmScreen = NULL;
    BITMAP bmpScreen;

    //generate a unique file name for the bitmaps
    static int file_number = 1;
    stringstream ss;
    ss << "all_capture_" << file_number++ << ".bmp";
    string filename = ss.str();
    wstring widestr = wstring(filename.begin(), filename.end());

    // Create a compatible DC which is used in a BitBlt from the window DC
    hdcMemDC = CreateCompatibleDC(hdcMonitor);

    if (!hdcMemDC)
    {
        MessageBox(hWnd, L"CreateCompatibleDC has failed", L"Failed", MB_OK);
        goto done;
    }

    // Get the client area for size calculation
    RECT rcClient = *lprcMonitor;

    // Create a compatible bitmap from the Window DC
    hbmScreen = CreateCompatibleBitmap(hdcMonitor, rcClient.right - rcClient.left, rcClient.bottom - rcClient.top);

    if (!hbmScreen)
    {
        MessageBox(hWnd, L"CreateCompatibleBitmap Failed", L"Failed", MB_OK);
        goto done;
    }

    // Select the compatible bitmap into the compatible memory DC.
    SelectObject(hdcMemDC, hbmScreen);

    // Bit block transfer into our compatible memory DC.
    if (!BitBlt(hdcMemDC,
                0, 0,
                rcClient.right - rcClient.left, rcClient.bottom - rcClient.top,
                hdcMonitor,
                0, 0,
                SRCCOPY))
    {
        MessageBox(hWnd, L"BitBlt has failed", L"Failed", MB_OK);
        goto done;
    }

    // Get the BITMAP from the HBITMAP
    GetObject(hbmScreen, sizeof(BITMAP), &bmpScreen);

    BITMAPFILEHEADER   bmfHeader;
    BITMAPINFOHEADER   bi;

    bi.biSize = sizeof(BITMAPINFOHEADER);
    bi.biWidth = bmpScreen.bmWidth;
    bi.biHeight = bmpScreen.bmHeight;
    bi.biPlanes = 1;
    bi.biBitCount = 32;
    bi.biCompression = BI_RGB;
    bi.biSizeImage = 0;
    bi.biXPelsPerMeter = 0;
    bi.biYPelsPerMeter = 0;
    bi.biClrUsed = 0;
    bi.biClrImportant = 0;

    DWORD dwBmpSize = ((bmpScreen.bmWidth * bi.biBitCount + 31) / 32) * 4 * bmpScreen.bmHeight;

    // Starting with 32-bit Windows, GlobalAlloc and LocalAlloc are implemented as wrapper functions that
    // call HeapAlloc using a handle to the process's default heap. Therefore, GlobalAlloc and LocalAlloc
    // have greater overhead than HeapAlloc.
    HANDLE hDIB = GlobalAlloc(GHND, dwBmpSize);
    char *lpbitmap = (char *)GlobalLock(hDIB);

    // Gets the "bits" from the bitmap and copies them into a buffer
    // which is pointed to by lpbitmap.
    GetDIBits(hdcMonitor, hbmScreen, 0,
              (UINT)bmpScreen.bmHeight,
              lpbitmap,
              (BITMAPINFO *)&bi, DIB_RGB_COLORS);




    // A file is created, this is where we will save the screen capture.
    HANDLE hFile = CreateFile(widestr.c_str(),
                              GENERIC_WRITE,
                              0,
                              NULL,
                              CREATE_ALWAYS,
                              FILE_ATTRIBUTE_NORMAL, NULL);

    // Add the size of the headers to the size of the bitmap to get the total file size
    DWORD dwSizeofDIB = dwBmpSize + sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER);

    //Offset to where the actual bitmap bits start.
    bmfHeader.bfOffBits = (DWORD)sizeof(BITMAPFILEHEADER) + (DWORD)sizeof(BITMAPINFOHEADER);

    //Size of the file
    bmfHeader.bfSize = dwSizeofDIB;

    //bfType must always be BM for Bitmaps
    bmfHeader.bfType = 0x4D42; //BM

    DWORD dwBytesWritten = 0;
    WriteFile(hFile, (LPSTR)&bmfHeader, sizeof(BITMAPFILEHEADER), &dwBytesWritten, NULL);
    WriteFile(hFile, (LPSTR)&bi, sizeof(BITMAPINFOHEADER), &dwBytesWritten, NULL);
    WriteFile(hFile, (LPSTR)lpbitmap, dwBmpSize, &dwBytesWritten, NULL);

    //Unlock and Free the DIB from the heap
    GlobalUnlock(hDIB);
    GlobalFree(hDIB);

    //Close the handle for the file that was created
    CloseHandle(hFile);

    //Clean up
    done:
    DeleteObject(hbmScreen);
    DeleteObject(hdcMemDC);
    return TRUE;
}

然而，事实证明它捕获了主屏幕两次。第二次截屏的屏幕大小与我的第二个显示器相同。我不知道代码哪里出了问题。有人能指出问题或建议更好的方法来完成任务吗？ 谢谢！

- Samuel

Answer 1

您需要从lprcMonitor提供的监视器坐标开始使用BitBlt，而不是从零点开始：

// Bit block transfer into our compatible memory DC.
if (!BitBlt(hdcMemDC,
            0, 0,
            rcClient.right - rcClient.left, rcClient.bottom - rcClient.top,
            hdcMonitor,
            lprcMonitor->left, lprcMonitor->top, // <<--- !!!
            SRCCOPY))

- Roman R.