使用 MSDN 库进行多屏幕捕获

Question

我正在使用多个显示单元进行屏幕捕获。由于 GetDesktopWindow() 仅获取主监视器的句柄，我尝试使用 EnumDisplayMonitors() 来完成这项工作。

看了MSDN网站，在main()中写了这些：

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，我复制了示例函数“ int CaptureAnImage(HWND hWnd)" from MSDN:Capturing an Image 并做了以下修改：

我没有读取 HWND 参数，而是在函数中声明了它并使用 GetDesktopWindow() 对其进行了初始化
删除了用于拉伸设备上下文的代码
使用参数 hdcMonitor 作为设备上下文
为 RECT 使用参数 lprcMonitor
添加了用于生成唯一文件名的代码

完整代码如下：

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。第二次捕获的屏幕尺寸与我的第二台显示器的屏幕尺寸相同。我不知道代码有什么问题。任何人都可以指出或建议更好的方法来完成任务吗？谢谢！

Answer 1

您需要从 lprcMonitor 中提供给您的监视器坐标 BitBlt，而不是从零点：

// 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))

使用 MSDN 库进行多屏幕捕获

multiple screen capture with MSDN library

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