在 Windows 上使用 C++ 裁剪 HBITMAP

Question

我有一个HBITMAP 持有一个窗口的屏幕截图。现在我想从中裁剪出某个区域/矩形并将其作为新的HBITMAP 返回。

然而，以下代码仅将图像压缩到正确的新矩形大小，但不会对其进行裁剪：

HBITMAP crop_image(const RECT rectangle, const HBITMAP source_image)
{
    const auto h_clone = static_cast<HBITMAP>(CopyImage(source_image, IMAGE_BITMAP, rectangle.right - rectangle.left,
                                                        rectangle.bottom - rectangle.top, LR_CREATEDIBSECTION));

    const auto hdc_mem = CreateCompatibleDC(nullptr);
    const auto hdc_mem2 = CreateCompatibleDC(nullptr);

    const auto h_old_bmp = static_cast<HBITMAP>(SelectObject(hdc_mem, source_image));
    const auto h_old_bmp2 = static_cast<HBITMAP>(SelectObject(hdc_mem2, h_clone));

    BitBlt(hdc_mem2, 0, 0, rectangle.right - rectangle.left, rectangle.bottom - rectangle.top,
           hdc_mem, rectangle.left, rectangle.top, SRCCOPY);

    SelectObject(hdc_mem, h_old_bmp);
    SelectObject(hdc_mem2, h_old_bmp2);

    DeleteDC(hdc_mem);
    DeleteDC(hdc_mem2);

    return h_clone;
}

如何修复我的代码以根据需要裁剪图像？

Answer 1

由于我很长时间没有在Win32上工作，我不确定这是否可行，所以让我们试试吧。

这是一个想法：

1. 创建一个新的内存 DC。
2. 创建一个新的位图，而不是从源中克隆。
3. BitBlt 将源位图的目标矩形放入刚刚创建的矩形中。

创建新的 DC 和位图

使用CreateCompatibleDC，就像创建内存 DC 一样。然后用CreateCompatibleBitmap 创建一个新的位图。位图的宽度和高度将是裁剪位图的大小。然后用SelectObject将其选入DC。

从源中复制目标矩形

现在您需要使用CreateCompatibleDC 为源位图创建一个新的DC，并使用SelectObject 将源位图选择到其中。然后使用BitBlt 将源位图的矩形复制到刚刚创建的位图中。目的地x 和y 将为零，因为我们想从左上角开始绘制。源x 和y 是您要从源位图开始复制的位置。

Answer 2

您的代码可以简化为：

HBITMAP crop_image(const RECT rectangle, const HBITMAP source_image)
{
    const auto h_clone = static_cast<HBITMAP>(CopyImage(source_image, IMAGE_BITMAP, rectangle.right - rectangle.left,
                                                        rectangle.bottom - rectangle.top, LR_CREATEDIBSECTION));    

    return h_clone;
}

删除的代码没有任何用处。测试后，我可以得到裁剪区域，而不是缩小的屏幕截图。因为你没有提供完整的代码，所以我无法得出更多结论。

截屏的过程，我们可以根据官方的例子轻松修改。

Capturing an Image

示例中详细描述了所使用的 API。

只需要修改部分代码，比如更改裁剪区域，示例中还展示了如何将裁剪区域保存为文件。

int CaptureAnImage(HWND hWnd)
{
    HDC hdcScreen;
    HDC hdcWindow;
    HDC hdcMemDC = NULL;
    HBITMAP hbmScreen = NULL;
    BITMAP bmpScreen;
    RECT rc;
    rc.left = 0;
    rc.top = 0;
    rc.right = 800;
    rc.bottom = 600;

    // Retrieve the handle to a display device context for the client 
    // area of the window. 
    hdcScreen = GetDC(NULL);
    hdcWindow = GetDC(hWnd);

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

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

    // Get the client area for size calculation
    RECT rcClient;
    GetClientRect(hWnd, &rcClient);

    //This is the best stretch mode
    SetStretchBltMode(hdcWindow, HALFTONE);

    //The source DC is the entire screen and the destination DC is the current window (HWND)
    if (!StretchBlt(hdcWindow,
        0, 0,
        rcClient.right, rcClient.bottom,
        hdcScreen,
        0, 0,
        GetSystemMetrics(SM_CXSCREEN),
        GetSystemMetrics(SM_CYSCREEN),
        SRCCOPY))
    {
        MessageBox(hWnd, L"StretchBlt has failed", L"Failed", MB_OK);
    }

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

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

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

    // Bit block transfer into our compatible memory DC.


    if (!BitBlt(hdcMemDC,
        rc.left, rc.top,
        rc.right - rc.left, rc.bottom - rc.top,
        hdcWindow,
        0, 0,
        SRCCOPY))
    {
        MessageBox(hWnd, L"BitBlt has failed", L"Failed", MB_OK);
    }



//  HBITMAP bmpnew = crop_image(rc, hbmScreen);

    // 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(hdcWindow, 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(L"captureqwsx.bmp",
        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
    DeleteObject(hbmScreen);
    DeleteObject(hdcMemDC);
    ReleaseDC(NULL, hdcScreen);
    ReleaseDC(hWnd, hdcWindow);

    return 0;
}

它会得到当前屏幕的截屏（800x600）文件。