将窗口内容保存到位图的C++窗口函数

Question

我正在尝试编写一个涉及处理从窗口捕获的数据的C++程序。我是有意用windows编写的，所以请不要推荐已经存在的库来简化这个任务。无论如何，为了进行测试，我编写了一个函数，它应该接受一个hwnd句柄和一个hdc设备上下文，并使用该信息创建由hwnd引用的窗口工作区的位图图像文件。我编写的代码很大程度上是基于此示例代码来自MSDN的。我只是暂时尝试捕捉数据，而不是在屏幕上显示或以任何方式修改它。我还对代码进行了大量的注释，以了解我认为我对每一行都做了什么。如果我在这里有什么根本的误解，请告诉我。

void screenshot(HWND hwnd, HDC hdc){
HDC clientarea = hdc; // The device context for the client area of the window
HDC memory = NULL; // client context for storing the bitmap in memory
RECT clientdim; // Defines memeory location for dimensions of client window
GetClientRect(hwnd, &clientdim); // Stores the client dimesnion info at clientdim location
BITMAP img; // Points to the where the bitmap handle will write it's data once it contains the bitmap data for the client window

HBITMAP clienthandle = CreateCompatibleBitmap(clientarea, clientdim.right - clientdim.left, clientdim.bottom - clientdim.top); // Bitmap handle, actual data of the rectangle defined at clientdim

SelectObject(memory, clienthandle); // Selecting the bitmap handle into the memory context allows us to bitblit the bitmap data to the bitmap handle

BitBlt(memory, 0, 0, clientdim.right - clientdim.left, clientdim.bottom - clientdim.top, clientarea, 0, 0, SRCCOPY);
// The bitmap handle now contains the data defined by the rectangle coordinates, which the dimensions of the hwnd client area
// The source and desitnation nXdest, nYdest, etc. fields are with respect the the window, not the screen, so they are 0

// Gets the bitmap data from the handle, and stores it to a memory location as an actual bitmap file
GetObject(clienthandle, sizeof(BITMAP), &img);

// Info head for the bitmap. Information about the dimesions and colors
BITMAPINFOHEADER bmih;
bmih.biSize = sizeof(BITMAPINFOHEADER); // Sets the size of the header literally to the size of an info header. 
bmih.biWidth = img.bmWidth; // Sets the width of the bitmap to the width of the bitmap in memory
bmih.biHeight = img.bmHeight; // Sets the height of the bitmap to the height of the bitmap in memory
bmih.biPlanes = 1; // number of planes must be 1 for bitmaps to save
bmih.biBitCount = 32; // 32-bit color. Means RGB dat is stored per pixel.
bmih.biCompression = BI_RGB; // Tells it to use the RGB compression, as defined by the 32bit pixels
bmih.biSizeImage = 0; // Size of compression buffer. RGB is uncompressed, so is 0 for now
bmih.biXPelsPerMeter = 0; // Defines the number of pixels per meter in the x axis. Only used by some programs to select images, we don't care.
bmih.biYPelsPerMeter = 0; // Same as above, but fore y axis.
bmih.biClrUsed = 0; // the number of color indexes the bitmat uses. 0 means it uses all of them
bmih.biClrImportant = 0; // The number of color indexes actually required to display a pixel. 0 means we need all of them.

DWORD bitmapsize = ((img.bmWidth * bmih.biBitCount + 31) / 32) * 4 * img.bmHeight; // Total size of the bitmap. Not sure why this works, will look into it

HANDLE hDIB = GlobalAlloc(GHND, bitmapsize); // A handle to heap memory of size bitmapsize where our bitmap is processed. GHND = Initializes memory to zero an
                                            //allows location to translate to pointer with globallock, called such because it locks the memory in place within the heap
                                            // hDIB because it is a Device Independent Bitmap

char *lpbitmap = (char *)GlobalLock(hDIB); // Stores the contents of the char at the memory locattion defined by GlobalLock(). Basically, this is the bitmap in the heap.

GetDIBits(clientarea, clienthandle, 0, (UINT)img.bmHeight, lpbitmap, (BITMAPINFO *)&bmih, DIB_RGB_COLORS); // Gets the bits from the bitmap and copies them to the buffer found at lpbitmap

HANDLE hFile = CreateFile(L"capture.bmp", GENERIC_WRITE, 0, NULL, CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL); // Creates file with various attributes. Check the documentation on MSDN.

DWORD dibSize = bitmapsize + sizeof(BITMAPFILEHEADER) + sizeof(BITMAPINFOHEADER); //Size of the dib is the size of the actual bitmap plus the size of its headers

BITMAPFILEHEADER bmfh; // Fileheader for the bitmap file
bmfh.bfType = 0x4D42; // Defines file as bitmap. 0x4D42 is the code for this
bmfh.bfSize = dibSize; // The total size is everything in the DIB
bmfh.bfReserved1 = 0; // These reserved values must be zero
bmfh.bfReserved2 = 0;
bmfh.bfOffBits = (DWORD)sizeof(BITMAPFILEHEADER) + (DWORD)sizeof(BITMAPINFOHEADER); // Offset to where the image begins. It begins after the header data

// File headers are written first, then the actual bitmap data.
DWORD byteswritten = 0; //Used in writefile commands to accept number of bytes written. I'd have to use an overlapped structure otherwise

WriteFile(hFile, (LPSTR)&bmfh, sizeof(BITMAPFILEHEADER), &byteswritten, NULL); // Look at documentation for remind on what the hell is happening here
WriteFile(hFile, (LPSTR)&bmih, sizeof(BITMAPINFOHEADER), &byteswritten, NULL);
WriteFile(hFile, (LPSTR)lpbitmap, bitmapsize, &byteswritten, NULL);

// Unlcok DIB from the heap, then free it's memory back up
GlobalUnlock(hDIB);
GlobalFree(hDIB);

// Close the file handle
CloseHandle(hFile);

// Delete all of the objects and release the device contexts
DeleteObject(memory);
ReleaseDC(hwnd, clientarea);

// Donezo!!

HDC hdc是由hwnd指示的窗口的设备上下文。我将这两种方法传递给函数，而不是仅仅在函数本身中使用GetDC(hwnd)，因为我已经从代码的其他地方获得了设备上下文。我想这更有效率。

任何指导都会很有帮助。谢谢。

编辑:有人指出，我在最初的帖子中忘记了实际的问题。我遇到的问题是，这段代码是保存一个仅仅是一个黑匣子的位图。它实际上并不捕获窗口的内容。

Answer 1

这是一个适合我的版本。

void screenshot(HWND hWnd,wchar_t* fileName){
    HBITMAP hbmScreen{NULL};
    BITMAP bmpScreen;
    DWORD dwBytesWritten = 0;
    DWORD dwSizeofDIB = 0;
    HANDLE hFile = NULL;
    char* lpbitmap = NULL;
    HANDLE hDIB = NULL;
    DWORD dwBmpSize = 0;

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

    // Create a compatible DC, which is used in a BitBlt from the window DC.
    const HDC hdcMemDC = CreateCompatibleDC(hdcScreen);

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

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

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


    // Create a compatible bitmap from the Window DC.
    hbmScreen = CreateCompatibleBitmap(hdcScreen, 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,
        hdcScreen,
        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;

    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.
    hDIB = GlobalAlloc(GHND, dwBmpSize);
    lpbitmap = (char*)GlobalLock(hDIB);

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

    // A file is created, this is where we will save the screen capture.
    hFile = CreateFile(fileName,
        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.
    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.

    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);
    ReleaseDC(hWnd, hdcScreen);
}