Win32 Create Bitmap from Device Context To File and/or BLOB

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You can use GetDIBits(). This function copies the image pixels data into your own allocated memory.

See GetDIBits and loop through pixels using X, Y for some more details and explanation.

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Blues76
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Blues76

Updated on June 04, 2022

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  • Blues76
    Blues76 almost 2 years

    I would like to get the context of my window into a bitmap. I use the window to draw basic lines with touch. The problem that I'm having is my bitmap is black. Which it is probably due to the fact that I'm not grabbing the device context properly or doing something else wrong.

    The functions CreateBitmapInfoStruct and CreateBMPFile are from this MSDN example.

    Also note that g_hWnd is a global variable that has the handle of the window for which I want to save the picture.

    My end goal is to be able to save the bitmap into a mysql field (BLOB) that I have. This is what my original problem was. Anyway, I started by first trying to create a BMP to file.

    I have searched here and in other places. The best solution I found was recommended here following this MSDN example. However, it is not working.

    Any help for this specific problem and/or in writing to the bitmap into a blob into mysql table, will be greatly appreciated.

    Here is my code:

    HDC hDC = GetDC(g_hWnd); 
    
    LPRECT rect = (LPRECT)malloc(sizeof(RECT)); 
    GetWindowRect(g_hWnd,rect); 
    int h = rect->right - rect->left;
    int w = rect->bottom - rect->top; 
    LPRECT rect = (LPRECT)malloc(sizeof(RECT)); 
    GetWindowRect(g_hWnd,rect); 
    HBITMAP hBmp = CreateCompatibleBitmap(hDC,w,h); 
    PBITMAPINFO pbmi;
    pbmi = CreateBitmapInfoStruct(g_hWnd,hBmp); 
    
    CreateBMPFile(g_hWnd, TEXT("c:\\TEMPO\\TestG2.bmp"), pbmi, 
                  hBmp, hDC) ;
    
    ReleaseDC(g_hWnd,hDC); 
    
    DeleteObject(hBmp); 
    DeleteObject(pbmi); 
    if (rect != nullptr)
        free(rect); 
    

    EDIT:

    The actual answer for capturing the screen (getDC) is to modify this sample in MSDN.

    I have modified the sample here (remove the stretch) Note that still uses the goto, which I will remove.
    A note about the GOTO... WHile I don't use it, I don't find it to be a problem either. I think too much has been made about the GOTO statement... like if in assembly, we wouldn't do goto (JUMPS)

    Here is the code:

    void saveBitmap()
    {
        HDC hdcScreen;
        HDC hdcWindow;
        HDC hdcMemDC = NULL;
        HBITMAP hbmScreen = NULL;
        BITMAP bmpScreen;
    
        // Retrieve the handle to a display device context for the client 
        // area of the window. 
        hdcScreen = GetDC(NULL);
        hdcWindow = GetDC(g_hWnd);
    
        // Create a compatible DC which is used in a BitBlt from the window DC
        hdcMemDC = CreateCompatibleDC(hdcWindow); 
        if(!hdcMemDC)
        {
        goto done;
        }
        RECT rcClient;
        GetClientRect(g_hWnd, &rcClient);
        hbmScreen = CreateCompatibleBitmap(hdcWindow, rcClient.right-rcClient.left, 
                                           rcClient.bottom-rcClient.top);
        if(!hbmScreen)
        {
            goto done;
        }
    
        SelectObject(hdcMemDC,hbmScreen);
    
        if(!BitBlt(hdcMemDC, 
                   0,0, 
                   rcClient.right-rcClient.left, rcClient.bottom-rcClient.top, 
                   hdcWindow, 
                   0,0,
                   SRCCOPY))
        {
            goto done;
        }
    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"c:\\tempo\\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
    done:
        DeleteObject(hbmScreen);
        DeleteObject(hdcMemDC);
        ReleaseDC(NULL,hdcScreen);
        ReleaseDC(g_hWnd,hdcWindow);
    }
    
    • Blues76
      Blues76 over 11 years
      I think I have to redraw the device context when I do it. Same as I'm doing it under WM_PAINT. I will not be able to check until Tuesday, but I have a feeling that is that. At the same time, I feel that If the content is drawn in the windows, there must be a direct way of getting it.
    • OJ.
      OJ. over 11 years
      if (rect != nullptr) <- what is nullptr ? Is that a local var? Or is this an indication that you're doing something with managed C++? ALso, you're declaring rect twice which would result in a compiler error. I would advise not creating an LPRECT using malloc, instead just declare a local RECT on the stack and pass &rect.
    • Blues76
      Blues76 over 11 years
      nullptr is part of c++ 11 which some of it works in vs 2010. No managed code oj
    • Blues76
      Blues76 over 11 years
      The lprect is declared twice because of a copy paste miskake.
    • OJ.
      OJ. over 11 years
      AH yes of course. C++11. My bad, thanks for the clarification.
    • Blues76
      Blues76 over 11 years
      if I type an answer to my question, should it be in my main post or a new one? I did it here.
    • Adrian McCarthy
      Adrian McCarthy over 11 years
      Note that your cleanup code is deleting the memory bitmap while it's still selected into the memory DC. In general, this is dangerous. You should select the bitmap out (or use a SaveDC/RestoreDC on the memory DC) before deleting the bitmap. This is a common error, and it seems newer versions of Windows have been hardened against it, but I've seen it cause problems on XP and older versions.
    • Blues76
      Blues76 over 11 years
      Can you give an example? This code was taken from msdn.
  • Blues76
    Blues76 over 11 years
    A question about the handle: HANDLE hSource = ?Should it be HBITMAP and would it be HBITMAP = CreateCompatibleBitmap(...) ?
  • Asaf
    Asaf over 11 years
    According to the documentation the second parameter is indeed HBITMAP