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

Question

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);
}

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. — Blues76, Jul 29 '12 at 02:49
`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`. — OJ., Jul 29 '12 at 03:26
nullptr is part of c++ 11 which some of it works in vs 2010. No managed code oj — Blues76, Jul 29 '12 at 04:09
The lprect is declared twice because of a copy paste miskake. — Blues76, Jul 29 '12 at 04:11
if I type an answer to my question, should it be in my main post or a new one? I did it here. — Blues76, Jul 31 '12 at 16:00
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. — Adrian McCarthy, Jul 31 '12 at 18:49

score 4 · Accepted Answer · edited May 23 '17 at 11:59

4

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.

edited May 23 '17 at 11:59

Community

1
1

answered Jul 29 '12 at 03:32

Asaf

4,317
28
48

A question about the handle: HANDLE hSource = ?Should it be HBITMAP and would it be HBITMAP = CreateCompatibleBitmap(...) ? – Blues76 Jul 29 '12 at 15:28
According to the documentation the second parameter is indeed HBITMAP – Asaf Jul 30 '12 at 10:48

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

1 Answers1