Thread: Windows GetDIBits Call Not Working - I'm desperate - please help - I'm begging!

Hi there,

I have a nightmare with the GetDIBits call. It is working 95% of the time, but occasionally fails inexplicably. I say inexplicably, because it returns zero (fail result) but the Windows GetLastError() call says there was no error afterwards.

Can anyone help? I have put some of my code below, where 'src' is the HBITMAP. Is possible that HBITMAP could be being used by another area of windows at the time? If so, how can I release it?

Thanks

Code:

  HDC hdc = 0;
  BITMAPINFO bmi;
  memset(&bmi, 0, sizeof(BITMAPINFO));
  bmi.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);

  // Make temporary compatible device
  hdc = CreateCompatibleDC(0);

  // Get dimensions first
  GetDIBits(hdc, (HBITMAP)src, 0, 0, 0, &bmi, DIB_RGB_COLORS);

  // We always want positive height (bottom up bmp)
  bmi.bmiHeader.biHeight = abs(bmi.bmiHeader.biHeight);

  bmi.bmiHeader.biBitCount = 24;
  bmi.bmiHeader.biCompression = BI_RGB;
  
  // Create memory
  data = new char[bmi.bmiHeader.biSizeImage];

   if (GetDIBits(hdc, (HBITMAP)src, 0, bmi.bmiHeader.biHeight, data,
       &bmi, DIB_RGB_COLORS) == 0)
   {
FAILS HERE
with GetLastError == 0 (OK?)
   }

Read the manual page disclaimer

biSizeImage
Specifies the size, in bytes, of the image. This may be set to zero for BI_RGB bitmaps.

Windows 98/Me, Windows 2000/XP: If biCompression is BI_JPEG or BI_PNG, biSizeImage indicates the size of the JPEG or PNG image buffer, respectively.

In both these cases, you're not counting pixels

Since you're passing "RGB" to GetDIBits, perhaps it's actually 0

[edit]We both quoted the manual in our answers, possibly there is a message in that somewhere.[/edit]

Code:

  bmi.bmiHeader.biBitCount = 24;

It is your responsibility to calculate the size of the bitmap bits as the native bitmap format may not be 24bpp and also:

Originally Posted by MSDN BITMAPINFOHEADER

biSizeImage
Specifies the size, in bytes, of the image. This may be set to zero for BI_RGB bitmaps.

The code to calculate the required size, taking into account DWORD alignment, is:

Code:

    bmi.bmiHeader.biSizeImage = ((bmi.bmiHeader.biWidth * bmi.bmiHeader.biBitCount + 31) & ~31) / 8
                                  * abs(bmi.bmiHeader.biHeight);

Originally Posted by Davros

I say inexplicably, because it returns zero (fail result) but the Windows GetLastError() call says there was no error afterwards.

Unfortunately, the bitmap functions seem not to call SetLastError() on many failure paths.

Originally Posted by Davros

Is possible that HBITMAP could be being used by another area of windows at the time? If so, how can I release it?

You'd have to tell us that. It is true that the bitmap can't be selected into any DC when you call GetDiBits().

Thanks for the replies

>In both these cases, you're not counting pixels

The code I put up was simplified, because I didn't expect people to read through reems of the stuff & made the mistake in the simplification. I've pasted my more complete code below.

>Unfortunately, the bitmap functions seem not to call SetLastError()

That's helpful. Thanks. It saves me wasting time trying to figure out why there is no error.


>You'd have to tell us that. It is true that the bitmap can't be selected into any DC when you call GetDiBits().

I'm not sure because the HBITMAP I am using is coming from a third-party control (TBitmap if that helps). But I think that is where the problem is. Is there any way I could unselect a HBITMAP from a device context?

I have in fact read the manual, but still gotta admit I'm not 100% sure about device contexts. For example, I am calling CreateCompatibleDC(0) to a device of the screen. Is this what I should be doing?

Code:

int Gfx::create(Gfx& dst, void* src, int cnt, int y)
{
  // Creates gfx data and fills it with contents of bitmap from
  // platform specific bitmap object src. At the moment, this method
  // only works under windows. Does nothing under any other platform.

  // Under windows, src is a HBITMAP & hdSrc is a HDC.
  // Gfx memory data which must be destroyed when finished by calling
  // destroy(). cnt is number of rows to retrieve. If this is <= 0,
  // full bmp height is retrieved. y is row to scan from which defaults
  // to zero if omitted. hdSrc maybe null, in which case screen is used.
  // If this method fails, rslt.data is NULL(0) & returns
  // platform specific error code. Error is -1 if operation
  // is not supported on platform.
  int err;
  if (y < 0) y = 0;

  // Ensure existing is destroyed.
  destroy(dst);

#ifdef AIDAWIN
  // WINDOWS IMPLEMENTATION
  // hndl must not be 'selected into a device'
  err = 0;
  HDC hdc = 0;
  BITMAPINFO bmi;
  memset(&bmi, 0, sizeof(BITMAPINFO));
  bmi.bmiHeader.biSize = sizeof(BITMAPINFOHEADER);

  try
  {
    // Make temporary compatible device
    hdc = CreateCompatibleDC(0);

    // Get dimensions first
    GetDIBits(hdc, (HBITMAP)src, 0, 0, 0, &bmi, DIB_RGB_COLORS);

    // We always want positive height (bottom up bmp)
    bmi.bmiHeader.biHeight = abs(bmi.bmiHeader.biHeight);

    // Work out full size count if needed
    if (cnt <= 0) cnt = bmi.bmiHeader.biHeight - y;

    // Fill data
    dst.pDim.x = bmi.bmiHeader.biWidth;
    dst.pDim.y = cnt;

    // Check we can handle range
    err = ERROR_INVALID_PARAMETER;

    if (y + cnt <= bmi.bmiHeader.biHeight && cnt > 0)
    {
      // Set pixel format to be retrieved
      bmi.bmiHeader.biBitCount = 24;
      bmi.bmiHeader.biCompression = BI_RGB;
      unsigned int sz = dst.rowsz() * dst.pDim.y;

      if (sz > 0)
      {
        // Create memory array
        dst.pData = new char[sz];

        err = 0;
        if (GetDIBits(hdc, (HBITMAP)src, (WORD)y, (WORD)cnt, dst.pData,
          &bmi, DIB_RGB_COLORS) == 0)
        {
          // Error - destroy memory and
          // error code will be set below
          destroy(dst);
ShowMessage("Y " + IntToStr(y));
ShowMessage("CNT : " +IntToStr(cnt));

        throw Exception("Error " + IntToStr(GetLastError()));
        }
      }
    }

    // If no data - set error code
    if (dst.pData == 0)
    {
      dst.pDim = Axy();
      if (err == 0) err = GetLastError();
    }

    // Free temporary device context
    DeleteDC(hdc);
  }
  catch(...)
  {
    // Clean-up
    destroy(dst);
    DeleteDC(hdc);
    throw;
  }
#else
  // NOT SUPPORTED
  err = -1;
#endif

  return err;
}
//---------------------------------------------------------------------------