Advanced MFC Programming

Chapter 10. Bitmap
pBmp->GetBitmap(&bm);
bi.biSize=sizeof(BITMAPINFOHEADER);
bi.biWidth=bm.bmWidth;
bi.biHeight=bm.bmHeight;
bi.biPlanes=bm.bmPlanes;
bi.biBitCount=bm.bmPlanes*bm.bmBitsPixel;
bi.biCompression=BI_RGB;
bi.biSizeImage=0;
bi.biXPelsPerMeter=0;
bi.biYPelsPerMeter=0;
bi.biClrUsed=0;
bi.biClrImportant=0;
dwSizeCT=GetColorTableSize(bi.biBitCount);
dwDibLen=bi.biSize+dwSizeCT*sizeof(RGBQUAD);
pPalOld=dc.SelectPalette(&m_palDraw, FALSE);
dc.RealizePalette();
hDib=::GlobalAlloc(GHND, dwDibLen);
lpBi=(LPBITMAPINFO)::GlobalLock(hDib);
……
lpBi->bmiHeader=bi;
We first fill the information obtained previously into a BITMAPINFOHEADER object. This is necessary
because when calling function ::GetDIBits(…), we need to provide a BITMAPINFOHEADER type pointer
which contains useful information. Here, some unimportant members of BITMAPINFOHEADER are assigned 0s
(biSizeImage, biXPelsPerMeter…). Then the size of the color table is calculated and a global memory
that is big enough for holding bitmap information header and color table is allocated, and the bitmap
information header is stored into the buffers. We will use these buffers to receive color table.
Although the memory size for storing bitmap data can be calculated from the information already
known, usually it is not done at this point. Generally the color table and the bitmap data are retrieved
separately, in the first step, only the memory that is big enough for storing structure BITMAPINFOHEADER and
the color table is prepared. When color table is being retrieved, the bitmap information header will also be
updated at the same time. Since it is more desirable to calculate the bitmap data size using the updated
information, in the sample, the memory size is updated after the color table is obtained successfully, and
the global memory is reallocated for retrieving the bitmap data.
We also need to select logical palette into the DC and realize it so that the bitmap pixels will be
intepreted by its own color table.
Function ::GetDIBits(…) is called in the next step to recieve BITMAPINFOHEADER data and the color
table. Because some device drivers do not fill member biImageSize (This member carries redunant
information with members biWidth, biHeight, and biBitCount), we need to calculate it if necessary:
……
……
VERIFY
(
::GetDIBits
(
dc.GetSafeHdc(),
(HBITMAP)pBmp->GetSafeHandle(),
0,
(WORD)bi.biHeight,
NULL,
lpBi,
DIB_RGB_COLORS
)
);
bi=lpBi->bmiHeader;
::GlobalUnlock(hDib);
if(bi.biSizeImage == 0)
{
bi.biSizeImage=WIDTHBYTES(bi.biBitCount*bi.biWidth)*bi.biHeight;
}
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