Advanced MFC Programming

Chapter 11. Sample: Simple Paint
We need to calculate the values of the following members for the new bitmap information header:
biWidth, biHeight and biImageSize. Here the dimension of the new image (biWidth, biHeight) can be
decided from the size of the tracker, and the image size can be calculated from member biBitCout along
with the new image dimension. In the sample, function CGDIDoc::CreateCopyCutDIB()creates a DIB that
is exactly the same with the image contained in the selected area. The following portion of this function
shows how the buffers are allocated and how the bitmap information header is created:
……
……
dwDIBSize=
(
sizeof(BITMAPINFOHEADER)+
GetColorTableSize(bi.biBitCount)*sizeof(RGBQUAD)+
WIDTHBYTES(rect.Width()*bi.biBitCount)*rect.Height()
);
hDIB=::GlobalAlloc(GHND | GMEM_SHARE, dwDIBSize);
ASSERT(hDIB != NULL);
lpBi=(LPBITMAPINFO)::GlobalLock(hDIB);
ASSERT(lpBi != NULL);
lpBi->bmiHeader=bi;
lpBi->bmiHeader.biWidth=rect.Width();
lpBi->bmiHeader.biHeight=rect.Height();
lpBi->bmiHeader.biSizeImage=WIDTHBYTES
(
bi.biBitCount*rect.Width()
)*rect.Height();
The buffer size of new DIB data is calculated and stored in variable dwDIBSize. Here rect stores the
normalized dimension of the current tracker. The current image’s bitmap information header is stored in
variable bi. After copying it into the new buffers, we change members biWidth, biHeight and
biImageSize to new values.
Then, we need to copy the current color table to the newly allocated buffers. Although we could
retrieve the palette from the original DIB data, it may not be up-to-date because the user may change any
entry of the palette by double clicking on the color bar. In the sample, function
CPalette::GetPaletteEntries(…) is called to retrieve the current palette, and is used to create the new
image:
……
……
nPalSize=GetColorTableSize(lpBi->bmiHeader.biBitCount);
lpPalEntry=(LPPALETTEENTRY)new BYTE[nPalSize*sizeof(PALETTEENTRY)];
m_palDraw.GetPaletteEntries(0, nPalSize, lpPalEntry);
for(i=0; ibmiColors[i].rgbRed=lpPalEntry[i].peRed;
lpBi->bmiColors[i].rgbGreen=lpPalEntry[i].peGreen;
lpBi->bmiColors[i].rgbBlue=lpPalEntry[i].peBlue;
lpBi->bmiColors[i].rgbReserved=NULL;
}
delete []lpPalEntry;
Because there is no restriction on the dimension of the tracker, the user can actually select an area that
some portion of it is outside the current image. In order to copy only the valid image, we need to adjust the
rectangle before doing the copy. In the sample, only the intersection between the tracker and the image is
copied, and the rest part of the target image will be filled with the current background color. In function
CGDIDoc::CreateCopyCutDIB(…), the actual rectangle that will be used to obtain the image bit values from
the source bitmap is stored in variable rectSrc, and sizeTgtOffset is used to specify the position where
the image will be copied to the target image. This variable is necessary because the tracker’s upper-left
corner may resides outside the image. The whole image is copied using a loop. Within each loop, one raster
line is copied to the target bitmap. In the function, two pointers are used to implement this copy: before
copying the actual pixels, lpRowSrc is pointed to the source image buffers and lpRowTgt is pointed to the
target image buffers. Their addresses are calculated by adding the bitmap information header size and the
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