Advanced MFC Programming

Chapter 10. Bitmap
handlers are added for both ID_FILE_SAVE and ID_FILE_SAVE_AS commands, and the corresponding
member functions are implemented as follows:
void CGDIDoc::OnUpdateFileSave(CCmdUI* pCmdUI)
{
pCmdUI->Enable(FALSE);
}
void CGDIDoc::OnUpdateFileSaveAs(CCmdUI* pCmdUI)
{
pCmdUI->Enable(m_bmpDraw.GetSafeHandle() != NULL);
}
It seems unnecessary to conver the DDB to DIB before saving the image to a disk file because its
original format is DIB. However, if the DDB is changed after being loaded (This is possible for a graphic
editor application), the new DDB is inconsistent with the original DIB data.
The DDB to DIB converting procedure is a little complex. If we are programming for Windows 95 or
Windows NT 4.0, we can create DIB section (will be introduced in later sections) to let the format be
converted automatically. If we are writing Win32 programs that will be run on Windows 3.1, we must use
the method discussed in this section to implement the conversion.
10.5 Drawing DIB Directly
The counterpart function of ::GetDIBits(…) is ::SetDIBits(…), it can be used to convert DIB data to
device dependent bitmap. The two functions provide us a way of implementing image editing: whenever
we want to make change to the image, we can first retrieve DIB data from the DDB, edit the DIB data, and
set it back to DDB.
New Functions
Sometimes it is easier to edit DDB directly instead of using DIB data. For example, if we want to
reverse every pixel of the image, we can just call one API funciton to let this be handled by lower level
driver instead of editting every single pixel by ourselves. This is why we need to handle both DIB and
DDB in the applications.
If our application is restricted on edittng only DIB data, we can call an API function directly to draw
DIB in the client window. By doing so, we eleminte the complexity of converting DIB to DDB back and
forth. This function is ::SetDIBitsToDevice(…), which has the following format:
int ::SetDIBitsToDevice
(
HDC hdc,
int XDest, int YDest, DWORD dwWidth, DWORD dwHeight,
int XSrc, int YSrc,
UINT uStartScan, UINT cScanLines,
CONST VOID *lpvBits, CONST BITMAPINFO *lpbmi, UINT fuColorUse
);
There are altogether 12 parameters, whose meanings are listed in the following table:
Parameter
hdc
Xdest, Ydest
dwWidth, dwHeight
uStartScan
UscanLines
lpvBits
Lpbmi
Meaning
Handle of target DC.
Position on the target DC where the image should be drawn.
Image size on the target DC.
Starting scan line of the source bitmap image.
Number of scan lines in the source bitmap image that will be output to the target.
Pointer to the DIB bits (image data).
Pointer to BITMAPINFO type object.
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