Advanced MFC Programming

Chapter 10. Bitmap
DIB Section
Both DIB and DDB are needed in order to implement transparent background drawing: we need DIB
format to generate mask image, and need DDB to draw the image. Of course we can call ::GetDIBits(…)
and ::SetDIBits(…) to convert between DIB and DDB format, however, there exists an easier way to let
us handle DIB and DDB simultaneously.
A DIB section can be created to manage the image so that we can have both the DIB and DDB features
without doing the conversion. A DIB section is a memory section that can be shared between the process
and the system. When a change is made within the process, it is automatically updated to the system. By
doing this, there is no need to update the data using functions ::GetDIBits(…) and ::SetDIBits(…).
We can call function ::CreateDIBSection(…) to create a DIB section. This function will return an
HBITMAP handle, which can be attached to a CBitmap variable by calling function CBitmap::Attach(…).
Function ::CreateDIBSection(…) has six parameters:
HBITMAP ::CreateDIBSection
(
HDC hdc,
CONST BITMAPINFO *pbmi, UINT iUsage, VOID *ppvBits, HANDLE hSection,
DWORD dwOffset
);
Parameter
hdc
pbmi
iUsage
ppvBits
hSection
dwOffset
Meaning
The handle of the device context.
A BITMAPINFO type pointer that provides the bitmap information.
Specifies data type contained in member bmiColors of structure BITMAPINFO, which is
pointed by pbmi. If the value is DIB_PAL_COLORS, array bmiColors contains logical palette
indices; if the value is DIB_RGB_COLORS, array bmiColors contains literal RGB values.
A pointer that can be used to receive device-independent bitmap’s bit values.
Specifies the section handle, we can pass a file handle to it to allow the bitmap to be mapped
to the file. If we pass NULL, operating system will automatically allocate memory for
bitmap mapping.
Specifies the offset from the beginning of the file to the bitmap data if the bitmap will be
mapped to a file. The parameter will be neglected if hSection is NULL.
After calling this function, we can access the buffers pointed by ppvBits and make change to the DIB
bits directly, there is no need for us to do any DIB to DDB conversion or vice versa. After the change is
made, we can draw the bitmap immediately by calling funciton CDC::BitBlt(…), this will draw the updated
image to the window.
New Variables
The following new variables are declared in class CGDIView for drawing bitmap with transparancy:
class CGDIView : public CScrollView
{
protected:
BOOL m_bBitmapLoaded;
CBitmap m_bmpBkd;
CBitmap m_bmpDraw;
CBitmap m_bmpMask;
CBitmap m_bmpBS;
CBitmap *m_pBmpOld;
CBitmap *m_pBmpMaskOld;
CBitmap *m_pBmpBSOld;
CDC m_dcMem;
CDC m_dcMemMask;
CDC m_dcMemBS;
CPalette *m_pPalOld;
CPalette *m_pPalMaskOld;
CPalette *m_pPalBSOld;
CGDIView();
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