Advanced MFC Programming

Chapter 11. Sample: Simple Paint
Using Region
Like any other GDI object, before using the region, we must first select it into the DC. The difference
between using region and other GDI objects is that we need to call function CDC::SelectClipRgn(…) to
select the region instead of calling function CDC::SelectObject(…).
Function CDC::SelectClipRgn(…) has two versions:
virtual int CRgn::SelectClipRgn(CRgn* pRgn);
int CRgn::SelectClipRgn(CRgn* pRgn, int nMode);
For the second version of this function, parameter nMode specifies how to combine the new region with
the region being currently selected by the DC. Again, it can be set to any of the following flags: RGN_AND,
RGN_COPY, RGN_DIFF, RGN_OR and RGN_XOR.
After using the region, we must select it out of the DC before deleting it. To select a region out of the
DC, we can call function CDC::SelectClipRgn(…) and pass a NULL pointer to it. For example, the
following statement selects the region out of DC (pointed by pDC):
pDC->SelectClipRgn(NULL);
A region can be deleted by calling function CGDIObject::DeleteObject(). Also, the destructor of
CRgn will call this function automatically, so usually there is no need to delete the region unless we want to
reinitialize it.
Sample
Sample 11.6\GDI is a standard SDI application generated by Application Wizard. In the sample, two
variables are declared in class CGDIView: CGDIView::m_rgnRect and CGDIView::m_rgnEllipse. They will
be used to create two regions, one is rectangular and one is elliptical. The two regions will be combined
together to create a new region that is the difference of the two. We will select this region into the client
window’s DC, and output text to the whole window. As we will see, only the area that is within the region
will have the text output.
The regions are created in the constructor of class CGDIView:
CGDIView::CGDIView()
{
m_rgnRect.CreateRectRgn(0, 0, 400, 400);
m_rgnEllipse.CreateEllipticRgn(50, 50, 350, 350);
m_rgnRect.CombineRgn(&m_rgnRect, &m_rgnEllipse, RGN_DIFF);
}
The final region will look like the shaded area shown in Figure 11-6.
Figure 11-6. Combine a rectangular region and an elliptical region together
In function CGDIView::OnDraw(…), string “Clip Region” is output repeatedly until all the client
window is covered by this text:
void CGDIView::OnDraw(CDC* pDC)
{
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