mfc, Drawing on CBitmap

microsoft.public.vc.mfc - Microsoft Visual C


mfc >> Drawing on CBitmap

by lynnzyeo » Tue, 21 Mar 2006 12:57:25 GMT

Hi, I have the following code extract:

In CView constructor:
m_Bitmap.CreateBitmap( 32, 32, 1, 16, NULL );

In OnDraw():
BITMAP BM;
m_Bitmap.GetObject( sizeof( BM ), &BM );

CPoint size( BM.bmWidth, BM.bmHeight );
pDC->DPtoLP( &size );

CPoint org( 0, 0 );
pDC->DPtoLP( &org );

CDC MemDC;
MemDC.CreateCompatibleDC( NULL );
CBitmap *POldBitmap = MemDC.SelectObject( &m_Bitmap );
MemDC.SetMapMode( pDC->GetMapMode() );

pDC->BitBlt( 0, 0, 32, 32, &MemDC, 0, 0, SRCCOPY );

MemDC.SelectObject( POldBitmap );

In OnTimer:
CClientDC ClientDC( this );

CDC MemDC;

CBitmap* pOldBitmap;

MemDC.CreateCompatibleDC( &ClientDC );

pOldBitmap = MemDC.SelectObject( &m_Bitmap );

MemDC.PatBlt( 0, 0, 32, 32, BLACKNESS );

MemDC.Ellipse( 2, 2, 30, 30 );

MemDC.MoveTo( 0, 0 );
MemDC.LineTo( 100, 500 );

However, nothing is drawn. The screen remains blank. I have no idea
where is thye problem. Can someone pls help? Thanks alot!


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mfc >> Drawing on CBitmap

by AliR. » Tue, 21 Mar 2006 14:13:20 GMT


There are a few things wrong with the code.

Most important one is in your OnTimer
You have to select the oldbitmap back into the Memory DC. And also you
want to probably invalidate the view so that the OnDraw method gets
called

In OnTimer:

CClientDC ClientDC( this );
CDC MemDC;
CBitmap* pOldBitmap;
MemDC.CreateCompatibleDC( &ClientDC );
pOldBitmap = MemDC.SelectObject( &m_Bitmap );
MemDC.PatBlt( 0, 0, 32, 32, BLACKNESS );
MemDC.Ellipse( 2, 2, 30, 30 );
MemDC.MoveTo( 0, 0 );
MemDC.LineTo( 100, 500 );

////////// select the old bitmap back into MemoryDC
MemDC.SelectObject(pOldBitmap);

//////// force window to redraw
Invalidate();
UpdateWindow();

The next thing is the bitmap creation, and the memory DC creation.

You are creating the Bitmap with the assumtion that the Device is in 16
bit color mode.

You might want to move the creation into OnInitialUpdate and change it
to

CDC *pDC = GetDC();
m_Bitmap.CreateCompatibleBitmap(pDC, 32, 32);
ReleaseDC(pDC);

and in your OnDraw you are creating your Memory DC with NULL parameter.
Should probably change it to:


MemDC.CreateCompatibleDC( pDC );

AliR.



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***************************************
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