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Problems painting into memory device context with GDI

Posted on 2004-08-26
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Last Modified: 2013-11-20
Here is a snippet of some GDI code, which uses a memory device context and copies it onto the view device context.  It appears to work fine but it's wrong since the old bitmap pointer is not saved:

CDC xdc;
CBitmap xbitmap;

xdc.CreateCompatibleDC(pDC);
xbitmap.CreateCompatibleBitmap(pDC, tex, wry);
xdc.SelectObject(xbitmap);
//...Draw onto xdc
//...BitBlt is used

Here is the code including the fixed resource leak.  It creates an error when SelectObject does not return a valid CBitmap pointer.

xdc.CreateCompatibleDC(pDC);
xbitmap.CreateCompatibleBitmap(pDC, tex, wry);
CBitmap* oldbitmap = xdc.SelectObject(xbitmap);
//...Draw onto xdc
//...BitBlt is used
//...
xdc.SelectObject(oldbitmap);

Of course, SelectObject takes a pointer to the object, so here is the fixed line.  Now it doesn't work at all; after the BitBlt operaton, the view is still empty.

CBitmap* oldbitmap = xdc.SelectObject(&xbitmap);

What am I doing wrong with this GDI code?
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Question by:Alkali_Guy
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11 Comments
 
LVL 6

Expert Comment

by:nabehs
ID: 11909327
i dont see anything wrong with the code, maybe the problem in BitBlt. can u post it?
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Author Comment

by:Alkali_Guy
ID: 11909501
pDC->BitBlt( 0, 0, mex, cry, &xdc, 0, 0, SRCCOPY );
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Author Comment

by:Alkali_Guy
ID: 11909517
I have only seen this code work when I pass the xbitmap object to SelectObject, instead of passing its pointer the way I am supposed to.
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LVL 5

Author Comment

by:Alkali_Guy
ID: 11909542
Other information:
Similar code later in the same function works fine, so what is currently happening is:

memdc   >-BitBlt->  xdc  >-BitBlt->  *pDC

I have also tryed CreateCompatibleBitmap passing &xdc instead of pDC.
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LVL 6

Expert Comment

by:nabehs
ID: 11909557
R U declaring xbitmap as CBitmap* xbitmap? It should be CBitmap xbitmap; I know that it is posted here correctly but maybe in your original code it is not.

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Author Comment

by:Alkali_Guy
ID: 11909600
xbitmap is declared correctly in the code.
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LVL 5

Author Comment

by:Alkali_Guy
ID: 11916041
To reiterate the code where the fault is:

xdc.CreateCompatibleDC(pDC);
xbitmap.CreateCompatibleBitmap(pDC, x, y);
CBitmap* pOldBitmap = xdc.SelectObject( &xbitmap );

//Draw stuff

pDC->BitBlt( 0, 0, x, y, &xdc, 0, 0, SRCCOPY );
xdc.SelectObject(oldbitmap);

Note that x and y are not necessarily the width and height of *pDC.  Do I have to convert coordinates before use?
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LVL 5

Author Comment

by:Alkali_Guy
ID: 11916058
[The last line in above code should read:
xdc.SelectObject(pOldBitmap);]
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Accepted Solution

by:
Alkali_Guy earned 0 total points
ID: 11916200
I found the problem:
I accidentally restored the pOldBitmap (oldbitmap) into xdc before BitBlting it onto the screen.  Since the above code is correct, there was no indication here of incorrect order in my own code.
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