Double Buffering, Reduce flicker of child control


I have CWnd that wraps a third party control (TX4OLE).  Currently the
third party control paints to the screen and our code draws on top of
it.  This process can produce a bit of flicker and I am trying to
eliminate this using double buffering.  Is there any way I can
redirect the output of the control into a memory buffer?  I already
handle WM_PAINT calls in the window proc, but there doesn't seem to be
any way to change the device context the control uses.  Any help or
ideas are appreciated.


3/5/2007 4:43:01 PM
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On Mar 5, 11:43 am, "Jay Hamilton" <> wrote:
> Hello,
> I have CWnd that wraps a third party control (TX4OLE).  Currently the
> third party control paints to the screen and our code draws on top of
> it.  This process can produce a bit of flicker and I am trying to
> eliminate this using double buffering.  Is there any way I can
> redirect the output of the control into a memory buffer?  I already
> handle WM_PAINT calls in the window proc, but there doesn't seem to be
> any way to change the device context the control uses.  Any help or
> ideas are appreciated.
> Thanks,
> Jay

Greets Jay,

We have had an identical issue. We are considering different
but the current 'fix'  (read:  hack)  makes a considerable difference
if you have
no other alternatives.

Note that this approach will only help providing:
- The image you are drawing on top of does not change very often
- You are able to detect when the image does change (without having to
examine pixels)
- A single flicker whenever the image changes is acceptable

Just by reading the above restrictions may give you some insight on
the method I
am proposing. In essense, you take a snapshot of the image whenever it
changed. For every other WM_PAINT you recieve (until the image changes
you perform double-buffering using your snapshot.

1)  Intercept WM_PAINT message before it reaches the 3rd party control
2)  Determine if the 3rd party control image has changed
   if yes:  allow 3rd party control to process WM_PAINT, then copy the
entire client rect to a memory buffer  (snapshot), then continue to
step 3
   if no :  do not redirect WM_PAINT to 3rd party control, continue to
step 3
3)  Copy snapshot to backbuffer
4)  Draw your custom content into backbuffer
5)  Blit backbuffer to window DC
6)  ValidateRect(hWnd, NULL)   to signify that the paint was handled
(otherwise you may get a never-ending flood of WM_PAINT)

*Note that blitting the backbuffer to the window DC is likely going to
be the slowest operation. If maintaining decent performance is
you can use clipping regions  (NOT update regions)  to give a
substantial speed boost to the backbuffer->winDC step;   we
experienced a 10x performance increase after generating clip regions
and selecting them into the winDC.

4a)  CLIP REGIONS   (Extra Credit)    ;)
  i)  Generate a combined HRGN that minimally contains all the pixels
that need to be drawn. For lines, polygons, ellipses, etc. you can
'record' a path when drawing and create a region from this using
PathToRegion(). For text or other complex shapes you can simply add a
rect that encapsulates the complex shape.
 ii)  Combine this HRGN with a previously saved one;  but keep the
uncombined one separate so you can store it later as the previously
saved for next WM_PAINT. This is for 'undrawing' the old contents.
iii)  Select the combined clip region into the window DC

If you are crazy or desperate enough to follow through with this
technique, which does work, I have one final piece of advice:
CombineRgn(RGN_OR)       in GDI+    Region::Union).
The algorithm is bugged and you will lose entire sections when the
regions are unioned. Proof of this bug at this URL:

To merge regions you will need to use them inverted  (ie:  instead of
starting with empty region and using RGN_OR/Union() to add to it,
start with infinite region and RGN_DIFF/Exclude() from it). Before
selecting into DC as a clip region, invert it  (XOR it with an
infinite region).

Best of luck,


dpeterson1 (10)
3/5/2007 9:20:47 PM
Will this article help?


Ravi Bhavnani

"Jay Hamilton" <> wrote in message
> Hello,
> I have CWnd that wraps a third party control (TX4OLE).  Currently the
> third party control paints to the screen and our code draws on top of
> it.  This process can produce a bit of flicker and I am trying to
> eliminate this using double buffering.  Is there any way I can
> redirect the output of the control into a memory buffer?  I already
> handle WM_PAINT calls in the window proc, but there doesn't seem to be
> any way to change the device context the control uses.  Any help or
> ideas are appreciated.
> Thanks,
> Jay

ravib (5)
3/5/2007 9:46:36 PM
I found a working solution after some trial and error, this approach
seems to work great for me.  I also had to remove all calls to the
text control's Refresh fxn, but otherwise this is pretty straight
forward.  Thanks to those who replied, it helped point me in the right

void MyDerivedTextEditor::OnPaint()
	CDC* pDC = GetDC();

	CRect rectClient(0,0,0,0);

	CDC memDC;

	CBitmap bmp;
	bmp.CreateCompatibleBitmap(pDC, rectClient.Width(),


	// Custom draw on top of memDC, e.g. circle some text

	// Paint to the buffer
	pDC->BitBlt(0, 0, rectClient.Width(), rectClient.Height(), &memDC, 0,


BOOL MyDerivedTexteditor::OnEraseBckgnd()
	return TRUE;

3/6/2007 11:05:54 PM

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