[Q] How do I reduced screen flicker in this small app, How n

Alain Barrette -- abarrette@sth1.maska.net
Tuesday, January 09, 1996

Using Msvc++2.0 & Mfc3.0 under w95 I generated using Appwizard a Dlg
based application.

In the application I set up a timer of 1 sec on which I call OnPaint()
And in Onpaint I put the following code.

CMyDlg::OnPaint() {	

CTime TmeNow 	= CTime::GetCurrentTime();
CString TxtAff 	= TmeNow.Format("%H:%M:%S");
CRect RectAff;	GetClientRect(RectAff);

CPaintDC dc(this);

dc.SetTextAlign(TA_BASELINE|TA_CENTER);
dc.SetBkMode(OPAQUE /*TRANSPARENT*/);
dc.TextOut(	RectAff.Width() / 2, 	RectAff.Height() / 2, 
   TxtAff, 		TxtAff.GetLength());
InvalidateRect(RectAff,FALSE);
}

However, as simple as the code may sound, there seem to be 2 problems
with this approach. 

First, there is lot of screen flicking on the redraw of the string via
InvalidateRect(). Yup, the screen refresh is not clean at all. 

Since I am a complete newbie in mfc prgming (but not to prgramming in
general) I can't thing of what to do next. So here I am, What should
be done in order to reduce the flicker ? There should be a better way
to do it...

I get out and buy the microsoft press book for mfc 3.0, and take a 
look at the member fct() of CDC, but woa... I am lost in the jungle 
here...

Second, my small app seem to take 100% cpu. :) And I can't find out
why. While system monitor is running, I can see that the processor
utilization jump to 100 when I start my app...  Any idea as why do I
get this behavior, and what is needed to correct this ? It seem to me
that the app is not doing much in each call to the onpaint(), so
what's causing this ?

Thanks in advance...
       ,,,         
      (o o)      
--oOOo-(_)-oOOo--abarrette@sib.qc.ca

---

Don.Irvine@net-tel.co.uk
Thursday, January 11, 1996

[Mini-digest: 6 responses]

> First, there is lot of screen flicking on the redraw of the string via
> InvalidateRect(). Yup, the screen refresh is not clean at all. 

Do all your drawing to an off screen DC, then BitBlt the results to the
screen DC.

There is a good article describing this on the MSDN under Technical
Articles/Windows Articles/GDI and OpenGL Articles/Flicker Free Displays using
an Off-screen DC.


Don Irvine

-----From: "David W. Gillett" 

> InvalidateRect(RectAff,FALSE);
> 
> First, there is lot of screen flicking on the redraw of the string via
> InvalidateRect(). Yup, the screen refresh is not clean at all. 
> 
> Second, my small app seem to take 100% cpu. :) And I can't find out
> why. While system monitor is running, I can see that the processor
> utilization jump to 100 when I start my app...  Any idea as why do I
> get this behavior, and what is needed to correct this ? It seem to me
> that the app is not doing much in each call to the onpaint(), so
> what's causing this ?

  BeginPaint(), called by the framework to set up the DC for your 
OnPaint method, validates the entire client area.  When you then 
InvalidateRect, you immediately give your app a new invalid region, 
which will cause the system to send you a WM_PAINT as soon as it has 
nothing else to do.  And when each time you handle a WM_PAINT, you 
generate one, you wind up with an app that is busy painting whenever 
there's a spare cycle.

  Try calling InvalidateRect from your timer callback or WM_TIMER 
handler, and let *that* trigger the WM_PAINT, rather than calling 
OnPaint() yourself.  If other processing is delaying updates of the 
displayed time -- probably not, since you're surprised to be using 
100% of the CPU -- then follow this InvalidateRect with a call to 
UpdateWindow ("send WM_PAINT right NOW").

Dave

-----From: darius@world.std.com (Darius Thabit)

> In the application I set up a timer of 1 sec on which I call OnPaint()
> And in Onpaint I put the following code.
> 
> CMyDlg::OnPaint() {	
> 
> CTime TmeNow 	= CTime::GetCurrentTime();
> CString TxtAff 	= TmeNow.Format("%H:%M:%S");
> CRect RectAff;	GetClientRect(RectAff);
> 
> CPaintDC dc(this);
> 
> dc.SetTextAlign(TA_BASELINE|TA_CENTER);
> dc.SetBkMode(OPAQUE /*TRANSPARENT*/);
> dc.TextOut(	RectAff.Width() / 2, 	RectAff.Height() / 2, 
>    TxtAff, 		TxtAff.GetLength());
> InvalidateRect(RectAff,FALSE);
> }

   I also have problems with flicker in my app using a timer to update a 
time value.  But I think first of all I don't think you want to call 
Invalidate Rect from inside OnPaint, because that will cause OnPaint to be 
called again - which will call InvalidaeRect again, and so on ... :-)  This 
may explain your 100% cpu usage and part of your flicker.  You might try 
calling InvalidateRect from OnTimer; I use an edit control, and I'm just 
calling pEditCtrl->SetWindowText(formatted_time) from OnTimer.

-----From: David Stidolph 

When you Invalidate a rectangle you are asking the system to cause a =
WM_PAINT message to be sent (and thus call OnPaint).  Get rid of your =
InvalidateRect call.  You might also want to override the =
OnEraseBackground message (or whatever it is called).  That clears the =
background before you begin painting it - thus the flicker.

-----From: Brad Wilson 

>> In the application I set up a timer of 1 sec on which I call OnPaint()

You shouldn't call OnPaint() directly to make a paint happen.  You
should instead call InvalidateRect() from OnTimer() to cause the
OnPaint() to happen.

>> And in Onpaint I put the following code.

>> InvalidateRect(RectAff,FALSE);

InvalidateRect() causes a repaint.  In your paint function you call
InvalidateRecT().  This would probably cause circular paint
messages to keep showing up all the time (although I'm not sure of
the effect of the implicit EndPaint(), which may end up validating
the rectangle).

So, you should

   1. Call InvalidateRect() from OnTimer() (don't call OnPaint())
   2. Don't call InvalidateRect() from OnPaint()

>> Second, my small app seem to take 100% cpu. :)

That's the circular paint thing I mentioned above.  The system is under
the impression that your screen is never painted, so your app eats the
system for lunch constantly repainting.

I would recommend that you pick up some books on beginning Windows
programming (even if they're not related to MFC).  Such as:

   Programming Windows
   Charles Petzold
   Microsoft Press
   ISBN 1-55615-264-7

The last version I saw was for Windows 3.1, but the techniques you
learn can be easily applied to Win32.  He is supposed to have a book
out on Win95, but every time I check the date keeps getting pushed
back for the book.  :-(

There are also some great Microsoft Press books available on using
MFC for Windows programming.  No definitive titles come to my
faltering mind :-) ... but I know Mike B. wrote one (which I have
not yet read).

Good luck!
Brad

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-----From: "Aravind Balakrishnan(abalakri@us.oracle.com)" 

The InvalidateRect() in your OnPaint() handler is the culprit!! When you use 
the CPaintDC() in OnPaint(), it ensures that all invalidated areas are 
validated.  You create a brand new invalidated region by callind 
InvalidateRect() in your OnPaint(), which causes a WM_PAINT, and hence your 
app is getting a whole lotta WM_PAINTS! 
 
One way of doing what you are doing: 
 
Move the InvalidateRect() from OnPaint() to OnTimer().  
 
Now, WM_PAINT is called once every second, which is not so bad.. 
 
Hope this helps! 
 
Cheers,   
Aravind.   
===   
 
Aravind Balakrishnan  
Systems Management Products, Oracle Corp. 
(415)506-0432 
 

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