Re: howto avoid cdc drawing done not in onPaint to be erased?


"Equilibrium" <ryariv@xxxxxxxxx> wrote in message news:1184150113.833579.37210@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

I'll try to explain.
The reason i need all of this to perform outside the WM_PAINT message
is because the control is updated each 20ms with new object to
draw. the static control main goal is to display a tracking data , so
i recive x.y coordinates each 20ms , and on those coordinates
i need to draw something.
Storing those x,y in a string or an array and then caling WM_PAINT
(via invalidate) , and there to draw them, seems to me quite
expensive / wasteful , especially that the data can flow to the system
for along time.

In othere word what you are saying is, that if we are using the mouse
to free draw on a control ' we need to save each and every point the
mouse x,y position was.

What i need , basically is a way , when the disaplay need to be
"refresh\repaing" , to save the DC , and then restore it on WM_PAINT.

I don't fully understand what you're trying to do, but this may be the reason why you DO need to paint by the standard mechanism.

If your owner draw button contains data members x, y, then you update (x,y) when you need to, and call InvalidateRect().

1. Windows is very clever with WM_PAINT messages: it queues them behind more important things, and amalgamates pending ones (IIRC). So if the program is busy doing other things you may not get a paint message every time you do an InvalidateRect(). So in fact you'll do less redrawing and it will be less "wasteful".

2. What makes you think that doing things by sending messages is such a big overhead? Sending a message to draw something is surely a minimal effort compared with actually drawing it!

In circumstances like this I take the advice (attributed to Donald Knuth) that "Premature optimisation is the root of all evil". Do it the proper way and think about it further if it doesn't work well enough.

Otherwise how much code are you going to write to detect whether someone else's window hasn't just covered yours? And do you really think it will be more efficient than letting Windows do what it was designed to do?

[ Oh, and BTW, in the old days of Windows 3.1, hanging on to a DC a moment longer than necessary was considered by many to be the worst crime in the book. I haven't checked recently but Windows does manage those resources carefully, and it is surely best to let it do so.]

Dave
--
David Webber
Author of 'Mozart the Music Processor'
http://www.mozart.co.uk
For discussion/support see
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