Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uflorida!haven!rutgers!att!pegasus!ech
From: ech@pegasus.ATT.COM (Edward C Horvath)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Text Drawing at speeds greater than 1200baud
Message-ID: <2534@pegasus.ATT.COM>
Date: 21 Jan 89 20:04:38 GMT
References: <1728@helios.ee.lbl.gov>
Organization: AT&T ISL Middletown NJ USA
Lines: 30

In article <24307@apple.Apple.COM> parent@Apple.COM (Sean Parent) writes:
>There is no way to tell for sure that there are not any windows in front of
>your window. The system owns the screen.

From article <1728@helios.ee.lbl.gov>, by beard@ux1.lbl.gov (Patrick C Beard):
> Not true.  It is very easy.  Just look at the visRgn of the window in
> question.  Clip your optimized drawing directly to the screen to this and
> you won't get in trouble.  I know, I've done it.  It is dirty pool, but
> it does work.

There is NOTHING "dirty pool" about this technique: the visRgn is, quite
precisely, the visible region of your port, and IM-I (Quickdraw) makes it
very clear that your drawing will be clipped to the visRgn (among other
things).  Limiting drawing to the visRgn is a perfectly wonderful (and
safe, presumably forever) means of limiting the cost of updates.

Multifinder takes the responsibility of removing the visRgn of each layer
from that of the layers beneath.  Makes sense, ne?

You never HAVE to draw anything that doesn't intersect the visRgn, nor will
it ever do you any good to draw stuff that isn't in the visRgn.

Also, please note that BeginUpdate "saves" the visRgn, and sets the visRgn
to the intersection of the visRgn and the updateRgn.  EndUpdate restores the
visRgn.  So Beard's "trick" properly limits the work you have to do whenever
there's an update event.

No trick, no dirty pool: good programming practice!

=Ned Horvath=