Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ncar!ames!oliveb!apple!Apple.COM!lsr
From: lsr@Apple.COM (Larry Rosenstein)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Flickering cursor
Message-ID: <1484@internal.Apple.COM>
Date: 20 Apr 89 21:41:55 GMT
Sender: usenet@Apple.COM
Organization: Objects-R-Us,  Apple Computer, Inc.
Lines: 27
References:<5522@microsoft.UUCP> <2735@carthage.cs.swarthmore.edu>

In article <2735@carthage.cs.swarthmore.edu> jackiw@cs.swarthmore.edu 
(Nick Jackiw) writes:
> I think it's more likely that the bits _beneath_ the cursor are being
> redrawn.  If this happens, they overwrite the image of the cursor, and
> then the VBL has to put the cursor back on top of them: flicker.  The

You are right about the cause of the flicker (I think), but your 
explanation is backwards.

The way things happens is that Quickdraw calls the routine ShieldCursor 
which hides the cursor if it intersects given rectangle.  It then calls 
ShowCursor when the drawing is finished.

I've also encountered one case in which Quickdraw calls ShieldCursor 
unnecessarily.  If you do a CopyBits on a Mac Plus (I never tried other 
machines, but think this was fixed), the cursor will be shielded even if 
the copy is entriely offscreen.  When I wrote my paint demo in MacApp, I 
couldn't figure out why the cursor would be hidden for certain operations, 
and this turned out to be the problem.  (It was especially noticeable 
because the CopyBits operation took 1.5 seconds and the cursor was hidden 
the whole time.)

Larry Rosenstein, Apple Computer, Inc.
Object Specialist

Internet: lsr@Apple.com   UUCP: {nsc, sun}!apple!lsr
AppleLink: Rosenstein1