Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!gatech!ncsuvx!ecemwl!jnh
From: jnh@ecemwl.ncsu.edu (Joseph N. Hall)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Offscreen Bitmap / MultiFinder
Message-ID: <3436@ncsuvx.ncsu.edu>
Date: 19 Jul 89 15:47:06 GMT
References: <14226@swan.ulowell.edu> <3427@ncsuvx.ncsu.edu> <42913@bbn.COM>
Sender: news@ncsuvx.ncsu.edu
Reply-To: jnh@ecemwl.UUCP (Joseph N. Hall)
Organization: North Carolina State University
Lines: 20

In article <42913@bbn.COM> levin@BBN.COM (Joel B Levin) writes:
 In article <3427@ncsuvx.ncsu.edu> jnh@ecemwl.UUCP (Joseph N. Hall) writes:
 |Well, first of all, look up SetPortBits in IM vol. 1.  You can use the window's
 |own GrafPtr; just save the value of its portBits and then
 |SetPortBits(yourBits).
 
 Well, no; I've been bitten by this.  You get into trouble with
 following IM I's advice here.
 ... My experience
 with trying to use SetPortBits to swap bitmaps gave me trouble because
 the offscreen bitmap got clipped (it was in the background under
 multifinder).

Yeah, true, this can happen ... I should have pointed this out. 
It also irritates me that there's no accessor function for OBTAINING the
value of your portBits, thus your code depends upon the window structure
remaining the same throughout antiquity ... not the most dangerous assumption
you could make, but still less advisable than finding a trap-only method
of implementing your algorithm.  Taking all this into consideration,
I guess I'd recommend starting off with a fresh GrafPtr ...