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From: ujmurphy@KING.MCS.DREXEL.EDU (Jim Murphy)
Newsgroups: comp.sys.apple2
Subject: QuickDraw II
Message-ID: <9105181855.AA17719@mcs.drexel.edu>
Date: 18 May 91 18:55:53 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 28

albert erroneously wrote:

> quickdraw does draw faster to off-screen grafports because it doesn't have
> to do the 2/4-bit conversion when drawing stuff on-screen (did i get this
> right dave?).

     Wrongo! When drawing to _any_ GrafPort (onscreen or otherwise), QuickDraw
II uses the portInfo record (LocInfo structure) of the GrafPort to determine
what kind of pixel map it is dealing with. It uses the portSCB word to draw to
the port's pixImage at the proper bit depth. This is at either 2 or 4 bit
depth, depending on the portSCB value.

     When QuickDraw II draws to a pixImage, it does not care where in memory
the image is. It has been designed to draw to pixMaps anywhere. Whether the
memory space that QD II has been asked to draw to does not correspond to 
screen memory (either $E1 or $01) does not matter. QD II makes no optimizations
when drawing to offscreen pixMaps.

     I'd suggest that you re-read the beginning of the QuickDraw chapter in
the Apple IIGS Toolbox Reference, Vol. II. You seem to be a bit confused as to
what QuickDraw II actually does. Checking out IIGS Technote #80, "QuickDraw II
Clipping" would also probably be a pretty good idea.


Jim Murphy                             "I know that you believe you understand
Internet:  ujmurphy@mcs.drexel.edu     what you think I said. But I am not sure
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