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From: c8637197@cc.nu.oz (Phil the Grk)
Newsgroups: comp.sys.mac.programmer
Subject: Alternate screen buffer ... How?
Message-ID: <8042@cc.nu.oz>
Date: 13 Jun 89 23:56:41 GMT
Organization: University of Newcastle
Lines: 26

Hi guys, I'm another new one on the net.  I have a question about the alternate
screen buffer.  How do you get to use it?  In IM, it says that the segment
loader makes it usable if a flag is set.  Great.  Beautiful.  But how?  IM
states that _Chain and _Launch check a value to see which screen buffers are
required, which implies (to me anyway) that there is some information that the
Finder uses to set the appropriate values up.  Once again, how?

I'm writing a game, you see, and I want to have a shoot-em-up thing as part of
the game and I use the whole screen for this.  So I have two 21888 byte blocks
which I use.  The first is the background scene.  This gets copied onto the
"canvas" and the "animated"  objects are drawn on to the canvas.  Then I do a
blockmove from the canvas to thePort's bitmap.  (I know, it won't work on a
different size screen, but I'm working on it.)  In any case I would like to
use the alternate screen buffer and install a little doodad as a Vertical
Blanking Task so that the alternate screen buffer will be the "canvas" until
I've finished drawing there, then the main screen buffer will be the canvas.
It would be a lot quicker doing a simple bitset or whatever than it is copying
22K, I think.

Thanks in advance for any information anyone has.  BTW, it costs us undergrads
money to receive mail from outside - about $160 per 1000 blocks. Neither are we
permitted to mail offsite.  So could any responses be short or posted?  Thanks. 
If (when?) I receive anything I'll summarize and post ... it if it's not already
there ...

						Phil.