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Path: utzoo!utgpu!water!watmath!clyde!rutgers!ames!amdahl!dlb!dana!rap
From: rap@dana.UUCP
Newsgroups: comp.sys.amiga
Subject: Re: Rapid Stimulus Displaying on the Amiga Help.  Solution
Message-ID: <152@dana.UUCP>
Date: Wed, 22-Apr-87 16:23:51 EST
Article-I.D.: dana.152
Posted: Wed Apr 22 16:23:51 1987
Date-Received: Sat, 25-Apr-87 07:37:19 EST
References: <8704200221.AA03819@cory.Berkeley.EDU>
Organization: Dana Computer, Inc., Sunnyvale, CA
Lines: 49
Summary: If re-rendering into non-displayed plane, it does matter...

To do rapid screen updating and not fool around with ScreenToFront and so on,
Matt suggests the following:

In article <8704200221.AA03819@cory.Berkeley.EDU>, dillon@CORY.BERKELEY.EDU (Matt Dillon) writes:
> 
> 	Use a two plane screen, then render bitmap A into plane 1, and 
> 	bitmap B into plane 2 .  The idea is that then all you need to
> 	do to switch bitmaps is set the color registers:
> 
> 	plane:	2 1	color register
> 		0 0	0	always background color
> 		0 1	1	depends 
> 		1 0	2	depends
> 		1 1	3	doesn't matter
> 
> 	Step 1:	Set all color registers to background color
> 	Step 2: Render bitmap A into plane 1
> 		Render bitmap B into plane 2
> (more)
>	(.... talking about using SetRGB4 to alternate which of
>	      two color registers gets the non-background color)

First, congratulations Matt... I was going to work out some whizbang
swapping of pointers (not necessarily, but possibly Intuition-compatible)
that would do the task, but you found a simpler and more general method.

But... (tiny one, but important), this must be a monocolor technique,
where the color in register 3 DOES matter if the user wants to effectively
be double-buffering.  That is, if he draws into the invisible plane
while displaying the visible one, then there could easily be places
where the bits of plane 2 and plane 1 would overlap, thereby selecting
color number 3.  Thus:

	color register 0 = background color always.

	While drawing into plane 1:
		color register 1 = background color (so is not seen)
		color register 2 = COLOR (being displayed)
		color register 3 = COLOR (being displayed)

					so that any bits coincident
					between plane 1 and plane 2
					select color 3 which is the
					same as the color currently
					being displayed.

But, again, a neat way of attacking the problem!

Rob Peck		...hplabs!dana!rap