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From: stc7@cunixb.cc.columbia.edu (Steven T Chiang)
Subject: Re: ANIMATION AND SCANLINES
Message-ID: <1993Feb11.011220.8858@news.columbia.edu>
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Reply-To: stc7@cunixb.cc.columbia.edu (Steven T Chiang)
Organization: Columbia University
References: <8549.61.uupcb@uttsbbs.uucp>
Date: Thu, 11 Feb 1993 01:12:20 GMT
Lines: 56

In article <8549.61.uupcb@uttsbbs.uucp> paul.parkhurst@uttsbbs.uucp (Paul Parkhurst)  writes:
>-> Does anybody out there have the code to detect where the current
>-> scanline on the GS screen is at so I could draw an image to the
>-> screen without scanline flicker. Currently, the C/asm code I have
>-> waits until a vertical blank before it draws to the screen, but that
>-> slows it down.

	I guess I missed this post the first time around.  Syncing, by
nature means that you are waiting for the beam to hit a certain line
or vblank.  It doesn't matter which way you sync, you'll end up
waiting for a certain amount of time.

	However, if the problem is you don't have enough time to
update the area, there are better ways of syncing, see below.

>   What you can do is set the scanline interrupt bit for a particular
>line (usually, a line after what it is you want to draw).  When you hit
>the interrupt, draw what needs to be drawn.  Since the interrupt occured
>on the line after what you are drawing, you should have plenty of time
>to update the upper part of the screen before its redrawn.
>
>   I did this in a long-lost project of mine and it worked fine.  No
>flicker in the animation at all.

	I suppose this would work.  However, scanline interrupts
aren't the highest priority in the interrupt manager's chain.  In DG,
we patch the interrupt manager to make it the highest priority.  Also,
there might be reasons why you wouldn't want interrupts enabled.

Here is a sync routine that will exit when beam has refreshed the line
# stored in location (word loc).

	(16 bit mode)

lup
	ldal 	$e1c02e
	and 	#$80ff
	dec
	xba
	asl
	cmp 	location ;(line number
	bne	lup

If you check the docs on c02e and c02f, you'll find that the LSB of
the counter is stored in c02f, which is why you AND off the
unimportant bits, and do the xba asl.

Basically, if the upper corner of your sprite is at line 30, you could
monitor the beam until it reaches line 30 and start drawing at that
point.  By doing this, you gain about 170 * amt of time to scan one
line worth of time.

Steven Chiang                  Order - DreamGrafix:  The ONLY 16/256/3200
stc7@cunixb.cc.columbia.edu       color paint program for the Apple IIGS.
America On_Line:  DWS Steve
Genie:  S.Chiang4              Apple IIGS Forever!