Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!bellcore!wind!sdh
From: sdh@wind.bellcore.com (Stephen D Hawley)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Alternate screen buffer ... How?
Message-ID: <16888@bellcore.bellcore.com>
Date: 17 Jun 89 01:09:34 GMT
References: <8042@cc.nu.oz> <227700007@uxa.cso.uiuc.edu>
Sender: news@bellcore.bellcore.com
Reply-To: sdh@wind.UUCP (Stephen D Hawley)
Organization: Bellcore, Morristown, NJ
Lines: 185


Here's how to do double buffering.  Remember that this is a hack.
Don't take the animation seriously --it runs just as smoothly on a
single buffered system, I just put it in for a quick demonstration.

This is for LSC, put in a project with mactraps and you should be all
hunky-dorey.

Enjoy.
Steve Hawley
sdh@flash.bellcore.com

PS - if anyone has written a better driver to multiplex sampled sounds
without that annoying 'pop', let me hear about it.

/*
 * Program to do double buffering.
 * This will crash with debuggers and the multifinder and
 * all kinds of other things.  Largely, it follows as many
 * rules as possible from IM, but is bound to break.
 * Steve Hawley
 */

#include <Quickdraw.h>
#define vPage2 6 /* VBL offsets from IM */
#define vBase 0xefe1fe
#define vBufA (512*15)
/* set page one */
#define ScreenOne() asm { BSET #vPage2,vBase+vBufA }
/* set page two */
#define ScreenTwo() asm { BCLR #vPage2,vBase+vBufA }
#define PAGEDIFF 0x8000
/* this is what inside mac says is the difference between
 * pages -probably a bad assumption.
 */

extern int CurPageOption: 0x936; /* current page in low memory */
extern char *MemTop: 0x108;

int *page1, *page2, *currpage;
unsigned int rwords, maxv;
GrafPort myPort;   /* globals used for my own grafport */

SwapPages()
{
	if (currpage == page1) {
		ScreenOne();
		currpage = page2;
	}
	else {
		ScreenTwo();
		currpage = page1;
	}
}

SetUpDouble()
/* set double buffering */
{
	int ref;
	Handle ahandle;
	Str255 ApplName;

	if (CurPageOption != -1) { /* no double paging, restart */
		GetAppParms(ApplName, &ref, &ahandle); /* get application name */
		Launch(-1, ApplName); /* sets up double buffers */
	}
	else {
		HideCursor();
		page1 = (int *)myPort.portBits.baseAddr; /* start of page */
		asm {
			move.l	page1, d0
			sub.l	#PAGEDIFF, d0
			move.l	d0, page2
		}
		maxv = myPort.portBits.bounds.bottom -
			myPort.portBits.bounds.top; /* how many scan lines? */
		rwords = myPort.portBits.rowBytes/2; /* words per row */
		clr(page1, 0x0000);
		clr(page2, 0x0000);
		currpage = page2;
	}
}

clr(ptr, what)
register int *ptr;
register int what;
{
	register long i;
	
	/* fill screen with "what" */
	i = maxv * rwords;
	do {
		*ptr++ = what;
	} while (--i);
}

blitblackbox(x, y, h, where)
register int x, y;
register long *where;
{
	register int sline = rwords * 2;
	
	asm {
		move.w	y, d0		; quick clipping test: is y + h > maxv?
		add.w	h, d0
		cmp.w	maxv, d0
		bls		@0
		move.w	maxv, h
		sub.w	y, h
	@0	muls.w	#4, x		; turn x into long word offsets
		ext.l	x
		add.l	x, where	; offset start
		clr.l	d0
		move.w	y, d0
		muls.w	sline, d0
		add.l	d0, where
	@1	move.l	#0xffffffff, (where)
		add.l	sline, where
		sub.w	#1, h
		bne		@1
	}
}

blitwhitebox(x, y, h, where)
register int x, y;
register long *where;
{
	register int sline = rwords * 2;
	
	asm {
		move.w	y, d0		; quick clipping test: is y + h > maxv?
		add.w	h, d0
		cmp.w	maxv, d0
		bls		@0
		move.w	maxv, h
		sub.w	y, h
	@0	muls.w	#4, x		; turn x into long word offsets
		ext.l	x
		add.l	x, where	; offset start
		clr.l	d0
		move.w	y, d0
		muls.w	sline, d0
		add.l	d0, where
	@1	clr.l	(where)
		add.l	sline, where
		sub.w	#1, h
		bne		@1
	}
}

slide()
{
	static int partial = 32, state = 0;
	register int x, y;
	
	for (x=0; x < 8; x++)
		if ((x + state) & 1) blitwhitebox(x, 0, partial, currpage);
		else  blitblackbox(x, 0, partial, currpage);
	for (y=0; y < 10; y++) {
		for (x=0; x < 8; x++) {
			if ((x + y + state) & 1)
				blitblackbox(x, 32 * y + partial, 32, currpage);
			else
				blitwhitebox(x, 32 * y + partial, 32, currpage);
		}
	}
	partial += 4;
	if (partial > 32) {
		partial = 1; state ^= 1;
	}
}

main()
{

	InitGraf(&thePort); /* set up quickdraw globals */
	OpenPort(&myPort);  /* get my own grafport */

	SetUpDouble();      /* do double buffering */
	while(!Button()) {
		slide();
		SwapPages();
	}
	ScreenOne(); /* put normal page for return to shell */
}