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From: psfales@ihlpe.ATT.COM (Pete Fales)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Graphics driver
Message-ID: <2000@ihlpe.ATT.COM>
Date: Fri, 28-Aug-87 10:51:09 EDT
Article-I.D.: ihlpe.2000
Posted: Fri Aug 28 10:51:09 1987
Date-Received: Sun, 30-Aug-87 01:34:12 EDT
References: <1096@ark.cs.vu.nl> <2436@bnrmtv.UUCP> <1099@ark.cs.vu.nl>
Organization: AT&T Bell Laboratories - Naperville, Illinois
Lines: 71

In article <1099@ark.cs.vu.nl>, lesmem@cs.vu.nl (Marco Lesmeister) writes:
> 
>   Can somebody post a fast line drawing algorithm that
> uses direct memory writes to CGA memory.
> 
> 		Thanks in advance,  Marco Lesmeister.

I won't make any claims about how fast this algorithm is relative to 
other potential candidates, but it is pretty good.  I didn't write it
(it was published in the IBM Journal of Research and Development), but
I have had several occasions to use it over the years, including some
assembly language implementations.  It uses only integer arithmetic
and multiplications only by powers of 2.

---------------------------- cut here ------------------------------

int xm1[] = { 1, 0, 1, 0,-1, 0,-1, 0 };
int ym1[] = { 0, 1, 0,-1, 0, 1, 0,-1 };
int xm2[] = { 1, 1, 1, 1,-1,-1,-1,-1 };
int ym2[] = { 1, 1,-1,-1, 1, 1,-1,-1 };

/* Bresenham (spelling?) algorithm to draw a line from (x1,y1) to (x2,y2) */

void 
absline(x1,y1,x2,y2,col)
{
	int m,deltax,deltay,e,cx,savedx,t;

	m=0;
	deltax=x2-x1;
	if ( deltax<0 ) {
		m=4;
		deltax = -deltax;
	}
	deltay=y2-y1;
	if (deltay<0 ) {
		m += 2;
		deltay = -deltay;
	}
	if ( deltax - deltay < 0 ) {
		m++;
		t=deltax;
		deltax=deltay;
		deltay=t;
	}

	deltay *= 2; /* OK, use a shift if you really want speed */
	e = deltay-deltax;
	savedx = deltax;
	deltax *= 2;

	for ( cx=savedx+1 ; cx >= 1 ; cx-- ) {
		scr_wdot(x1,y1,col);
		if ( e > 0 ) {
			x1 += xm2[m];
			y1 += ym2[m];
			e += deltay-deltax;
		}
		else {
			x1 += xm1[m];
			y1 += ym1[m];
			e += deltay;
		}
	}
}

-- 
Peter Fales		UUCP:	...ihnp4!ihlpe!psfales
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