From jar92@ecs.southampton.ac.uk  Wed May  8 15:59:34 1996
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Date: Wed, 8 May 1996 15:59:34 +0200
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From: Jon Rijk <jar92@ecs.southampton.ac.uk>
Message-Id: <Pine.LNX.3.91.960508144811.17774A-100000@soolin>
To: d94marka@isy.liu.se
In-Reply-To: <199605081144.NAA07455@bastet.isy.liu.se>
Subject: Re: Gupta-Sproull!
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On Wed, 8 May 1996 d94marka@se.liu.isy wrote:

> I just wanted to ask if you please could tell
> me how this sort of antialiasing is done. 
> I've heard a little about it, and it sounds
> interesting.

It's all in the Foley, Van Dam book, Computer Graphics
Principles and Practices (or whatever), but just in case
you don't have that or ain't yet understood it here's my
prog I did awhile back:

/*
 *  Antialised line drawing on Silicon Graphics
 *  (C) Jon Rijk, Dec 1995
 */

/*#define TIFF*/

#include <stdlib.h>
#include <stdio.h>
#include <math.h>

#ifdef TIFF
unsigned char **image;
extern void TiffOut(unsigned char **Image, int width, int height, char *name);
#else
#include <gl/gl.h>
#include <gl/device.h>
#endif

#define PI M_PI /* from math.h */

#define swap(a,b) (a)=(a)^(b);(b)=(b)^(a);(a)=(a)^(b)

int alias;
int filter[192];

void GSline(int x0, int y0, int x1, int y1)
{
  long p[2];
  int  i, dx, dy, xi, yi, x=0, y=1, dE, dNE, d;

  int    vdx;
  double rdx, rvdx;

  if((dx=x1-x0)<0) { dx=-dx; xi=-1; } else xi=1;
  if((dy=y1-y0)<0) { dy=-dy; yi=-1; } else yi=1;

  if(dy>dx) { swap(dx,dy); swap(xi,yi); swap(x,y); }

  d=2*dy-dx;
  dE=2*dy;
  dNE=-2*dx; /* dE component has been taken out */

  vdx=0;
  rvdx=1.0/(2.0*sqrt(dx*dx + dy*dy));
  rdx=2.0*dx*rvdx;

  for(i=dx, p[x]=x0, p[y]=y0; i>0; i--) {
#ifdef TIFF
    if(alias) {
      image[p[0]][p[1]]=filter[(int) (128*fabs(vdx*rvdx))];     p[y]+=yi;
      image[p[0]][p[1]]=filter[(int) (128*fabs(rdx-vdx*rvdx))]; p[y]-=yi*2;
      image[p[0]][p[1]]=filter[(int) (128*fabs(rdx+vdx*rvdx))]; p[y]+=yi;
    }
    else image[p[0]][p[1]]=255;
#else
    bgnpoint();
    if(alias) {
      color(filter[(int) (128*fabs(vdx*rvdx))]);     v2i(p); p[y]+=yi;
      color(filter[(int) (128*fabs(rdx-vdx*rvdx))]); v2i(p); p[y]-=yi*2;
      color(filter[(int) (128*fabs(rdx+vdx*rvdx))]); v2i(p); p[y]+=yi;
    }
    else v2i(p);
    endpoint();
#endif

    if(d>0) {
      vdx=d-dx;
      d+=dNE;
      p[y]+=yi;
    }
    else vdx=d+dx;
    d+=dE;
    p[x]+=xi;
  }
}

int main(int argc, char *argv[])
{
  double ang;
  int    i;

  if(argc==1) alias=1; else alias=atoi(argv[1]);
  if(alias<0 || alias>10) alias=1;

#ifdef TIFF
  image = (unsigned char**)calloc(250,sizeof(unsigned char*));
  for (i=0;i<250;i++){
      image[i] = (unsigned char*)calloc(250,sizeof(unsigned char));
  }
#else
  prefsize(250,250);
  if(winopen("Line Drawing")==-1) {
    fprintf(stderr, "Error: Could not open graphics window");
    exit(EXIT_FAILURE);
  }

  for(i=0; i<256; i++) mapcolor(i, i, i, i);
  gflush();
  color(0);
  clear();

  if(!alias) color(255);
#endif

  if(alias==1)            /* Linear Filter function */
    for(i=0; i<192; i++)
      filter[i]=255-(255*i/192);

  else if(alias>1)        /* Gaussian Filter function */
    for(i=0; i<192; i++)
      filter[i]=(int) (255.0*exp(-1.0*pow(i/((alias-1.0)*20.0), 2.0)));

  if(argc==6) {
    GSline(atoi(argv[2]), atoi(argv[3]), atoi(argv[4]), atoi(argv[5]));
  }
  else if(argc==3) switch(atoi(argv[2])) {
    case 1:
      for(ang=0,i=3; ang<=PI/4; ang+=PI/(4*20),i+=5)
        GSline(3, i, (int)(170*cos(ang))+3, (int)(170*sin(ang))+i);
      break;

    case 2:
      GSline(10, 10, 190, 20);
      GSline(10, 30, 190, 40);
      GSline(10, 40, 190, 30);
      break;
  }
  else
    for(ang=0; ang<2*PI; ang+=PI/30)
      GSline(100, 100, (int)(90*cos(ang))+100, (int)(90*sin(ang))+100);

#ifdef TIFF
  TiffOut(image, 250, 250, "out/tif");
#else
  while(!getbutton(ESCKEY));
  gexit();
#endif
}

Sorry it ain't the commented version, if you still don't get what's
going on, I could send you the PostScript of my report on it, soz but
I'm incredibly busy at the mo, I don't really have the time to remind
myself of all the intracies.

BTW how did you discover moi?

-- Jon Rijk --
email : jar92@ecs.soton.ac.uk  or  jar@mail.soton.ac.uk
WWW   : http://whirligig.ecs.soton.ac.uk/~jar92