From: "Michael Stewart" <mike AT reggin DOT freeserve DOT co DOT uk>
Newsgroups: comp.os.msdos.djgpp
Subject: Polygon scan problem
Date: Sun, 17 Jan 1999 23:36:18 -0000
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To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

I have been having an annoying problem whilst writing a scan convertor.
Hopefully someone will be able to see what I am doing wrong.

The problem I have been having concerns the accuracy of fixed point values.
The routine works as it should while using doubles, however when converting
to fixed point values I loose an enormous amount of accuracy, to much to be
accounted for by the accuracy of a 16.16 fixed point value.

I have tried using Allegros fixed point routines instead of my own and I get
the exactly the same problem. Can anyone see what I am doing wrong ?

The routine I am using is as follows:

struct s_pscan {
   int left, right;
}

void fill_scan_line (int x, int y, int x2, int y2, s_pscan *scan) {
   fixed xinc, ydif, fx, fy;
   int yinc;
   // check for special case first
   if (y == y2) { // horizontal
      if (scan[y].left > x) scan[y].left = x;
      if (scan[y].left > x2) scan[y].left = x2;
      if (scan[y].right < x) scan[y].right = x;
      if (scan[y].right < x2) scan[y].right = x2;
   } else { // any other line
      int y1;
      fixed x1, xinc;

      if (y2 < y) { // make sure we're going in the right direction
         y1 = y;
         y = y2;
         y2 = y1;

         y1 = x;
         x = x2;
         x2 = y1;
      }

     x1 = x;
     xinc = (x2 - x) / (y2 - y);

     for (y1 = y; y1 <= y2; y1++) {
         if (scan[y1].left >int(x1)) scan[y1].left = int(x1);
         if (scan[y1].right < int(x1)) scan[y1].right = int(x1);
         x1 += xinc;
      }
   }
}