Path: utzoo!utgpu!cunews!cognos!alzabo!andras
From: andras@alzabo.ocunix.on.ca (Andras Kovacs)
Newsgroups: comp.sys.acorn
Subject: Re: Drawing lines in C
Message-ID: <1991Feb25.081646.15077@alzabo.ocunix.on.ca>
Date: 25 Feb 91 08:16:46 GMT
References: <8671@castle.ed.ac.uk>
Reply-To: andras@alzabo.UUCP (Andras Kovacs)
Organization: Brian's XENIXlings, Ottawa, Canada
Lines: 141

In article <8671@castle.ed.ac.uk> ecwu61@castle.ed.ac.uk (R Renwick) writes:
>	I want to plot lines on the screen in C, not under the wimp. 
>How can I do it???  I can't seem to find an example anywhere.

  Here is an implementation of Bresenham's algorithm for line drawing.
Please don't judge my programming from this example :-); it is just to pass
on some code to the needy. Anyway, this program will draw random lines on
the screen. It does not do clipping; if your x coordinates are out of screen,
then it will wrap around; if the y ones are out of bounds then BOMB!BOMB!
I put the screen pointer into a global 'cause I would hate to pass it
everytime on the stack. As you observe there is some code duplicated in the
Line routine; it is an attempt to optimize by trading size for speed.
The plot is inlined to speed things up a bit.
  This implementation draws around 1400 lines per second (including the loop
overhead and the - significant - time required to generate the random
coordinates); a carefully coded and little bit cheating assembly version does
approx. 2200 lines/sec (in the same test bed). (John Kortink will understand
this; Tiggr and some others will try to squeeze the same performance out of
their C compilers but they are doomed :-) ).
  There is lots of ways to do clipping on different levels; you have to
choose for yourself the best one for the particular problem - but it will
slow down the line drawing (obviously).

---- cut here ----
#include <Arthur.h>
#include <stdio.h>
#include <stdlib.h>

void Line(int x1, int y1, int x2, int y2, char col);

int InputBlock[] = {149, -1};
char *screen;

int main(int argc, char *argv[])
{
   int i, limit;
   reg_set regs;

   if (argc != 2)
   {
      printf("Usage: Lines <# of lines>\n");
      return -1;
   }

   mode(13);

   regs.r[0] = (int)InputBlock;
   regs.r[1] = (int)(&screen);

   swi(XOS_MASK | OS_ReadVduVariables, &regs);

   swi(XOS_MASK | OS_RemoveCursors, &regs);

   limit = atoi(argv[1]);

#define  r(n)  rand() % n

   for (i = 0; i < limit; ++i)
      Line(r(320), r(256), r(320), r(256), r(256));

   getchar();
   mode(0);

   return 0;
}

void Line(int x1, int y1, int x2, int y2, char col)
{
   int dx, dy, e, yincr, xincr, tmp;

   if (abs(x2 - x1) >= abs(y2 - y1))
   {
      if (x1 > x2)
      {
         tmp = x1;
         x1 = x2;
         x2 = tmp;
         tmp = y1;
         y1 = y2;
         y2 = tmp;
      }
      dx = x2 - x1;
      dy = y2 - y1;
      if (dy > 0)
         yincr = 1;
      else
         yincr = -1;
      dy = abs(dy);
      e = (dy << 1) - dx;
      for ( ; x1 <= x2; ++x1)
      {
         *(screen + x1 + y1 * 320) = col;
         if (e > 0)
         {
            y1 += yincr;
            e -= dx << 1;
         }
         e += dy << 1;
      }
   }
   else
   {
      if (y1 > y2)
      {
         tmp = x1;
         x1 = x2;
         x2 = tmp;
         tmp = y1;
         y1 = y2;
         y2 = tmp;
      }
      dx = x2 - x1;
      dy = y2 - y1;
      if (dx > 0)
         xincr = 1;
      else
         xincr = -1;
      dx = abs(dx);
      e = (dx << 1) - dy;
      for ( ; y1 <= y2; ++y1)
      {
         *(screen + x1 + y1 * 320) = col;
         if (e > 0)
         {
            x1 += xincr;
            e -= dy << 1;
         }
         e += dx << 1;
      }
   }
}
---- cut here ----

  Oh, I just realized that there is no comments in this source; I suppose
I'll never be a CS student... :-)

  Andras
-- 
Andras Kovacs 
andras@alzabo.ocunix.on.ca
Nepean, Ont.