=========================================================================
EXAMPLE PROGRAM LISTING 1
=========================================================================

/*==============================================*/
/* EXAMPLE 1                                    */
/* Using slope intercept method to plot a line. */
/* Very simplified.                             */
/*==============================================*/
#define swap(x,y) {register int temp; temp = x; x = y; y = temp;}

drawline( x1, y1, x2, y2)
int x1, y1, x2, y2;
{
    float slope, intercept;
    float x,y;

    if(x1 > x2){
        swap(x1, x2);
        swap(y1, y2);
    }
    slope = (float)(y1 - y2)/(float)(x1 - x2);
    intercept = (float)y1 - (slope * (float)x1);

    for( x = (float)x1; x <= (float)x2; x+=1.0){
        y = ((slope * x) + intercept);
        plot((int)x, (int)y);
    }
}


=========================================================================
EXAMPLE PROGRAM LISTING 2
=========================================================================
/*=====================================================*/
/* EXAMPLE 2                                           */
/* Using slope intercept method to plot a line.        */
/* Improved by not using floats.                       */
/* Very simplified.                                    */
/*=====================================================*/
#define swap(x,y) {register int temp; temp = x; x = y; y = temp;}

drawline( x1, y1, x2, y2)
int x1, y1, x2, y2;
{
    long delta_x, delta_y;
    long intercept;
    long x,y;

    delta_x = x1 - x2;
    delta_y = y1 - y2;
    intercept = y1 - (delta_y * x1)/delta_x;

    if(x1 > x2){
        swap(x1, x2);
        swap(y1, y2);
    }
    for( x = (long)x1; x <= (long)x2; x++){
        y = (x * delta_y)/delta_x + intercept;
        plot((int)x, (int)y);
    }
}


=========================================================================
EXAMPLE PROGRAM LISTING 3
=========================================================================
/*==============================================*/
/* EXAMPLE 3                                    */
/* Using slope intercept method to plot a line. */
/* Improved by not using floats.                */
/* Further improved by checking for Horizontal  */
/* and verticle lines, and by incrementing on   */
/* the axis which changes most.                 */
/*==============================================*/
#define swap(x,y) {register int temp; temp = x; x = y; y = temp;}

drawline( x1, y1, x2, y2)
int x1, y1, x2, y2;
{
    long delta_x, delta_y;
    long intercept;
    long x,y;

    delta_x = x1 - x2;
    delta_y = y1 - y2;

    if(delta_x == 0){
        v_line(x1, y1, y2); /* draw a verticle line */
        return;
    }
    if(delta_y == 0){
        h_line(x1, x2, y1); /* draw a horizontal line */
        return;
    }

    intercept = y1 - (delta_y * x1)/delta_x;

    if(delta_y > delta_x){ /* X increases faster than Y */
        if(x1 > x2){
            swap(x1, x2);
            swap(y1, y2);
        }
        for( x = (long)x1; x <= (long)x2; x++){
            y = (x * delta_y)/delta_x + intercept;
            plot((int)x, (int)y);
        }
    }
    else{                  /* Y increases faster than X */
        if(y1 > y2){
            swap(x1, x2);
            swap(y1, y2);
        }
        for( y = (long)y1; y <= (long)y2; y++){
            x = ((y - intercept) * delta_x)/ delta_y;
            plot((int)x, (int)y);
        }
    }
}

=========================================================================
EXAMPLE PROGRAM LISTING 4
=========================================================================

/*----------------------------------------------------------------*/
/*  Program Listing 4                                             */
/*                                                                */
/*    MODULE: line.c                                              */
/*  CONTENTS: drawline(), h_line(), v_line(), clip_chk()          */
/*    AUTHOR: John T. Bell                                        */
/*      DATE: 03 June, 1989                                       */
/*----------------------------------------------------------------*/
#include "graph.h"

/*----------------------------------------------------------------*/
/* This macro swaps the value of integer X with integer Y.        */
/*----------------------------------------------------------------*/
#define swap(x,y) {register int temp; temp = x; x = y; y = temp;}

/*--------------------------------------------------------------------------*/
/*   Function: drawline(int strt_x, int strt_y, int end_x, int end_y);      */
/*    Purpose: Draw a line from the starting coordinates to the             */
/*             ending coordinates.                                          */
/*    Returns: 0 if line can be drawn, -1 if error.                         */
/*      Setup: Computer must be in graphics mode.                           */
/* Attributes: NONE                                                         */
/*      Notes: Uses the "Octantal Digital Differential Analizer" method     */
/*             to draw the line.                                            */
/*             Ref: Artwick, "Microcomputer Displays, Graphics,             */
/*                  and Animation",Prentice Hall, 1985. pp 20-30            */
/*--------------------------------------------------------------------------*/
int drawline(strt_x,strt_y,end_x,end_y)
int strt_x,strt_y,end_x,end_y;
{
    int dx,dy;              /* delta x and delta y */
    int err,count;          /* error value, and point counter */

    /* test for data in range */
    if(clip_chk(strt_x,strt_y) || clip_chk(end_x,end_y))
        return(0);

    if(strt_x == end_x)     /* if verticle line do faster v_line() */
        return(v_line(strt_x,strt_y,end_y));

    if(strt_y == end_y)     /* if horizontal line do faster h_line() */
        return(h_line(strt_x,end_x,strt_y));

    if(end_x < strt_x){     /* swap points to map octants 3-6 into 1,2,7,8 */
        swap(strt_x,end_x);
        swap(strt_y,end_y);
    }
    dx = end_x - strt_x; /* calc change in x (delta x) */
    dy = end_y - strt_y; /* calc change in y (delta y) */

    if(dy < 0){             /* Select Octant 7,8 or 1,2 */
        if( -dy > dx){      /* Octant 7 Line */
            err = dy/2;
            for(count = -dy+1; count >= 0; count--){
                plot(strt_x,strt_y);
                strt_y--;
                err += dx;
                if(err >= 0){
                    strt_x++;
                    err += dy;
                }
            }
        }
        else{           /* Octant 8 Line */
            err = -dx/2;
            for(count = dx+1; count >= 0; count--){
                plot(strt_x,strt_y);
                strt_x++;
                err -= dy;
                if(err >= 0){
                    strt_y--;
                    err -= dx;
                }
            }
        }
    }
    else{               /* Octant 1 or 2  */
        if(dy > dx){    /* Octant 2 Line */
            err = -dy/2;
            for(count = dy+1; count >= 0; count--){
                plot(strt_x,strt_y);
                strt_y++;
                err += dx;
                if(err >= 0){
                    strt_x++;
                    err -= dy;
                }
            }
        }
        else{           /* Octant 1 Line */
            err = -dx/2;
            for(count = dx+1; count >= 0; count--){
                plot(strt_x,strt_y);
                strt_x++;
                err += dy;
                if(err >= 0){
                    strt_y++;
                    err -= dx;
                }
            }
        }
    }
    return(1);
}

int v_line(x, strt_y, end_y)
int x;
int strt_y, end_y;
{
    if(strt_y > end_y){
        swap(strt_y, end_y);
    }

    for(; strt_y <= end_y; strt_y++)
        plot(x, strt_y);
    return(1);
}

int h_line(strt_x, end_x, y)
{
    if(strt_x > end_x){
        swap(strt_x, end_x);
    }
    for(;strt_x <= end_x; strt_x++)
        plot(strt_x, y);
    return(1);
}

/*--------------------------------------------------------------------*/
/* clip_chk(int x, int y);                                            */ 
/* returns TRUE if x or y is out of the screen plotting range.        */
/* This needs the globals G_maxx and G_maxy which correspond to       */
/* the maximum X and Y axis values.                                   */
/*--------------------------------------------------------------------*/
clip_chk(x,y)
int x,y;
{
    return((((x < 0) || (x > G_maxx) || (y < 0) || (y > G_maxy)) ? 1 : 0));
}

=========================================================================
LISTING 5 graph.h
=========================================================================

#ifndef EXTRN
extern int G_maxx, G_maxy;

#else
int G_maxx, G_maxy;
#endif

#define AZTEC
#ifdef AZTEC
extern int _maxx;
extern int _xaspect, _yaspect;
extern _oldx, _oldy;
#endif

=========================================================================
LISTING 6 demo.c
=========================================================================

/*----------------------------------------------------------------*/
/*                Copyright 1989 by John T. Bell                  */
/*                                                                */
/*    MODULE: DEMO.C                                              */
/*   PURPOSE: DEMO program for line drawing routines.             */
/*  CONTENTS:                                                     */
/*    AUTHOR: John T. Bell                                        */
/*   HISTORY: Created 05/31/89                                    */
/*----------------------------------------------------------------*/
#include <stdio.h>

#define EXTRN
#include "graph.h"

/*--------------------------------------------------------------------------*/
/* Here we need to define;                                                  */
/*                                                                          */
/* G_maxx - Maximum value for the X axis.                                   */
/* G_maxy - Maximum value for the Y axis.                                   */
/*                                                                          */
/* We should also put the screen into graphics mode and clear the screen    */
/* to our background color.                                                 */
/*                                                                          */
/* The following implemenation was written for Aztec C and an IBM CGA card. */
/*--------------------------------------------------------------------------*/
graph_init()
{
    mode('H'); /* sets the graphic mode to 640 by 200 */
    G_maxx = _maxx;
    G_maxy = 200;
    color('W'); /* sets plotting color to white */
}

/*--------------------------------------------------------------------------*/
/* This just undoes what graph_init did.                                    */
/*--------------------------------------------------------------------------*/
graph_restore()
{
    mode(0);
}

/*-------------------------------------------*/
/* use Aztec point() function to plot points */
/*-------------------------------------------*/
int plot(x,y)
{
    return(point(x, G_maxy - y));
}

main()
{
    int x, y;
    int cx, cy;

    graph_init();

    cx = G_maxx/2;
    cy = G_maxy/2;

    for(x = 0; x <= G_maxx; x += (G_maxx/30))
        drawline(cx, cy, x, 0);

    for(y = 0; y <= G_maxy; y += (G_maxy/20))
        drawline(cx, cy, G_maxx, y);

    for(x = G_maxx; x >= 0; x -= (G_maxx/30))
        drawline(cx, cy, x, G_maxy);

    for(y = G_maxy; y >= 0; y -= (G_maxy/20))
        drawline(cx, cy, 0, y);

    getchar();	/* pause for keypress */
    graph_restore();
}