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I am trying to do vertical lines using Bresenham's Line Algorithm. But when I put coordinate for a vertical line, it is printing a point only, not showing a vertical line.

#include <graphics.h>
#include <stdlib.h>
#include <stdio.h>
#include <conio.h>

     int main( )
     {
         int x1,y1,x2,y2,dx,dy,ds,dt,d,x,y;
         /* request auto detection */
         int gdriver = DETECT, gmode, errorcode;
      
        /* initialize graphics and local variables */
         initgraph(&gdriver, &gmode, "c:\\tc\\bgi");

         x1=200;
         x2=200;
         y1=200;
         y2=300;

         x=x1;
         y=y1;
         dx=x2-x1;
         dy=y2-y1;
         dt=2*(dy-dx);
         ds=2*dy;
         d=2*dy-dx;

         printf("Using Bresenham's Line Algorithm");
         putpixel(x,y,7);

         while(x<=x2)
            {
            x=x+1;
            if(d<0)
                 d=d+ds;
            else
                 {
                y=y+1;
                d=d+dt;
                 }
            putpixel(x,y,7);
            }

           getch();
       closegraph();
       return 0;
}

When I put x1=200 x2=200 it gives me an error. Why am I getting the error? But in normal line function, I am getting the right result, but when putting in Bresenham, I am getting the wrong result.

genpfault
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Faisal Mahamud
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  • After `x= x + 1;` the loop ends immediately. – Damien May 23 '22 at 14:53
  • should i comment it?how can i solve? – Faisal Mahamud May 23 '22 at 14:56
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    _"It gives me error"_ is not a helpful information. Please tell us more about the error? What has happened? Did you get an error message? Did it format your hard drive? Did it burn your house down? Please [edit] and clarify. Also read this: [ask] – Jabberwocky May 23 '22 at 15:03
  • What do you know about Bresenham's line algorithm? – user253751 May 23 '22 at 15:15
  • Goswin von Brederlow is correct this is only for first octant so rendering any line outside its range will result in wrong render... You need to handle all 8 cases or if you clever only 2 or 1. However Why not use [DDA](https://stackoverflow.com/a/24682318/2521214) which is simpler and faster since i80x386 and also easily portable to higher dimension (old books on CG say Bresenham is faster but that was true many years ago) ... – Spektre May 24 '22 at 06:42

1 Answers1

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Bresenham like you implemented can only draw lines with a slope between 0° and 45° since every loop increases x by one and conditionally increases y by one.

What you have to do is first check if the line goes left to right. If not you have to switch the endpoints.

Next if the line slopes down instead of up you have to decrement y instead of incrementing it. You can store 1 or -1 in a temp variable depending on whether the lines slopes up or down and add that to y when needed.

And if the change in y is greater than the change in x you have to swap the coordinates around in the algorithm incrementing y every loop and x conditionally. For this you actually have to duplicate the whole loop.

Goswin von Brederlow
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