Bresenham’s line algorithm is an algorithm that determines the points of an n-dimensional raster that should be selected in order to form a close approximation to a straight line between two points. It is commonly used to draw lines on a computer screen, as it uses only integer addition, subtraction and bit shifting.

(Via Wikipedia)

Here is C++ implementation of this algorithm.

#include <graphics.h>
#include <stdlib.h>
#include <stdio.h>
#include <conio.h>
#include <iostream.h>
int main(void)
{
   /* request auto detection */
   int gdriver = DETECT, gmode, errorcode;

   /* initialize graphics and local variables */
   initgraph(&gdriver, &gmode, "C:\\tc\\bgi");

   cout<<"\n Enter X1,Y1,X2,Y2";
   int x1,y1,x2,y2;
   cin>>x1>>y1>>x2>>y2;

   int dx=x2-x1;
   int dy=y2-y1;
   int x=x1;
   int y=y1;

   int e=(2*dy)-dx;
   for (int i=0;i<=dx;i++)
   {
	putpixel(x,y,15);
	while(e>=0)
	{
		y=y+1;
		e=e-(2*dx);
	}
	x=x+1;
	e=e+(2*dy);
   }

   /* clean up */
   getch();
   closegraph();
   return 0;
}

Output: