Bresenham's Algorithm

Do you want to draw line segments at the speed of light? Just use Bresenham's Algorithm:

void bresenham(x1,y1,x2,y2)
int x1,y2,x2,y2
{
 int dx,dy,i,e;
 int incx,incy,inc1,inc2;
 int x,y;
 int locx,locy;

 dx = x2-x1;
 dy = y2-y1;
 if(dx < 0) dx = -dx;
 if(dy < 0) dy = -dy;
 incx = 1;
 if(x2 < x1) incx = -1;   
 incy = 1;
 if(y2 < y1) incy = -1;
 x = x1;
 y = y1;
 if(dx > dy)
  locx = x;
  locy = y;
  set_pixel(locx,locy);
  e = 2*dy - dx;
  inc1 = 2*(dy-dx);
  inc2 = 2*dy;
  for(i = 0 ; i < dx ; i++)
  {
   if(e >= 0)
   {
    y += incy;
    e += inc1;
   } 
   else e += inc2;
   x += incx;
   locx = x;
   locy = y;
   set_pixel(locx,locy);
  }
 }
 else
 {
  locx = x;
  locy = y;
  set_pixel(locx,locy);
  e = 2*dx - dy;
  inc1 = 2*(dx-dy);
  inc2 = 2*dx;
  for(i = 0 ; i < dy ; i++)
  {
   if(e>=0)
   {
    x += incx;
    e += inc1;
   }
   else e += inc2;
   y = += incy;
   locx = x;
   locy = y;
   set_pixel(locx,locy);
  }
 
 }
}