Implementation of DDA line drawing algorithm in OpenGL

In computer graphics, a digital differential analyzer (DDA) is hardware or software used for linear interpolation of variables over an interval between start and end point. DDAs are used for rasterization of lines, triangles and polygons. For more on DDA : Visit Digital Differential Analyzer - Wikipedia page. The following code is a DDA line drawing algorithm that draws a line between two given points.

#include <stdio.h>
#include <math.h>
#include <GL/glut.h>

double X1, Y1, X2, Y2;

float round_value(float v)
{
  return floor(v + 0.5);
}
void LineDDA(void)
{
  double dx=(X2-X1);
  double dy=(Y2-Y1);
  double steps;
  float xInc,yInc,x=X1,y=Y1;
  /* Find out whether to increment x or y */
  steps=(abs(dx)>abs(dy))?(abs(dx)):(abs(dy));
  xInc=dx/(float)steps;
  yInc=dy/(float)steps;

  /* Clears buffers to preset values */
  glClear(GL_COLOR_BUFFER_BIT);

  /* Plot the points */
  glBegin(GL_POINTS);
  /* Plot the first point */
  glVertex2d(x,y);
  int k;
  /* For every step, find an intermediate vertex */
  for(k=0;k<steps;k++)
  {
    x+=xInc;
    y+=yInc;
    /* printf("%0.6lf %0.6lf\n",floor(x), floor(y)); */
    glVertex2d(round_value(x), round_value(y));
  }
  glEnd();

  glFlush();
}
void Init()
{
  /* Set clear color to white */
  glClearColor(1.0,1.0,1.0,0);
  /* Set fill color to black */
  glColor3f(0.0,0.0,0.0);
  /* glViewport(0 , 0 , 640 , 480); */
  /* glMatrixMode(GL_PROJECTION); */
  /* glLoadIdentity(); */
  gluOrtho2D(0 , 640 , 0 , 480);
}
void main(int argc, char **argv)
{
  printf("Enter two end points of the line to be drawn:\n");
  printf("\n************************************");
  printf("\nEnter Point1( X1 , Y1):\n");
  scanf("%lf%lf",&X1,&Y1);
  printf("\n************************************");
  printf("\nEnter Point1( X2 , Y2):\n");
  scanf("%lf%lf",&X2,&Y2);
  
  /* Initialise GLUT library */
  glutInit(&argc,argv);
  /* Set the initial display mode */
  glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
  /* Set the initial window position and size */
  glutInitWindowPosition(0,0);
  glutInitWindowSize(640,480);
  /* Create the window with title "DDA_Line" */
  glutCreateWindow("DDA_Line");
  /* Initialize drawing colors */
  Init();
  /* Call the displaying function */
  glutDisplayFunc(LineDDA);
  /* Keep displaying untill the program is closed */
  glutMainLoop();
}