Draw the pattern using DDA Line drawing algorithm with KeyBoard in OpenGL

Program:
#include<stdio.h>
#include<GL/glut.h>
#include<math.h>
int xx1,xx2,yy1,yy2;
int sign(int x)
{
    if(x > 0) return 1;
    if(x < 0) return -1;
    return 0;
}
void drawline(int X1,int Y1, int X2,int Y2)  
{
    float x,y,dx,dy,length;
    int i;
    dx=abs(X2-X1);
    dy=abs(Y2-Y1);
    if(dx>=dy)
        length=dx;
    else
        length=dy;
    dx=(X2-X1)/length;
    dy=(Y2-Y1)/length;
    x=X1 + 0.5*sign(X1);
    y=Y1 + 0.5*sign(Y1);
    i=1;
    while(i<=length)
    {
   
                 glBegin(GL_POINTS);
             glVertex2i(x,y);
             glEnd();
                 glFlush();
        x=x+dx;
        y=y+dy;
        i=i+1;
    }
    glFlush();
}
  
void display(void)                            
{
    glClear(GL_COLOR_BUFFER_BIT);
    glColor3f(1.0,0.0,0.0);
    drawline(xx1,yy1,xx1,yy2);
    drawline(xx1,yy2,xx2,yy2);
    drawline(xx2,yy2,xx2,yy1);
    drawline(xx2,yy1,xx1,yy1);
     glColor3f(0.0,1.0,0.0);
    drawline(xx1,(yy1+yy2)/2,(xx1+xx2)/2,yy2);
    drawline((xx1+xx2)/2,yy2,xx2,(yy1+yy2)/2);
    drawline(xx2,(yy1+yy2)/2,(xx1+xx2)/2,yy1);
    drawline((xx1+xx2)/2,yy1,xx1,(yy1+yy2)/2);  
     glColor3f(0.0,0.0,1.0);
    drawline((xx1+(xx1+xx2)/2)/2,(yy2+(yy1+yy2)/2)/2,(xx2+(xx1+xx2)/2)/2,(yy2+(yy1+yy2)/2)/2);
    drawline((xx2+(xx1+xx2)/2)/2,(yy2+(yy1+yy2)/2)/2,(xx2+(xx1+xx2)/2)/2,(yy1+(yy1+yy2)/2)/2);
    drawline((xx2+(xx1+xx2)/2)/2,(yy1+(yy1+yy2)/2)/2,(xx1+(xx1+xx2)/2)/2, (yy1+(yy1+yy2)/2)/2);
    drawline((xx1+(xx1+xx2)/2)/2, (yy1+(yy1+yy2)/2)/2,(xx1+(xx1+xx2)/2)/2,(yy2+(yy1+yy2)/2)/2);  

}
void init(void)
{
       glViewport(0,0,640,480); 
       glMatrixMode(GL_PROJECTION); 
       glLoadIdentity(); 
       gluOrtho2D(0.0,640.0,0.0,480.0); 
       glMatrixMode(GL_MODELVIEW);    
}
int main(int argc,char** argv)
{
    printf("\n Enter vertex x1 ");
        scanf("%d",&xx1);
        printf("\n Enter vertex y1 ");
        scanf("%d",&yy1);
        printf("\n Enter vertex x2 ");
        scanf("%d",&xx2);
        printf("\n Enter vertex y2 ");
        scanf("%d",&yy2);
        glutInit(&argc,argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
    glutInitWindowSize(640,480);
    glutInitWindowPosition(0,0);
    glutCreateWindow("DDA Line");
    init();
    glClear(GL_COLOR_BUFFER_BIT);
    glutDisplayFunc(display);
    glFlush();
    glutMainLoop();
    return 0;
}
Output: