OpenGL - Simple Drawing Box

#include <GL/gl.h>
#include <GL/glu.h>
#include <GL/glut.h>

#include <iostream>
using namespace std;

#define LIMIT 20.0      // limit
#define CENTER (LIMIT - LIMIT - 1)


// prototype
void setup();   // initialization of the program
void display(); // drawing method
void createCoordinate();  // certasian coordinate
void draw();    // draw the object

void setup(){
    glClearColor(1.0, 1.0, 1.0, 1.0);
    gluOrtho2D(CENTER, LIMIT, CENTER, LIMIT); // -x1, x2, -y1, y2
}

void display(){
    glClear(GL_COLOR_BUFFER_BIT);
    glColor3f(1.0, 0.0, 0.0);

    draw();
    createCoordinate();

    glFlush();
}


void draw(){
    // drawing a box
    float x1 = 1, x2=2, y1=1, y2=2;
    glBegin(GL_POLYGON);
        glVertex2f(x1, y1);
        glVertex2f(x1, y2);
        glVertex2f(x2, y2);
        glVertex2f(x2, y1);
    glEnd();

}

void createCoordinate(){
    glBegin(GL_LINES);
        // horizontal lines
        glVertex2f(-LIMIT, 0.0);
        glVertex2f(LIMIT, 0.0);

        // vertical lines
        glVertex2f(0.0, -LIMIT);
        glVertex2f(0.0, LIMIT);
    glEnd();
}

int main(int argc, char *argv[]){
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
    glutInitWindowPosition(200, 100);
    glutInitWindowSize(400, 300);
    glutCreateWindow("Hello World");
    glutDisplayFunc(display);

    setup();
    glutMainLoop();



    return 0;

}

OpenGL - Simple Drawing Box