Graph(+x,-x), (+y,-y)

#include <windows.h>
#include <GL/glut.h>
#include <stdlib.h>
#include <stdio.h>

float Color1=0, Color2=1, Color3=0;
int r,p,q;


void keyboard(unsigned char key, int x, int y)
{
  switch (key)
  {
  case 27:             // ESCAPE key
	  exit (0);
	  break;

  case 'r':
	 Color1=1.0, Color2=0.0, Color3=0.0;
	 break;
  case 'g':
	 Color1=0.0, Color2=1.0, Color3=0.0;
     break;
  case 'b':
      Color1=0.0, Color2=0.0, Color3=1.0;
	  break;
  }
  glutPostRedisplay();

}

void Init(int w, int h)
{
    glClearColor(1.0, 1.0, 1.0, 1.0);
    glViewport(0,0, (GLsizei)w,(GLsizei)h);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D( (GLdouble)w/-2,(GLdouble)w/2, (GLdouble)h/-2, (GLdouble)h/2);

}

static void display(void)
{

    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    int i=0;

    glPointSize(1);
    glLineWidth(2);
    glColor3f(Color1,Color2, Color3);

    glBegin(GL_LINES);
        glVertex2f(0, -240);
        glVertex2f(0, 240);
        glVertex2f(-320, 0);
        glVertex2f(320, 0);
    glEnd();

    int j=-240,k=-320,l=0;

    glBegin(GL_POINTS);
     while(j<=240){
        for( i=-320;i<=320;i++)
        glVertex2f(i,j);
        j+=20;
     }
     while(k<=320){
        for( l=-240;l<=240;l++)
        glVertex2f(k,l);
        k+=20;
     }
    glEnd();

    glFlush();
}


int main(int argc, char *argv[])
{

    glutInit(&argc, argv);
    glutInitWindowSize(640,480);
    glutInitWindowPosition(400,200);
    glutInitDisplayMode(GLUT_RGB);

    glutCreateWindow("Polygon drawing");

    Init(640, 480);
    glutKeyboardFunc(keyboard);
    glutDisplayFunc(display);


    glutMainLoop();

    return 0;
}