/* grid.c   basic grid  */

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

static int winWidth = 600;
static int winHeight = 600;
static double Pi = 3.14159265;


static void reshape(int w, int h)
{
  glViewport(0, 0, w, h);
  winWidth = w;
  winHeight = h;
  glutPostRedisplay();
}

static void display(void)
{
  int i, j, n;
  double theta, dtheta, r, dr;
  double x1, y1, x2, y2;

  n = 60;
  dtheta = Pi/30.0;
  theta = 0.0;
  glClear(GL_COLOR_BUFFER_BIT);
  glLineWidth(1.0);
  glColor3f(0.0,0.0,0.0);
  /* radial lines */
  glBegin(GL_LINES);
    for(i=0; i<n; i++)
    {
      x1 = cos(theta);
      y1 = sin(theta);
      x2 = -x1;
      y2 = -y1;
      glVertex2f(x1, y1);
      glVertex2f(x2, y2);
      theta = theta + dtheta;
    }
  glEnd();

  n = 64;
  dtheta = Pi/32;
  r = 0.05;
  dr = 0.05;

  glLineWidth(1.0);
  glColor3f(0.0,0.0,0.0);
  /*  circles */
  for(j=0; j<20; j++)
  {
    theta = 0.0;
    glBegin(GL_LINES);
      for(i=0; i<n; i++)
      {
        x1 = r*cos(theta);
        y1 = r*sin(theta);
        x2 = r*cos(theta+dtheta);
        y2 = r*sin(theta+dtheta);
        glVertex2f(x1, y1);
        glVertex2f(x2, y2);
        theta = theta + dtheta;
      }
      r = r + dr;
    glEnd();
  }
  glFlush();
}

int main(int argc, char *argv[])
{
  glutInit(&argc, argv);
  glutInitDisplayMode(GLUT_RGB);
  glutInitWindowSize(winWidth, winHeight);
  glutInitWindowPosition(50,50); 
  glutCreateWindow(argv[0]);
  glutReshapeFunc(reshape);
  glutDisplayFunc(display);
  gluOrtho2D(-1.1, 1.1, -1.1, 1.1);
  glClearColor(1.0, 1.0, 1.0, 0.0);
  glutMainLoop();
  return 0;
} /* end grid_gl.c */