/*************************************/
/* A Simple OpenGL Example           */
/*************************************/

// this code just creates a window and draws three rectangles in it

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

// set up projection
void init()
{
  glClearColor(0.0, 0.0, 0.0, 0.0); // clear -> black R,G,B,A = 0 (A=alpha channel)

  glMatrixMode(GL_PROJECTION);      // applies subsequent matrix ops to
                                    // projection matrix
  glLoadIdentity();
  glOrtho(-10.0, 10.0, -10.0, 10.0, -10.0, 10.0); // view box

  glMatrixMode(GL_MODELVIEW);

  // all matrix operations that follow apply to modelview matrix
}

// display content (called automatically in main loop)

void display()
{
  glClear(GL_COLOR_BUFFER_BIT);  // clear color buffer

  // big green rectangle at a certain z coordinate
  
  glColor3f(0.0, 1.0, 0.0);    // green
  //float z = 11;                 // clipped (outside box)
  float z = 5;                 // visible

  // draw quad first
  glBegin(GL_QUADS);
  glVertex3f(-10.0,  10.0, z);
  glVertex3f(-10.0, -10.0, z);
  glVertex3f( 10.0, -10.0, z);
  glVertex3f( 10.0,  10.0, z);               
  glEnd();
  
  // big red rectangle at z=0
  glColor3f(1.0, 0.0, 0.0);      // red
  glRectf(-8.0, 8.0, 8.0, -8.0); // draw rectangle  

  // small blue rectangle at z=0
  glColor3f(0.0, 0.0, 1.0);      // blue
  glRectf(-5.0, 5.0, 5.0, -5.0); // draw rectangle
  
  glutSwapBuffers();             // bring draw buffer to front
}


int main (int argc, char **argv)
{
  // setup window
  glutInit(&argc, argv);
  glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB); // double buffering, rbg mode
  glutInitWindowSize(250, 250);
  glutInitWindowPosition(100, 100);
  glutCreateWindow("My First OpenGL Application");

  init();

  // register call-back function
  glutDisplayFunc(display);

  // glut takes over - calling display() repeatedly
  glutMainLoop();
  return 0;
}