/******************************************************
**  Demo Program for Timing 
**  Object rotates once every 4 seconds - exactly.
**  Uses a signal sent at regular intervals to call PostRedisplay.
**
**  R.S. Dannelly
*****************************************************/
#include <GL/glut.h>
#include <stdlib.h>
#include <sys/time.h>
#include <time.h>
#include <signal.h>

#define TRANS_PER_SEC 25

GLfloat spin = 0.0;    

/*****************************************************/
void display(void)
{
   static int i=0;

   // send an alarm in N microseconds
   struct itimerval old, new;
   new.it_interval.tv_usec = 0;
   new.it_interval.tv_sec = 0;
   new.it_value.tv_usec = (long int) (4000000 / TRANS_PER_SEC);
   new.it_value.tv_sec = 0;
   if (setitimer (ITIMER_REAL, &new, &old) < 0)
      printf("timer not set\n");

   // clear the screen
   glClearColor (0.0, 0.0, 0.0, 0.0); 
   glClear(GL_COLOR_BUFFER_BIT);

   // draw the spun teapot
   glLoadIdentity();
   glRotatef (spin, 0,1,0);
   if (i++ % TRANS_PER_SEC == 0)   // draw red when spin == 0
       glColor3f (1.0, 0.0, 0.0); 
   else
       glColor3f (0.5, 0.5, 0.5); 
   glutSolidTeapot(15);

   glutSwapBuffers();
   glFlush();
}

/*****************************************************/
void reshape(int w, int h)
{
   glViewport (0, 0, (GLsizei) w, (GLsizei) h);
   glMatrixMode(GL_PROJECTION);
   glLoadIdentity();
   glOrtho(-50.0, 50.0, -50.0, 50.0, -50.0, 50.0);
   glMatrixMode(GL_MODELVIEW);
   glLoadIdentity();
}

/****** use the mouse to exit  *************************/
void mouse(int button, int state, int x, int y)
{
   if (state == GLUT_DOWN)
      exit(1);
}

/********************************************************/
void signal_handler()
{
   // be sure to re-register this handler 
   signal (SIGALRM, signal_handler);

   // move a little bit
   spin += (360.0 / TRANS_PER_SEC);
   glutPostRedisplay();
}
/********************************************************/
 
int main(int argc, char** argv)
{
   signal (SIGALRM, signal_handler);   // handle the alarm signal

   glutInit(&argc, argv);              // standard startup stuff
   glutInitDisplayMode (GLUT_RGB | GLUT_DOUBLE);
   glutInitWindowSize (450, 450); 
   glutInitWindowPosition (100, 100);
   glutCreateWindow (argv[0]);
   glutDisplayFunc(display); 
   glutReshapeFunc(reshape); 
   glutMouseFunc(mouse);               // exit on mouse click
   glutMainLoop();
   return 0;  
}