Warn illumination model

 /*
* OpenGL implementation of Warns illumination model(spotlight)
* for better understanding read Computer Graphics by Hill
* page 426
*/
 
#include <GL/glut.h>
 
void init(void)
{

    GLfloat mat_specular[] = { 1.0, 1.0, 0.0, 1.0 };
    GLfloat mat_shininess[] = { 50.0 };
    //set spotlight’s position
    GLfloat dir_light[]={2.0, 1.0, -4.0};
    GLfloat light_position[] = { 2.0, 1.0, 5.0, 1.0 };
 
 
    glClearColor (1.0, 0.0, 0.0, 0.0);
    glShadeModel (GL_SMOOTH);

    glMaterialfv(GL_FRONT, GL_SPECULAR, mat_specular);
    glMaterialfv(GL_FRONT, GL_SHININESS, mat_shininess);
    glLightfv(GL_LIGHT0, GL_POSITION, light_position);
 
    /*
    * a cutoff angle of 45 degrees
    * default value is 180 degrees
    */
    glLightf(GL_LIGHT0,GL_SPOT_CUTOFF, 45.0);
 
    /*
    * The desired falloff of light with angle(E) is 1
    * As E increases the spotlight disappears
    * default value is 0
    */
    glLightf(GL_LIGHT0,GL_SPOT_EXPONENT, 1.0);
     
    glLightfv(GL_LIGHT0,GL_SPOT_DIRECTION,dir_light);
 
    //enables light
    glEnable(GL_LIGHTING);
    
    //enables this particular source.
    glEnable(GL_LIGHT0);
 
    glEnable(GL_DEPTH_TEST);
}
void display(void)
{
      glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
      glutSolidSphere (1.0, 20, 16);
      glutSwapBuffers();
    
}
void reshape (int w, int h)
{
    glViewport (0, 0, (GLsizei) w, (GLsizei) h);
    glMatrixMode (GL_PROJECTION);
    glLoadIdentity();
    if (w <= h)
        glOrtho (-1.5, 1.5, -1.5*(GLfloat)h/(GLfloat)w, 1.5*(GLfloat)h/(GLfloat)w, -10.0, 10.0);
    else
        glOrtho (-1.5*(GLfloat)w/(GLfloat)h, 1.5*(GLfloat)w/(GLfloat)h, -1.5, 1.5, -10.0, 10.0);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
}
int main(int argc, char **argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB | GLUT_DEPTH);
    glutInitWindowSize (500, 500);
    glutInitWindowPosition (100, 100);
    glutCreateWindow (”Single-Point Light Source”);
    init();
    glutDisplayFunc(display);
    glutReshapeFunc(reshape);
    glutMainLoop();
    return 0;
}