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

#define PIXEL_COLOR 0.8

GLfloat normal[3], position[4], direction[3];

void init(void)
{
    GLfloat buf[4];

    glClearColor(0.0, 0.0, 0.0, 0.0);
    glDisable(GL_DITHER);
    glEnable(GL_LIGHTING);

    glEnable(GL_LIGHT0);

    glLightf(GL_LIGHT0, GL_SPOT_CUTOFF, 80);

    // Setup polymode and twoSided switch. 

    glPolygonMode (GL_FRONT_AND_BACK, GL_POINT);
    glLightModeli (GL_LIGHT_MODEL_TWO_SIDE, GL_FALSE);

    direction[0] = 0;
    direction[1] = 0;
    direction[2] = -1;
    glLightfv(GL_LIGHT0, GL_SPOT_DIRECTION, direction);

    // Setup light position.

    position[0] = sqrt(1.0 - PIXEL_COLOR*PIXEL_COLOR);
    position[1] = 0.0;
    position[2] = PIXEL_COLOR;
    position[3] = 0.0;
    glLightfv(GL_LIGHT0, GL_POSITION, position);

    glLightf(GL_LIGHT0, GL_SPOT_EXPONENT, 1);

    buf[0] = 1.0;
    buf[1] = 1.0;
    buf[2] = 1.0;
    buf[3] = 1.0;
    glMaterialfv(GL_LIGHT0, GL_AMBIENT, buf);
    glLightfv(GL_FRONT, GL_AMBIENT, buf);

    buf[0] = 0.0;
    buf[1] = 0.0;
    buf[2] = 0.0;
    buf[3] = 1.0;
    glMaterialfv(GL_FRONT, GL_DIFFUSE, buf);
    glMaterialfv(GL_FRONT, GL_SPECULAR, buf);
    glLightModelfv(GL_LIGHT_MODEL_AMBIENT, buf);
}

static void test(void)
{    
    glBegin(GL_POINTS);
        glVertex2f(0.5, 0.5);
    glEnd();
}

void display(void)
{
    GLfloat buf[3];

    glViewport(0, 0, 10, 10);
        glMatrixMode(GL_PROJECTION);
        glLoadIdentity();
    gluOrtho2D(0, 10, 0, 10);
        glMatrixMode(GL_MODELVIEW);
        glLoadIdentity();

    test();

    glReadPixels(0, 0, 1, 1, GL_RGB, GL_FLOAT, (GLvoid *)buf);

    printf ("actual buf[0]: %f, expected: %f\n", buf[0], PIXEL_COLOR);
}

int main(int argc, char** argv)
{
   glutInit(&argc, argv);
   glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
   glutInitWindowSize(10, 10);
   glutInitWindowPosition(20, 20);
   glutCreateWindow(argv[0]);
   init();
   glutDisplayFunc(display);
   glutMainLoop();
   return 0; 
}