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

#define WINDSIZEX 100
#define WINDSIZEY 100


void
init (void)
{
    int i;
    GLfloat white[] = { 1.0, 1.0, 1.0, 1.0 };

    glClearColor (0.0, 0.0, 0.0, 1.0);
    glColor4fv (white);
    glDisable (GL_DITHER);
}


static void
test (void)
{
    GLfloat defaultColor[] = { 0.0, 0.0, 0.0, 1.0 };
    int i, j;
    GLfloat buf;
    GLfloat n, f;
    GLubyte img[1] = {0xff};	

    glClearDepth (0);
    glClearColor (0.0, 0.0, 0.0, 1.0);
    glClear (GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glEnable (GL_DEPTH_TEST);
    glDepthFunc (GL_ALWAYS);
    glColor3f (0.0, 0.0, 0.0);

    n = 0.0;
    f = 1.0;
    glDepthRange(n, f);
    GLfloat nD = 0.25;

    glRasterPos3f (0.2, 0.2, nD);
    glBitmap(1, 1, 0, 0, 0, 0, img);
    //glDrawPixels(1, 1, GL_RGB, GL_FLOAT, defaultColor);

    glReadPixels (0, 0, 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &buf);
    printf ("read out pixels: (0, 0) = %f, ", buf);
    printf ("should be %f\n", (f-n)/2*nD+(f+n)/2);
}

void
display (void)
{
    glMatrixMode (GL_PROJECTION);
    glLoadIdentity ();
    glOrtho (0, WINDSIZEX, 0, WINDSIZEY, 1.0, -1.0);
    test ();
}

int
main (int argc, char **argv)
{
    glutInit (&argc, argv);
    glutInitDisplayMode (GLUT_SINGLE | GLUT_RGBA | GLUT_DEPTH);
    glutInitWindowSize (WINDSIZEX, WINDSIZEY);
    glutInitWindowPosition (100, 100);
    glutCreateWindow (argv[0]);
    init ();
    glutDisplayFunc (display);
    glutMainLoop ();
    return 0;
}