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

#define WINDSIZEX 100
#define WINDSIZEY 100


void init(void)
{
   glClearColor(0.0, 0.0, 0.0, 1.0);
   glClearDepth(0.0);
}

static void test(void)
{    
   GLfloat buf[4];

   glEnable(GL_DEPTH_TEST);
   glClearDepth(1.0);
   glClear(GL_DEPTH_BUFFER_BIT);

   //set the polygon mode to fill
   glPolygonMode(GL_FRONT_AND_BACK, GL_FILL);

   glColor3f(0.0, 1.0, 0.0);
   glBegin(GL_POLYGON);
   glVertex3f(25.0, 25.0, 0.00);
   glVertex3f(25.0, 75.0, 0.00);
   glVertex3f(75.0, 75.0, 0.5);
   glVertex3f(75.0, 25.0, 0.5);
   glEnd();


   glEnable(GL_POLYGON_OFFSET_FILL);
   glPolygonOffset(0.0, -4.0);   // dz/dx = 0.25/50 use Scale of -100.0 

   glColor3f(1.0, 0.0, 0.0);
   glBegin(GL_POLYGON);
   glVertex3f(25.0, 25.0, 0.00);
   glVertex3f(25.0, 75.0, 0.00);
   glVertex3f(75.0, 75.0, 0.5);
   glVertex3f(75.0, 25.0, 0.5);
   glEnd();

   glReadPixels(50, 50, 1, 1, GL_RGBA, GL_FLOAT, buf);
   printf("draw pixel at (50, 50): %f %f %f\n", buf[0], buf[1], buf[2]);
}

void display(void)
{
   glClear(GL_COLOR_BUFFER_BIT);

   glMatrixMode(GL_MODELVIEW);
   glLoadIdentity();
   glMatrixMode(GL_PROJECTION);
   glPushMatrix();
   glLoadIdentity();
   glOrtho(0.0, WINDSIZEX, 0.0, WINDSIZEY, 1.0, -1.0);

   test();
}

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