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

#define WINDSIZEX 100
#define WINDSIZEY 100

#define X1   5.5
#define Y1   6.5
#define X2  10.5
#define Y2  12.5

void init(void)
{
   glDisable (GL_DITHER);
}

static void test2(void)
{    
   GLfloat pixelBuf[3];
   char buf[12];
   static GLfloat vbuf[] = {X1, Y1, X1, Y2, X2, Y1, X2, Y2};

   glPolygonMode (GL_FRONT_AND_BACK, GL_POINT);
   glBegin(GL_POLYGON);
       glColor3f(0.2, 0.5, 0.8);
       glVertex2f(X1, Y1);
       glColor3f(0.2, 0.5, 0.8);
       glVertex2f(X1, Y2);
       glColor3f(0.2, 0.5, 0.8);
       glVertex2f(X2, Y1);
       glColor3f(0.2, 0.5, 0.8);
       glVertex2f(X2, Y2);
   glEnd();

   glReadPixels(X1, Y1, 1, 1, GL_RGB, GL_FLOAT, pixelBuf);
   printf("pixels is: %f %f %f\n", pixelBuf[0], pixelBuf[1], pixelBuf[2]);
   printf("pixels should be: %f %f %f\n", 0.2, 0.5, 0.8);
}

void display(void)
{
   glViewport(0, 0, WINDSIZEX, WINDSIZEY);
   glMatrixMode(GL_PROJECTION);
   glLoadIdentity();
   gluOrtho2D(0, WINDSIZEX, 0, WINDSIZEY);
   glMatrixMode(GL_MODELVIEW);
   glLoadIdentity();

   test2();
}

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