#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, 0.0);
    glClear(GL_COLOR_BUFFER_BIT);
}

static void test(void)
{   
   GLfloat buf[10000];

    glClear(GL_COLOR_BUFFER_BIT);

    glColor4f(1.0, 1.0, 1.0, 0.0);
    glBegin(GL_POLYGON);
        glVertex2i(WINDSIZEX/4, WINDSIZEY/4);
        glVertex2i(WINDSIZEX*3/4, WINDSIZEY/4);
        glVertex2i(WINDSIZEX*3/4, WINDSIZEY*3/4);
        glVertex2i(WINDSIZEX/4, WINDSIZEY*3/4);
    glEnd();

    glReadPixels(WINDSIZEX/2, WINDSIZEY/2, 1, 1, GL_RGB, GL_FLOAT, buf);

}

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

    test();
}

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; 
}