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

#define WINDSIZEX 100
#define WINDSIZEY 100


void init(void)
{
   GLfloat     blackColor[4] = {0.0, 0.0, 0.0, 1.0};
   GLfloat     whiteColor[4] = {1.0, 1.0, 1.0, 1.0};

   glDisable (GL_DITHER);

   glEnable (GL_LIGHT0);   //LIGHT0 diffuse = WHITE
   glEnable (GL_LIGHTING);

   glMaterialfv (GL_FRONT_AND_BACK, GL_AMBIENT,  blackColor);
   glMaterialfv (GL_FRONT_AND_BACK, GL_SPECULAR, blackColor);
   glMaterialfv (GL_FRONT_AND_BACK, GL_DIFFUSE,  whiteColor);
}

static void test(void)
{    
   GLfloat buf[4];
   GLfloat *allbuf;
   GLfloat vbuf[] = {5.5, 6.5, 5.5, 12.5, 10.5, 6.5, 10.5, 12.5};
   GLfloat position[4] =  {0, 0.6, 0.8, 0};
   GLfloat nbuf[][3] = {{0.3, 0.4, 0.7}, {0.3, 0.4, 0.7}, {0.3, 0.4, 0.7}, {0.3, 0.4, 0.7}};

   glPolygonMode (GL_FRONT_AND_BACK, GL_POINT);
   glVertexPointer (2, GL_FLOAT, 0, vbuf);
   glEnableClientState (GL_VERTEX_ARRAY);
   glEnableClientState (GL_NORMAL_ARRAY);
   glNormalPointer (GL_FLOAT, 0, nbuf);

   glLightfv (GL_LIGHT0, GL_POSITION, position);

   glClear (GL_COLOR_BUFFER_BIT);
   glDrawArrays (GL_QUAD_STRIP, 0, 4);

   allbuf = malloc(WINDSIZEX * WINDSIZEY * sizeof(GLfloat) * 3);
   glReadPixels(0, 0, WINDSIZEX, WINDSIZEY, GL_RGB, GL_FLOAT, allbuf);


   glReadPixels(5.5, 6.5, 1, 1, GL_RGBA, GL_FLOAT, buf);
   // pixel should be 0*0.3+0.6*0.4+0.8*0.7=0.8
   printf("draw pixel at (5.5, 6.5): %f %f %f, it should be 0.8\n", buf[0], buf[1], buf[2]);
}

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; 
}