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

#define WINDSIZEX 100
#define WINDSIZEY 100


GLfloat *buf;
GLubyte *clearBuf;
GLubyte *bitmap;

void init(void)
{

}

static void test(void)
{    
   int i, bufHeight = 3;
   GLint mask = 189;

   clearBuf = malloc(sizeof(GLubyte) * WINDSIZEX * bufHeight);
   bitmap = malloc(sizeof(GLubyte) * WINDSIZEX * bufHeight);
   for (i = 0; i < WINDSIZEX * bufHeight; i++) {
      clearBuf[i] = i % 256;
      bitmap[i] = 0xFF;
   }

   glClear(GL_COLOR_BUFFER_BIT);

   glEnable(GL_STENCIL_TEST);
   glStencilMask(255); //stencil bits is 8
   glColor4f(1.0, 1.0, 0.0, 1.0);
   glRasterPos2f((float)0.20, (float)0.20);
   glDrawPixels(WINDSIZEX, bufHeight, GL_STENCIL_INDEX , GL_UNSIGNED_BYTE, clearBuf);

   glStencilMask(mask); 
   glStencilFunc(GL_NEVER, 0, ~0);
   glStencilOp(GL_ZERO, GL_KEEP, GL_KEEP);
   //glStencilOp(GL_REPLACE, GL_KEEP, GL_KEEP);

   glRasterPos2f(0.25, 0.25);
   glBitmap(WINDSIZEX, bufHeight, 0, 0, 0, 0, bitmap);

   buf = malloc(sizeof(GLfloat) * WINDSIZEX * bufHeight);

   glReadPixels(0, 0, WINDSIZEX, bufHeight, GL_STENCIL_INDEX, GL_FLOAT, buf);
   for (i = 0; i < 20; i++)
   {
//      printf("expected at (%d): %f, actual: %f\n", i, (float)(clearBuf[i] & ~mask), buf[i]);
	GLint pre = clearBuf[i];
        GLint expected = pre & ~mask;
        printf("expected at (%d): %d, actual: %f\n", i, expected, buf[i]);
   }
}

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_DOUBLE | GLUT_RGB | GLUT_DEPTH);
   glutInitWindowSize(WINDSIZEX, WINDSIZEY);
   glutInitWindowPosition(100, 100);
   glutCreateWindow(argv[0]);
   init();
   glutDisplayFunc(display);
   glutMainLoop();
   return 0; 
}