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

#define WINDSIZEX 100
#define WINDSIZEY 100

GLubyte *clearBuf;
GLint *readBuf;

GLint bufHeight = 3;
GLint value, maxValue;

void init(void)
{
   int i;
   int stencilBits;

   glGetIntegerv(GL_STENCIL_BITS, &stencilBits);

   if (stencilBits == 0)
      exit(1);

   value = 1 << stencilBits;
   maxValue = value -1;

   clearBuf = (GLubyte *)malloc(WINDSIZEX * bufHeight * sizeof(GLubyte));
   readBuf =  (GLint*)malloc(WINDSIZEX * bufHeight * sizeof(GLfloat));

   for (i = 0; i<WINDSIZEX* bufHeight; i++) {
      clearBuf[i] = i %  value;
   }

   glFrontFace(GL_CCW);
  // glFrontFace(GL_CW);
}

void RenderState(void)
{
   /*! \brief Triangles (drawn in clockwise direction - backfacing)  */
   glBegin(GL_TRIANGLES);
   glVertex2f(0, 0);
   glVertex2f(0, bufHeight);
   glVertex2f(WINDSIZEX, bufHeight);

   glVertex2f(WINDSIZEX, 0);
   glVertex2f(0, 0);
   glVertex2f(WINDSIZEX, bufHeight);
   glEnd();
}


void RenderState2(void)
{
   /*! \brief Triangles (drawn in clockwise direction - backfacing)  */
   glBegin(GL_TRIANGLES);
   glVertex2f(0, 0);
   glVertex2f(WINDSIZEX, bufHeight);
   glVertex2f(0, bufHeight);

   glVertex2f(WINDSIZEX, 0);
   glVertex2f(WINDSIZEX, bufHeight);
   glVertex2f(0, 0);
   glEnd();
}

static void test(void)
{    
int mask = 1;

   glClear(GL_COLOR_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);

   glEnable(GL_STENCIL_TEST);                           // Clear stencil with known value
   glEnable(GL_DEPTH_TEST);                             // Enable depth test

   // Force everything to be written
   glStencilMaskSeparate(GL_BACK, maxValue);
   glStencilMaskSeparate(GL_FRONT, maxValue);

   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);

   // Setup write mask
   glStencilMaskSeparate(GL_BACK, mask);
   glStencilMaskSeparate(GL_FRONT, mask);

   glDisable(GL_DEPTH_TEST);

   glStencilFuncSeparate(GL_FRONT_AND_BACK, GL_ALWAYS,
	 0, ~0);

   glStencilOpSeparate(GL_BACK, GL_KEEP, GL_KEEP,
	 GL_INVERT); //GL_KEEP

   glStencilOpSeparate(GL_FRONT, GL_KEEP, GL_KEEP, GL_KEEP); //INVERT);

   RenderState();

   glReadPixels(0, 0, WINDSIZEX, bufHeight, GL_STENCIL_INDEX, GL_INT, readBuf);
   printf("draw pixel at : %d %d %d\n", readBuf[0], readBuf[1], readBuf[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 | GLUT_DEPTH | GLUT_STENCIL);
   glutInitWindowSize(WINDSIZEX, WINDSIZEY);
   glutInitWindowPosition(100, 100);
   glutCreateWindow(argv[0]);
   init();
   glutDisplayFunc(display);
   glutMainLoop();
   return 0; 
}