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

#define WINDSIZEX 100
#define WINDSIZEY 100

static GLfloat grayColor[4] = {
   0.5f, 0.5f, 0.5f, 1.0f
};
static GLfloat envColors[][4] = {
   {0.8f, 0.7f, 0.8f, 0.10f},
   {0.3f, 0.7f, 0.8f, 0.25f},
   {0.1f, 0.0f, 0.9f, 0.35f},
   {0.3f, 0.4f, 0.0f, 0.55f},
};

static GLuint texture;
static GLint  numTextureUnits = 2, maxNumTextUnits;

static void DrawPoly(void)
{
   int i;

   glBegin(GL_QUADS);
   for (i = 0; i < numTextureUnits; i++) {
      glMultiTexCoord2fARB(GL_TEXTURE0_ARB + i, 0, 0);
   }
   glVertex2f(0, 0);

   for (i = 0; i < numTextureUnits; i++) {
      glMultiTexCoord2fARB(GL_TEXTURE0_ARB + i, 0, 1);
   }
   glVertex2f(0, 1.0);

   for (i = 0; i < numTextureUnits; i++) {
      glMultiTexCoord2fARB(GL_TEXTURE0_ARB + i, 1, 1);
   }
   glVertex2f(1.0, 1.0);

   for (i = 0; i < numTextureUnits; i++) {
      glMultiTexCoord2fARB(GL_TEXTURE0_ARB + i, 1, 0);
   }
   glVertex2f(1.0, 0);
   glEnd();
}



static void CreateTexture()
{
   glBindTexture (GL_TEXTURE_2D, texture);

   glTexImage2D(GL_TEXTURE_2D, 0, GL_INTENSITY, 1, 1, 0,
	 GL_RGBA, GL_FLOAT, grayColor);

   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP);
   glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP);

   glBindTexture (GL_TEXTURE_2D, 0);
}

void test()
{
   GLfloat buf[4];
   GLint unitNum, j;

   for(unitNum = 0; unitNum < numTextureUnits; unitNum++){
      glActiveTextureARB (GL_TEXTURE0_ARB + unitNum);
      glBindTexture(GL_TEXTURE_2D, texture);

      glTexEnvfv(GL_TEXTURE_ENV, GL_TEXTURE_ENV_COLOR, envColors[unitNum % 4]);

      glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_BLEND);
   }

   for(unitNum = 0; unitNum < numTextureUnits; unitNum++){
      glActiveTextureARB (GL_TEXTURE0_ARB + unitNum);
      glEnable(GL_TEXTURE_2D);
   }

   glClear(GL_COLOR_BUFFER_BIT);
   glEnable(GL_BLEND);
   DrawPoly();
   glReadPixels(0, 0, 1, 1, GL_RGBA, GL_FLOAT, buf);
   //printf("pixel: %f %f %f\n", buf[0], buf[1], buf[2]);

   glDisable(GL_BLEND);
   DrawPoly();

   glReadPixels(0, 0, 1, 1, GL_RGB, GL_FLOAT, buf);
   printf("pixel: %f %f %f\n", buf[0], buf[1], buf[2]);
}

void init(void)
{
   glGetIntegerv (GL_MAX_TEXTURE_UNITS_ARB, &maxNumTextUnits);
   printf("num texture units: %d\n", maxNumTextUnits);
   glDisable(GL_DITHER);
   glClearColor(1.0, 1.0, 1.0, 1.0);
   glBlendFunc (GL_ZERO, GL_SRC_ALPHA);
   glShadeModel (GL_FLAT);

   glGenTextures(1, &texture);
   CreateTexture();

   glColor4f (0.6, 0.6, 0.6, 0.0);
}

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; 
}