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

#define WINDSIZEX 100
#define WINDSIZEY 100


void init(void)
{
   GLfloat     sizeRange[2], sizeStep;

   glClearColor(0.0, 0.0, 0.0, 0.0);
   glColor4f(1.0, 0.0, 0.0, 1.0);

   glDisable(GL_DITHER);

   glEnable(GL_POINT_SMOOTH);

   glEnable(GL_BLEND);
   glBlendFunc(GL_SRC_ALPHA, GL_ONE);

   // Get min and max point sizes
   glGetFloatv(GL_ALIASED_POINT_SIZE_RANGE, sizeRange);
   if (glGetError() != GL_NO_ERROR) {
      glGetFloatv(GL_POINT_SIZE_RANGE, sizeRange);
   }

   // Get granularity between min and max
   glGetFloatv(GL_POINT_SIZE_GRANULARITY, &sizeStep);

   printf("size range: %f %f; size step: %f \n", 
	sizeRange[0], sizeRange[1], sizeStep);
}

static void test(void)
{    
   GLfloat buf[25];
   int row, col;

   glPointSize(3);
   glClear(GL_COLOR_BUFFER_BIT);
   glBegin(GL_POINTS);
   glVertex2f(2.5, 2.5);
   glEnd();
   glReadPixels(0, 0, 5, 5, GL_RED, GL_FLOAT, buf);
   for (row = 0; row < 5; row++)
      for (col = 0; col < 5; col++)
	 printf("draw pixel at (%d, %d): %f\n", row, col, buf[row*5+col]);
}

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; 
}