#include<GL/gl.h>
#include<GL/glu.h>
#include<GL/glut.h>
#include<stdlib.h>

const int screenWidth = 500;
const int screenHeight = 500;

/* draw a 5000-point 2D Sierpinski gasket in the square (0,0) to (500,500) */
void drawGasket( void )
{

/* define a point data type */

    typedef GLfloat point2[2];     

    /* use it to define the three vertices of a triangle */
    point2 vertices[3]={{0.0,0.0},{250.0,500.0},{500.0,0.0}}; 

    int i, j, k;
    int rand();       /* standard random number generator */
    point2 p ={75.0,50.0};  /* arbitrary initial point inside triangle */

    /* compute and plot 5000 points on the gasket: */
    for( k=0; k<5000; k++)
    {
      /* first, pick a vertex at random */
         j=rand()%3; 

      /* then, compute point halfway between selected vertex and old point */
         p[0] = (p[0]+vertices[j][0])/2.0; 
         p[1] = (p[1]+vertices[j][1])/2.0;
   
     /* plot new point */
          glBegin(GL_POINTS);
               glVertex2fv(p); 
          glEnd();

     /* and so on. */
     }
}

/* the mouse callback: */

void myMouse(int button, int state, int x, int y)
{
  if(button==GLUT_LEFT_BUTTON&state==GLUT_DOWN)    exit(0);
  if(button==GLUT_MIDDLE_BUTTON&state==GLUT_DOWN)  exit(0);
  if(button==GLUT_RIGHT_BUTTON&state==GLUT_DOWN)   exit(0);
}

/* the display callback: */

void myDisplay(void)
{
    glClear(GL_COLOR_BUFFER_BIT);  // clear the screen 
    // NB: if omit that line, will get a see-through window.  Try it.
    glViewport(0, 0, 500, 500);
    drawGasket();
    glFlush(); 
}

/* initialization: */

void myInit(void)
{
  glClearColor(1.0, 1.0, 1.0, 0.0); // background
  glColor3f(1.0, 0.0, 0.0);   // drawing color
  glMatrixMode(GL_PROJECTION);   // random necessary wierdness
  glLoadIdentity();              // to set up world coordinates      
  glOrtho(0.0, 500.0, 0.0, 500.0, -1.0, 1.0);  
}

int main(int argc, char** argv)
{
  glutInit(&argc, argv);
  glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
  glutInitWindowSize(screenWidth,screenHeight); 
  glutInitWindowPosition(0,0); 
  glutCreateWindow("Sierpinski Gasket");
  glutMouseFunc(myMouse);
  glutDisplayFunc(myDisplay);
  myInit();
  glutMainLoop();
}