/* gasket.c   */

/* E. Angel, Interactive Computer Graphics */
/* A Top-Down Approach with OpenGL, Third Edition */
/* Addison-Wesley Longman, 2003 */
/* Updated:  Bary W Pollack */

/* Two-Dimensional Sierpinski Gasket          */
/* Generated Using Randomly Selected Vertices */
/* And Bisection                              */

#include <GL/glut.h>

void myinit (void)
{
    /* attributes */
    glClearColor (1.0, 1.0, 1.0, 1.0); /* white background */
    glColor3f (1.0, 0.0, 0.0); /* draw in red */

    /* set up viewing */
    /* 500 x 500 window with origin lower left */
    glMatrixMode (GL_PROJECTION);
    glLoadIdentity ();
    gluOrtho2D (0.0, 500.0, 0.0, 500.0);
    glMatrixMode (GL_MODELVIEW);
}

void display (void)
{
    /* define a point data type */
    typedef GLfloat point2[2];     

    point2 vertices[3] = {{0.0,0.0}, {250.0,500.0}, {500.0,0.0}}; /* A triangle */

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

    glClear (GL_COLOR_BUFFER_BIT);  /* clear the window */

    /* compute and plots 5000 new points */
    for (k = 0; k < 5000; k++)
    {
        int j = rand () % 3; /* pick a vertex at random */

        /* 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 ();
    }
    glFlush (); /* clear buffers */
 }

void main (int argc, char** argv)
{
    /* Standard GLUT initialization */
    glutInit (&argc,argv);
    glutInitDisplayMode (GLUT_SINGLE | GLUT_RGB);  /* default, not needed */
    glutInitWindowSize (500, 500);          /* 500 x 500 pixel window */
    glutInitWindowPosition (0, 0);          /* place window top left on display */
    glutCreateWindow ("Sierpinski Gasket"); /* window title */
    glutDisplayFunc (display);              /* display callback invoked when window opened */

    myinit();                               /* set attributes */

    glutMainLoop();                         /* enter event loop */
}