/* sierpinski.c */

/*
	Purpose:
 
	Demonstrate sierpinski gasket generation by recursion in the simplest possible manner.
	This code does _not_ implement efficiency measures such as display lists, vertex arrays, etc
	so as to clearly demonstrate the recursive fractal generation.
*/

#ifdef __APPLE__
#include <GLUT/glut.h>
#else
#include <GL/glut.h>
#endif

#include <stdlib.h>

#define KEY_ESC 27      /* glut doesn't define this one for some reason */


/* 
	Generate the sierpinski gasket by subdividing a given triangle into subtriangles.
	Then recursively do the same to the subtriangles.
	When we have finished recursing then draw a tiny little solid triangle.
*/
void generateGasket( float* p1, float* p2, float* p3, int level )
{
	if( 0 == level )
	{
		glVertex2fv( p1 );
		glVertex2fv( p2 );
		glVertex2fv( p3 );
	}
	else
	{
		/* Figure out the midpoints between the given points */
		float midp1p2[2];
		float midp2p3[2];		
		float midp1p3[2];
		
		midp1p2[0] = ( p1[0] + p2[0] ) * 0.5;
		midp1p2[1] = ( p1[1] + p2[1] ) * 0.5;

		midp2p3[0] = ( p2[0] + p3[0] ) * 0.5;
		midp2p3[1] = ( p2[1] + p3[1] ) * 0.5;

		midp1p3[0] = ( p1[0] + p3[0] ) * 0.5;
		midp1p3[1] = ( p1[1] + p3[1] ) * 0.5;
		
		/* Now generate the subtriangles.  Remember to leave out the middle subtriangles. */
		generateGasket( p1, midp1p2, midp1p3, level - 1 );
		generateGasket( midp1p2, p2, midp2p3, level - 1 );
		generateGasket( midp1p3, midp2p3, p3, level - 1 );
	}
}


void display( void )
{
	/* Select an initial triangle that fills the default orthographic projection */
	float p1[2] = { -1.0 , -1.0 };
	float p2[2] = { 1.0 , -1.0 };		
	float p3[2] = { 0.0 , 1.0 };

	glClear( GL_COLOR_BUFFER_BIT );
	
	glBegin( GL_TRIANGLES );
		/* Now subdivide the triangle and make the sierpinski gasket */
		generateGasket( p1, p2, p3, 8 );
	glEnd();
		
	glFlush();
}


void keyboard( unsigned char key, int x, int y)
{	
	switch (key) 
	{
	case KEY_ESC:   /* exit when escape key is pressed */
    	exit(0);
   	break;
	}
}


int main( int argc, char *argv[] )
{
	glutInit( &argc, argv );
	glutCreateWindow( argv[0] );

	glutDisplayFunc( display ); 
	glutKeyboardFunc( keyboard );

	glutMainLoop();
	
	return 0;
}