IMPLEMENTATION OF NOISE

	From [Watt & Watt, p.208]

	#include <math.h>

	#define SIZE 575


/* noise is randomly assigned values in [0,1] */ 
	extern float	noise[SIZE][SIZE];
	extern float	scale, pixel_size;


/* Perlin's turbulence function */ 

	float turbulence(float u, float v)
	{
	  float t = 0.;
	  float linear_noise(   );
	
	  while (scale > pixel_size) {
	
		t += abs(linear_noise(u/scale, v/scale) * scale);
		scale /= 2.;

	  }
	  return( t );
	}


	
/* linear_noise interpolates the values of noise[][] to 
		non-integer coordinates */ 


	float linear_noise( float u, float v)
	{
	  int iu, iv, ip, iq;
	  float du, dv, bot, top;

	  iu = u;
	  iv = v;
	  du = u - iu;
	  dv = v - iv;

	  iu = iu % SIZE ;
	  iv = iv % SIZE ;
	  ip = (iu+1) % SIZE ;
	  iq = (iv+1) % SIZE ;


	  bot = noise[iu][iv] + du * (noise[ip][iv] - noise[iu][iv]) ;
	  top = noise[iu][iq] + du * (noise[ip][iq] - noise[iu][iq]) ;
	
	  return( bot + dv * (top - bot) );
	}