/*
 * C code used to produce images appearing in:
 *
 * "Texturing and Modeling: A Procedural Approach," by David S. Ebert, ed.,
 * F. Kenton Musgrave, Darwyn Peachey, Ken Perlin, and Steven Worley.
 * Academic Press, 1998.  ISBN 0-12-228730-4.
 *
 * Note that this is the original C texture code, on which the Larry Gritz's
 * RenderMan shaders that appear in the text were based.
 *
 * Copyright 1998 F. Kenton Musgrave
 * All rights reserved.
 */

/*
 * A cheesy hurricane model.
 *
 * Eye of the storm is centered on z axis.
 *
 * Called as:
 * texture cyclone 1 .5 0.7 0.5 0.675 4 0.7 0.3 3.5 0.025 1.5 0.25
 * ("colour" is a pale yellow, not specified above)
 */
void Cyclone( 	Vector texture, Vector intersect,
			Colour *colour,
			double max_radius, double twist, double scale, 
			double offset, double omega, double octaves, 
			double p0, double p1, double p2, double p3, double p4,
			double p5 )
{
	double	radius, dist, angle, sine, cosine, eye_weight, value;
	Vector	point;
	double	power, detail_height;

	/* rotate hit point to "cyclone space" */
	radius = sqrt(intersect.x*intersect.x + intersect.y*intersect.y);

	if ( radius < max_radius ) {   /* inside of cyclone */
		/* invert distance from center */
		dist = max_radius - radius;
		dist *= dist;
		dist *= dist;
		angle =  PI + twist*TWOPI * (max_radius-dist)/max_radius;
		sine = sin( angle );
		cosine = cos( angle );
		point.x = texture.x*cosine - texture.y*sine;
		point.y = texture.x*sine + texture.y*cosine;
		point.z = texture.z;
		/* add "eye" of storm */
		if (radius < p3*max_radius) {
			/* normalize */
			eye_weight = (p3*max_radius-radius)/p3;
			eye_weight = 1.0 - eye_weight;	/* invert */
			eye_weight *= eye_weight; /* make nonlinear */
			eye_weight *= eye_weight;
		} else	eye_weight = 1.0;
		
		/* raise density towards center */
		value = (1.0 - radius/max_radius);
		value *= p0 * value;
	} else	{
		eye_weight = 1.0;
		point = texture;
		value = 0.0;
	}

	if ( eye_weight ) {
		/* "VLfBm()" is fBm built from "VLNoise()" */
		value += offset + scale*VLfBm( point, omega, 2., octaves );
		/* blacken the "eye" */
		value *= eye_weight;
	} else 	value = 0.0;

	/* add fine-grain, undistorted cloud texture */
	power = pow( 2., p2 );
	point = texture;
	SCALE( power, point );
	detail_height = p4*VLfBm( point, omega, 2., octaves );
	if ( detail_height < 0.0 ) detail_height = - detail_height;
	value *= value-p5 + value*detail_height;
	if ( value < 0.0 || !eye_weight ) value = 0.0;
	if ( value > 1.0 ) value = 1.0;

	colour->red *= value + (1.0-value)*Background.red;
	colour->green *= value + (1.0-value)*Background.green;
	colour->blue *= value + (1.0-value)*Background.blue;

} /* Cyclone() */