/* Copyrighted Pixar 1989 */
/* From the RenderMan Companion p.367 */
/* Listing 16.24  Displacement shader providing light-bulb threads to cylinder */

/*
 * threads(): wrap threads around a cylinder 
 */
displacement 
threads ( 
	float   Km		=  .1,
		frequency	= 5.0,
		phase		=  .0,
		offset		=  .0,
		dampzone	=  .05 )
{
	float magnitude;
 
	/* Calculate the undamped displacement */
	magnitude = (sin( PI*2*(t*frequency + s + phase))+offset) * Km;

	/* Damp the displacement to 0 at each end */
	if( t > (1-dampzone)) 
		magnitude *= (1.0-t) / dampzone;
	else if( t < dampzone )
		magnitude *= t / dampzone;

	/* Do the displacement */
	P += normalize(N) * magnitude;
	N = calculatenormal(P);
}