surface 
filament ( 
	float	frequency	= 5.0,
		phase 		= 0.0, 
		width		= 0.15 )
{
	/* Calculate the distance of (s,t) from a spiral as a fraction [0,1] */
	float offset = mod((t*frequency + s + phase), 1.0);

	/* Threshold the fraction against the fractional filament width */
	if (offset < width) {
		Ci = 0.0;
		Oi = 1.0;
	} else {
		Ci = 0.0;
		Oi = 0.0;
	}
}