surface
thirds(
	float	Ks = 0.5,				// specular coefficient
		Kd = 0.5, 				// diffuse  coefficient
		Ka = .1, 				// ambient  coefficient
		NoiseAmp  = 0.00,			// noise amplitude
		NoiseFreq = 1.00,			// noise frequency
		roughness = 0.1;			// specular roughness
	color	SpecularColor = color( 1, 1, 1 )	// specular color
)
{
	float RAMP = 0.5;				// over how much of the width to ramp up

	float TheHeight = 0.;			// how much displacement to apply

	varying vector Nf = faceforward( normalize( N ), I );
	vector V = normalize( -I );

	point PP = point "shader" P;
	float magnitude = 0.;
	float size = 1.;
	float i;
	for( i = 0.; i < 6.0; i += 1.0 )
	{
		magnitude += ( .5 - noise( NoiseFreq * size * PP ) ) / size;
		size *= 2.0;
	}

        color TheColor = Cs;

	float up = 2. * u;
	float vp = v;

	float WIDTH = 0.10;
	float numinu = floor( up / WIDTH );
	float numinv = floor( vp / WIDTH );
	float ucenter = numinu*WIDTH + WIDTH/2.;
	float vcenter = numinv*WIDTH + WIDTH/2.;
	point center = point( ucenter, vcenter, 0. );
	point upvp = point( up, vp, 0. );

	vector delta = upvp - center;
	float oldrad = length( delta );
	float newrad = oldrad + NoiseAmp*magnitude;
	float factor = newrad / oldrad;
	point uppvpp = center + factor * ( delta );
	float upp = xcomp(uppvpp);
	float vpp = ycomp(uppvpp);
	
	if( mod( numinu+numinv, 2. ) == 0. )
	{
		float umin = numinu*WIDTH;
		float umax = umin+WIDTH;
		float vmin = numinv*WIDTH;
		float vmax = vmin+WIDTH;
		float distu = min( upp-umin, umax-upp );
		float distv = min( vpp-vmin, vmax-vpp );
		float dist = min( distu, distv )/WIDTH;
		float t = smoothstep( 0., RAMP, dist );	   // 0. if dist <= 0., 1. if dist >= RAMP
		TheHeight = t;				// apply the blending
	}
	
	if( TheHeight > 0. )
		TheColor = color( 1., .5, 0. );
	
	Oi = Os;	// use whatever the rib file gave us

	Ci =        TheColor * Ka * ambient();
	Ci = Ci  +  TheColor * Kd * diffuse(Nf);
	Ci = Ci  +  SpecularColor * Ks * specular( Nf, V, roughness );
}