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surface Rust(float smooth = 25;float severity = .5;float multipass = 10;float age = 60; ) {
//float smooth = 20; // Higher numbers for less blocky rust. ~ 12 - 45
//float severity = .8; //for mixing, 0-1.
//float multipass = 10; //1-inf
//float age = 60; //You start to see rust around 25.  Objects disappear around 90.  Max 112

float displace = 0; // Whether or not to displace this pixel.

//This will give us the patchy effect.
float phase = noise(s*(smooth),t*(smooth));  

//We don't want age-phase to go below 1.
if (age - phase < 1) phase = age - 1;

  normal Nf;
    
  Nf = faceforward( normalize(N), I );
//From Dave's example
	vector NS = vtransform( "shader" , N ); 
	point PS  = transform( "shader" , P ); 

	float i; //Really...

//Like blinn, only more convenient. :)
	color thispixel = Oi * ( (Cs * (diffuse(Nf) - .3)) + 
		(specular(normalize(N),normalize(-I),8) *15)
                 * 6 /*.025*/);

//Now rust it.
	//Blend a random color out of the "ramp" texture.
	
	for (i=0;i<multipass;i = i + 1) {
		//No particular numeric significance here, but I'm trying to adjust the 
		//amount of rust vs. metal based on age, and this does a good job.
		if (phase > ((1/112) * age))break;

		//Different lighting for the rusted metal...
		if (displace = 0) { //Always do this only once per pixel
			thispixel = Oi * ((thispixel*(diffuse(Nf)*12)) + 
				(specular(normalize(N),normalize(-I),2)*4)*9);
		}

		//Rust. -- This will blend a random rust color up to element <age> in 
		//our ramp, <multipass> times.
		thispixel = mix(thispixel,
				/*Lookup from our color ramp*/ 
				color texture("rust.tex",
				(1/112)*round(random() * age),.5), 
						severity);
		displace = 1;
	}

	if(displace == 1) {
		//Displace a little, based on age, but only if the spot is rusted.
		PS += (phase * (.0005 * age)) * normalize(NS);
	}	

	Oi = Os;
	Ci = Oi * thispixel;

	P = transform( "shader", "current", PS ); 
	N = calculatenormal(P);
}