float square(float x, y, minx, maxx, miny, maxy, taper)
{
return	smoothstep(minx, minx + taper, x) *
		(1 - smoothstep(maxx, maxx + taper, x)) *
 		smoothstep(miny, miny + taper, y) *
		(1 - smoothstep(maxy, maxy + taper, y));
}

displacement
repeatbump(float 	Km = 0.1,
					min_s = 0.1, max_s = 0.8,
					min_t = 0.1, max_t = 0.8,
					fuzz = 0.1, 
					s_repeats = 4, t_repeats = 4)
{
float	hump = 0;
normal	n = normalize(N);

float ss = mod(s * s_repeats, 1);
float tt = mod(t * s_repeats, 1);

hump = 	square(ss, tt, min_s, max_s, min_t, max_t, fuzz);

P = P - n * hump * Km;
N = calculatenormal(P);
}