function curlNoise2D(x, y){
var n1 = simplex.noise2D(x + eps, y);
var n2 = simplex.noise2D(x - eps, y);
var a = (n1 - n2)/(2 * eps);
var n1 = simplex.noise2D(x, y + eps);
var n2 = simplex.noise2D(x, y - eps);
var b = (n1 - n2)/(2 * eps);
return [b, -a];
}