public void buildShell(final double k, final double width) { build( new Function3D() { public double eval(double x, double y, double z) { x = Math.min(x, _lx - x); y = Math.min(y, _ly - y); z = Math.min(z, _lz - z); double r = Math2.hypot(x, y, z); return (Math.abs(k - r) < width / 2.) ? 1 : 0; } }); }