/**
     *  Figure out which ring a particular point corresponds to.
     *
     * @param x,y  Coordinates of the point
     * @returns Index of ring
     */
    public int ringID(int x, int y) {
	    int id;
	    int r = diameter/2;
	    int rp = (int)Math.sqrt((x-r)*(x-r)+(y-r)*(y-r));
	    if (rp <= r) {
	        id = (rp*color.length)/r;
	    } else {
	        id = color.length;
	    }
	    id = color.length-1-id;
	    return id;
    }