function getRelativeOffset(x, y) {
  x = x | 0;
  y = y | 0;
  const xx = (x - this.cx) / this.cs;
  const yy = (y - this.cy) / this.cs;
  return ({
    x: xx,
    y: yy
  });
}
@param {Number} x
@param {Number} y
@return {Object}