function prepareBgPos(/* prop, */_, value) {
    if (value instanceof Array) {
      var x = trueDimension(value[0]).v;
      var y = trueDimension(value[1]).v;
      return [!Number.isNaN(x * 1) ? x : 50, !Number.isNaN(y * 1) ? y : 50];
    }

    var posxy = value.replace(/top|left/g, 0)
      .replace(/right|bottom/g, 100)
      .replace(/center|middle/g, 50);

    posxy = posxy.split(/(,|\s)/g);
    posxy = posxy.length === 2 ? posxy : [posxy[0], 50];
    return [trueDimension(posxy[0]).v, trueDimension(posxy[1]).v];
  }
Returns the property tween object.
@param {string} _ the property name
@param {string} value the property value
@returns {number[]} the property tween object