Google maps api 3 谷歌地图-如何对齐标记图像，使左下角指向标记的纬度/高度

给定w=图像的宽度，h=图像的高度，如何设置它，使url中指定的图像左下角指向标记定义的纬度和经度

...
'icon': new google.maps.MarkerImage(url,
  new google.maps.Size(w, h),
  new google.maps.Point(?, ?),
  new google.maps.Point(?, ?)
),
...

...
'icon': new google.maps.MarkerImage(url,
  new google.maps.Size(w, h),
  new google.maps.Point(0,0),
  new google.maps.Point(0,h)
),
...