Parameters for cropping

[smartnews-weitao]

Hi team, i am currently trying to use thumber for image cropping. For example, the original image size is 20002000, i am trying to produce a 1200628 version by cropping. I think i can use following parameters for easy cropping.

&w=1200&h=628&cm=1

However i am wondering how can i specify a starting position? For example i want to crop from a certain point? Appreciate your help.