text: Title: how to crop the template patch
  Body: ![Screenshot from 2019-11-13 17-05-17](https://user-images.githubusercontent.com/10957718/68748755-f4e77980-0637-11ea-99aa-8c3a39e7b41b.png)
  
  
  I just want to know why not crop directly using target's bbox(w*h).
  Does the context of target matter? 
  