Is there a way to sample " below" for sbsar or something?

For example I want to sample  all  the height beneath  of  a curent layer , blur it  and add the current height  then apply  compare mask   that wouldn't see that blur on "below" .    How can I do it?

By anckor maybe?  But looks like it sends higher only content of current layer and not combined image of whats bellow.  

Like this?   Can I do my own compare  filter   and how can I sample "bellow"?