Thanks!

It works! 
Though I need to additionally right-click -> "Group selected" on the expression value, and then expand it to get to the
value selection. So if I could directly copy the expression result to clipboard then it would be absolutely perfect.
It would be also  nice to remember the expression somehow when I open and crop another image.
Can it be done?


About my workflow: I process a lot of videos, and I create "job" tables in Excel. Then I have a Python script that parses the job 
table with all parameters and passes it to FFmpeg commands.

One of the job parameters is crop value. Other parameters I get from video player, like time cut points.

So when I look through videos, I capture needed frame and then I need to define the crop region.
Many programs can define region, but those I've tried before cannot compose and copy a string with crop region easily and
I need to manually write them down in Excel table.

The string for FFmpeg crop filter is in format w:h:x:y.
So I can minimize the manual input and make the workflow more pleasant.


Mikhail