Motion Tracking: How to detect features inside grease pencil?

Hi there,
I’m using version 2.79RC and can’t get the “detect features” button to place the features just inside the grease pencil layer. It’s driving me crazy… Can anyone help me please?!
Thanks
Pete

Solved: Paint the area with grease pencil. Press button “detect features”. Press F6. Select “Inside Grease Pencil”.

Cool thanks!