So do i have to use prim option no matter what ? Yes, rug was less than 0,5 m. Then as you told, i added a hidden surface making it 0,53 m. i uploaded and set physics to prim. Omg, it ended up with 450 LI. Also i didn't get the logic @ "add some extra geometry slightly above the rug somewhere to trick the sim server to beleive the walking surface is slightly lower than it actually is." You put extra geometry above but it reads surface lower ?