  This paper presents a robotic manipulation technique for dexterous
ungrasping. It refers to the capability of securely transferring a grasped
object from the gripper to the robot's environment, i.e. the inverse of
grasping or picking, through dexterous manipulation. The game of Go offers an
example: consider how the player would typically place an initially
pinch-grasped stone onto the board through the dexterous interaction between
the fingers, the stone, and the board. Likewise, dexterous ungrasping addresses
the necessity of changing the object's configuration relative to the gripper or
the environment in order to securely keep hold of the object. In particular, we
present a planning framework for determining a feasible minimum-cost motion
path that completes dexterous ungrasping. Digit asymmetry in a gripper, i.e.
difference in digit lengths, is discovered as the key to feasible and secure
ungrasping. A set of experiments show the effectiveness of dexterous ungrasping
in practical placement tasks.
