This document describes techniques and systems that enable a radar-image shaper for radar-based applications. A radar field enables an electronic device to accurately determine a characteristic disposition (e.g., a location, orientation, velocity, or direction) of an object in the radar field. The characteristic disposition is determined by detecting a radar cross-section (or radar signature) of a radar-image shaper that is included in the object. The shape of the radar-image shaper produces a known signature when illuminated by the radar field. Using these techniques, the electronic device can determine a characteristic disposition of the object, which allows the object to be used to interact with the electronic device using gestures and other position-based techniques. Because the radar-image shaper enables a passive object to control applications on the electronic device, users have an interaction method with a rich library of gestures and controls that does not require additional components or a battery.