## 4 Methodology
### Gesture Definition
#### 4.1.2 Gesture Vocabulary  
In a previous user study and survey [32], we tested two sets of gesture vocabularies, a finger-based and a palm-based vocabulary in a simulated drone navigation environment. Users were required to navigate a virtual drone in an outdoor and an indoor environment. In the finger-based mode, each gesture is defined by which fingers are extended, and the velocity value is defined by the pointing direction of a selected extended finger. In the palm-based mode, the operator uses the positioning and orientation of his/her open palm (all finders extended) to control the drone. The subjective study results showed a clear user preference for the palm-based control, which was described as more comfortable and easier to learn and use. The objective metrics showed faster task completion using the palm-based controls, however, finger-based control offered more precise navigation. The overall objective score showed that the palm-based control has a slightly better performance. The study results are aligned with findings in [33] that intuitive NUIs should be based on a common metaphor. In this case, the metaphor chosen is that the drone corresponds to the user's hand, and hence mimics and follows all its movements. Therefore, raising the hand up higher will correspond to an ascend (increase throttle) command; a tilt of the hand will correspond to the drone assuming a similar angle (pitch and/or roll); a horizontal rotation will correspond to yaw; and so on. This metaphor, validated by the user study, addresses the first aim of gesture control. The second aim of gesture control is to avoid non-ergonomic, physically stressing hand gestures. It may be noted that even easy-to-perform gestures can be tiring when performed repeatedly or may oblige users to hold their hand/arm steady in the same height for a long time. In order to avoid the latter, we applied a relative gesture vocabulary based on a varying reference position. One key gesture is used as a reset and calibration command. When the user performs this key hand gesture, the current hand pose is set as the neutral (resting) position, and all following gestures are interpreted relative to this. This allows users to chose the resting position that is most comfortable to them and even define new resting positions in preparation for certain maneuvers. Hence, a user wishing to command the drone to ascend may define a low resting position, allowing her to move her hand higher with no discomfort. Later, in preparation for a descent, a higher resting position may be chosen so that the user can lower her hand easily. The reset and calibration command is tied to the user opening the palm and extending the fingers. Hence, after a brake and stop command (closed fist), a new restingposition is always defined before the user issues a new command. While the hand is closed and no commands are issued, users can position their hand in preparation for the next gestures safely. This palm-based gesture set was expanded in later work to include take-off and landing gestures [35]. Similar to a handheld remote, these gestures are mapped to hand positions assumed and held over a couple of seconds. To differentiate them from the other UAV control commands, take-off and landing use an upwards-facing palm. The full palm gesture vocabulary used for drone navigation is shown in Figure 6. Moreover, the user's other hand can be used to control the camera gimbal, where the same set of gestures are mapped to camera pitch and yaw. It can be noted that the selected gesture set can be used to perform any UAV motion achievable with the traditional handheld remote. Velocities of UAV commands are mapped directly to the angles or distances relative the resting position, allowing for a continuous control ranging from no motion at all to fast motion. Combination motions, such as pitching forward while yawing a little to the left and slowly descending are easily achievable.