## 2 Materials and Methods
### Interaction Devices and Gestures  
The current trend in the automotive industry is to have multiple screens that provide information to drivers and passengers, usually reacting to the human touch. Interacting with hands on a car windshield is not feasible, and using remote buttons, levers, or knobs increases the number of iterations needed to access various functions. Consequently, we selected two gesture-based interfaces to compare their effectiveness in controlling different functionalities displayed on the windshield. The former is the UltraLeap Stratos [16]. Using two infrared cameras, this device can track the hand's position in 3D space and provide haptic feedback in mid-air using an array of ultrasonic emitters. It can enable a mid-air gesture with force feedback where the users are not required to touch any surface. The latter device, instead, is a standard capacitive trackpad that receives the touch input through a series of capacitors that sense the changes of potential charge across the screen when a finger is positioned above the pad's surface. For the development, we used a tablet with an opaque screen cover. We considered this second interaction a reference for the test due to its more comprehensive application, even outside the automotive field. To interact with the information shown on the windshield and to evaluate the different input techniques, six different interaction gestures (see Figure 5) were developed for the Stratos and the trackpad:  
Figure 4: Grid matches with the layout of the items inside the menu. * Finger interaction: By keeping the palm and fingers parallel to the floor, the users can activate a specific menu when a single finger is blended. For the trackpad, due to the impossibility of determining the specific finger doing the interaction, a different methodology has been developed: main sections are activated according to the number of fingers positioned on the trackpad;
* Swipe interaction: The hand movement in the 3D space from right to left gets detected and interpreted to return a direction; this movement is used to swipe between different menu elements on the windscreen. The same interaction is used but performed in the 2D space for the trackpad;
* Grid interaction: The information on the windshield is selected by moving the hand through a virtual grid positioned parallel to the floor and perpendicular to the windscreen. The interaction is the same for the trackpad, but fingers should swipe in the direction of the element to select it;
* Confirm: To confirm the selected menu elements, the users should perform a grab gesture by bending all the fingers from an open position to a closed one (\"fist\"). For the trackpad, they only need to touch and release the surface of the pad with a finger;
* Back: To return from a menu to a previous one, the user should rapidly swipe up, returning to the initial position. For the trackpad, they should perform a quick swipe with their fingers from the center of the pad to the bottom part;
* Volume: To modify the music volume, the users should put two fingers near each other (\"pinch\") and then move their hand on the axis perpendicular to the floor to turn up and down the volume. For the trackpad, they should put two fingers on the pad and then drag them up or down to increase or decrease the volume. Every time the Stratos registers an interaction, mid-air haptic feedback notifies the user; this notification consists of a brief (200 ms) push of air on the center of the user's palms. The feedback was immediately given when the gesture was considered complete  
Figure 5: Different gesture interactions for the Stratos (**left**) and trackpad interfaces (**right**). (**a**) finger interaction, (**b**) swipe interaction, (**c**) grid interaction, (**d**) confirm, (**e**) back, (**f**) volume. for all the interactions except the grid and volume. At the same time, in the case of the grid interaction, the feedback was given each time the hand changed the quadrant, and in the volume gesture, it was given continuously for its duration.