OVR Stylus: Designing Pen-Based 3D Input Devices for Virtual Reality Bret Jackson* Macalester College Saint Paul, Minnesota, USA Figure 1: The OVR Stylus is a pen-shaped 3D input device for VR/AR environments. Wireless communication transmits button clicks and values from a sliding touch pad. A haptic motor provides vibrotactile feedback. ABSTRACT are developing pen-shaped 3D input devices. Examples include the Logitech VR Ink [22], Massless Pen [24], and Holo-Stylus [16]. We present the OVR Stylus, an open-source, tangible, haptic prop for 3D input in virtual environments. The design offers an alterna- In this paper, we present the OVR Stylus, a pen-shaped, tangible tive to commercial 3D pen input devices, and represents a second- prop for 3D input. The stylus features two buttons and a touch pad generation hardware design based on six years of use with a previous slider for input that communicate wirelessly with a host computer. prototype. With a touch pad, two input buttons, LRA-based vibrotac- A linear resonant actuator (LRA) motor enables vibrotactile haptic tile haptic feedback, and Bluetooth communication, the 3D-printed feedback. The design presented here is a second-generation proto- prop’s light-weight design (35g) enables creative and precise interac- type, refining an initial model that has been used in our lab with VR tion. Through a discussion of design considerations and motivations, applications for the last six years. components, manufacturing processes, software, and lessons learned Although pen-shaped 3D input devices are not in themselves we enable others to create their own OVR Stylus or develop similar novel, to our knowledge, no other fully-realized, open source im- designs. plementation exists using modern hardware. Our primary contri- bution is the hardware and software implementation, which other Index Terms: Human-centered computing—Human com- researchers can use to produce their own OVR Stylus. Furthermore, puter interaction—Interaction devices—Pointing devices; Human- the hardware design can serve as a building block for modifications centered computing—Human computer interaction—Interaction to create other formats of novel 3D input devices. In addition, we paradigms—Virtual reality provide design criteria, discussion, and lessons learned to guide this future work. 1 INTRODUCTION 2 RELATED WORK In virtual reality (VR) and augmented reality (AR) users typically interact with a hand-held device (e.g. HTC Vive Controllers) to Many stylus input devices have been created, but they are used perform common operations like selection, manipulation, and system predominantly for surface computing or touch displays (e.g Wacom control. Driven primarily by the gaming industry, these devices are Pen [15], Elastylus [23], Haptylus [27], ImpAct [35]). Here, we commonly held using a power grip, similar to how a user might hold limit our discussion to those relevant to mid-air 3D input for use a handle or lift a coffee mug. For many games, this type of grip and with VR or AR displays. controller makes sense. It closely mimics how someone might hold Notable examples include work by Teather and Stuerzlinger [33] a tool, and predominantly uses the force of the hand rather than the and Pham and Stuerzlinger [28] that uses an un-powered pen-like strength of individual fingers. device fitted with optical tracking markers to explore 3D pointing. However, holding a controller using a power grip limits preci- Similarly, Brown et al. [8] use a chopstick tracked with a Leap sion [9]. Indeed, recent work by Pham and Stuerzlinger [28] found Motion [34] for pointing. SymbiosisSketch [3] uses a traditional that in a Fitts’ Law task, a pen-shaped prop using a precision grip writing pen fitted with motion capture markers to support mid-air outperformed a standard VR controller in pointing speed, mouse con- sketching. Each of these examples demonstrates the utility of pen- trol, errors, and throughput; it reached comparable input to a mouse. shaped input devices for VR, but their focus is on studying 3D It is no surprise that with increasing use of VR/AR technologies pointing performance or application design rather than creating a for consumer applications beyond gaming, commercial companies functional input device for everyday use. With the exception of Pham and Stuerzlinger [28], which includes a short description *e-mail: [email protected] of the VR/AR pen design space, these references provide further motivation for the need for pen-shaped VR input devices, but little in the way of concrete designs. Most existing functional 3D stylus designs are tethered with a cord to a host computer to facilitate communication of button presses Figure 2: OVR Stylus circuit schematic. The hardware contains a battery charging circuit, variable selection of the power source, and a voltage regulator to output 3.3v power. The Redbear MB-N2 module houses a Nordic nRF52832 BLE chip and antenna. and provide power. Examples include the Fasttrak Polyhemus Dig- more closely mimics the shape of a pen or marker and avoids ambi- itizer [11], and the zSpace stylus [36]. The Tactylus stylus [21] guity in how the device should primarily be held using a precision and the stylus presented by Kamuro et al. [19] are similar, but add grip (while still allowing for a power grip if needed). additional haptic feedback to the devices. However, the tether limits The wireless connectivity is a primary functional characteristic. mobility for use in room-scale displays (e.g. a VR CAVE), and can Without a tether providing power, this design decision necessitates cause increased fatigue due to balance issues from the cord [21]. that the electronic components have low consumption to avoid the This prior work influenced the design decision to make the OVR weight and size impact of heavier batteries. Along these lines, Stylus fully wireless. the first prototype required the user to disassemble the stylus to The SenStylus [12] is perhaps the most similar to the OVR Sty- remove the battery for recharging. To improve the utility of the OVR lus design and holds comparable motivations with our prior work Stylus, the current design needs to include a battery charging circuit, in developing pen-based input devices for immersive 3D model- allowing the device to be recharged through a USB port without ing [17] and other VR applications. The SenStylus consists of a disassembly. stylus containing buttons and analog input linked to a wireless Log- For 3D tracking, we considered adding an inertia measurement itech Rumblepad game controller worn on the user’s arm. In contrast unit (IMU) to calculate orientation. However, this approach would to this 15 year old design, the OVR Stylus provides a modern imple- require an external way to measure the absolute spatial position of mentation. It does not require the cumbersome wrist-worn device the stylus. Instead, the design integrates optical tracking markers and is both lighter and thinner. for use with common motion capture systems (e.g. NaturalPoint OptiTrack). 3 DESIGN CONSIDERATIONS The last functional consideration is to limit the number of buttons Our design considerations were informed by six years of use with on the device. Although it is a common practice in user interface a first generation stylus prototype used for input in a VR CAVE design to map each new function to a separate button; this approach with applications for scientific visualization and 3D modeling. This does not scale. Instead, we follow the recommendation of Jackson et experience instructed our design goals for both the physical charac- al. [18] to minimize the number of buttons and instead use context- teristics of the device and functional characteristics. based interaction to interpret button input. Our experience with the One of the main advantages of the first prototype is its light first prototype, which also included only two buttons, has been that weight. We have used it in multiple 2–5 hour VR sessions with this characteristic improves learnability of the interface and makes it relatively little discomfort compared to using traditional and heavier more intuitive to use. wand input devices, like the HTC VIVE Controllers. This is particu- larly important for 3D input devices that are used for large mid-air sweeping input to avoid the common gorilla arm effect [7]. Manag- 4 OVR STYLUS ing weight was a key requirement for the new design. Prior work has also found that the weight distribution of a stylus can impact The OVR Stylus is made from the components shown in Figure 3. fatigue with use [28]. As a result, the weight distribution in the OVR A custom-designed printed circuit board (PCB) houses the elec- Stylus is centered on the user’s hand as much as possible. The total tronics and micro-controller with a Bluetooth Low-Energy (BLE) assembled weight is about 35 grams. module and antenna. For user input, there are two buttons and a Six years ago, our first prototype was also quite thick, with a touch pad created by a sliding soft membrane potentiometer that diameter of 25mm. At the time, the smallest off-the-shelf micro- gives analog values. A LRA haptic motor and controller board is controller development boards available were 18mm wide. Although mounted at the front of the stylus for vibrotactile feedback. The the first prototype was cylindrical in the shape of a pen, some users 3D printed case allows interchangeable tips and optical tracking would initially hold it like a baton with a power grip. A primary goal configurations. In the sections below, we describe the hardware and of the redesign is to keep the OVR Stylus as thin as possible. This software implementations in more detail. Table 1: Summary of primary hardware components. Component Part Description Micro-Controller & BLE RedBear Labs MB-N2 nRF52832 Module.