## 1 Introduction  
Augmented reality (AR) has been widely utilized in various fields because it can visually superimpose different information, such as text, images, videos, and 3D virtual objects, onto corresponding physical artifacts. In the early stage of AR, it was mainly applied to entertainment, such as games, but recently it has been expanded and applied in various other fields, such as assembly, maintenance, training, remote collaboration, and smart manufacturing [1; 2; 3; 4; 5]. In different manufacturing fields, an industrial worker can perform a variety of tasks, such as monitoring, maintenance, assembly, and inspection of equipment and products [6]. In performing these tasks, the worker must review the dynamically changing status of equipment and products in real-time and take appropriate actions [7]. AR can support task assistance by overlaying machine status information onto a related physical artifact and by providing step-by-step instructions. Thus, the worker can perform their tasks more effectively. However, the worker must use a separate handheld device, such as a smartphone or tablet, to acquire task assistance. In particular, novice workers are required to have other training and instruction manuals when performing such tasks in an actual manufacturing environment. Thus, the worker cannot observe the equipment, products, and handheld device at the same time while performing tasks because their attention would be distracted. For this reason, wearable AR with smart glasses is considered to help industrial workers to more effectively perform their tasks [7; 8; 9]. Different types of AR smart glasses have been developed and applied to various industrial fields. Figure 1 shows wearable AR-based task assistance to workers wearing AR smart glasses, such as visualizing the simulation of assembly, monitoring production information, and inspecting the status information of a robot. AR smart glasses, such as Epson Moverio [10] and Microsoft HoloLens [11], have been developed and commercialized to provide users with wearable AR applications in mobile environments. In the wearable AR, it is essential to help the worker to understand and interact with the AR-based visual instructions more intuitively with a less cognitive burden. For example, when the worker must follow different types of annotations augmented on the display of the smart glasses (right of Figure 1), it is necessary to provide user-friendly visual augmentation forms on the optical see-through display of the AR smart glasses. However, AR smart glasses with a see-through display have a limited display size and viewing angle [6]. For example, Epson Moverio BT-200 has a 0.42 inch wide LCD panel, a dual display with \(960\times 540\) resolution, and a field of view (FOV) of approximately 23 degrees [10]. The FOV in current smart glasses is not large enough to display all the necessary information, which limits user-centric task assistance. Furthermore, the method for user interactions in the wearable AR environment is not natural and limited not to be applied to manufacturing services since it cannot support two-handed operations, and interaction is only supported by a multi-touchpad (e.g., Epson Moverio). Recently, two-handed operations have been supported in other smart glasses, such as MS HoloLens, but it can only support simple interactions, such as clicking and selection. It is crucial to analyze user interactions concerning not only visual contexts but also interaction metaphors to help the worker to effectively utilize AR smart glasses with limited capabilities for task assistance [12]. Note that usability is the extent to which a product can be used by specified users to achieve specified goals with effectiveness, efficiency, and satisfaction in a specified context of use. On the other hand, user experience is a person's perceptions and responses that result from the use or anticipated use of a product, system or service, and thus it includes all the users' emotions, beliefs, preferences, perceptions, etc. [12].