# Virtual Gymnasium: Personalized Weight Perception Interface in Lifting Virtual Objects
## 1 Introduction  
The immersive technology of augmented virtuality (AV) is widely used in many applications related to serious games, such as sports, healthcare, and rehabilitation [1, 2, 3, 4, 5, 6]. To add real-world information to the virtual world, the process of serious games can consist of three fields: recognition, calculation, and representation [7, 8]. Recognition in terms of human body information tracks (or identifies and classifies) the joints of the human body, in what is called the human pose estimation method, using computer vision-based technology [9]. Although recent human pose estimation methods provide high accuracy using deep learning-based algorithms, they are difficult to apply to augmented virtuality serious games because of performance issues. Hence, augmented virtuality serious games generally use conventional computer vision tracking methods [6] or controllers for performance. A serious game calculates the state based on recognized information and delivers results of the recognition to a user in the representation process. In the representation process, it is necessary to provide various perception methods to achieve the goal of augmented virtuality. In this study, we focus on the representation of virtual object manipulation using a human body tracking method, which is one of the challenging issues in augmented virtuality serious games. A user can grasp a virtual object and manipulate it with his/her real hands using a human body tracking method. Virtual perception of a virtual object is essential for perceiving the weights of virtual objects to increase the sense of presence and virtual experience in augmented virtuality serious games, such as sports and health care. Several researchers have focused on simulating mass in virtual objects by using haptic and visual senses. To obtain the sensation of mass in VR through haptic senses, exoskeleton-based haptic feedback interfaces have been used, where the devices are attached to body parts to provide forces on different parts of the body [10; 11]. Although haptic devices provide accurate and direct weight perception, they are generally expensive and difficult to wear and use in augmented virtuality serious games. Alternatively, a pseudo-haptic feedback approach has been used to simulate virtual weights to reduce the complexity of the haptic device. To provide weight perception of virtual objects with pseudo-haptic feedback, visualization of different heaviness of virtual objects is combined with simplified device feedback, such as tactile, skin pressure, and vibration [7; 12; 13; 14; 15]. Although pseudo-haptic feedback has established the feasibility of weight perception of virtual objects without complex haptic devices, the user still wears or carries some devices. Furthermore, conventional research restricts possible weight perception areas to the human hand. It is difficult to extend to other areas of the human body because of limited device configurations. Because of the above, recent virtual object weight perception research has focused on extending the perception area from the hand to the human body [16; 17; 18]. According to the experimental results, specific body postures and gestures can increase weight perception in terms of lifting objects. Another pseudo-weight perception method, which controls the lifting force of virtual objects with visual feedback, showed enhanced results of weight perception [16; 19; 20]. Although conventional weight perception methods improve the representation of virtual weights, the effect of weight perception is less effective for people with different body sizes, because the conventional weight perception module is mostly designed with a bias toward the median adult male body size. Furthermore, the material-weight illusion (MWI) issue can be raised by differences in the visual appearances of virtual objects of the same mass [21; 22; 23; 24]. Users may feel higher when they lift a heavier-looking object and heavier when they lift a lighter-looking object.