Hirohito Sato

This paper addresses the challenge of generating natural, interactive motion for embodied agents in social roles. We argue that such motion requires three key properties: Adaptability to dynamic environments and user interactions, switchability between different motions, and directability for expressing intentions and emotions. We propose a novel method that leverages overlapping minimum jerk trajectories to represent the motion of body parts, achieving both switchability and adaptability. Switchability is enabled by the ability to begin transitioning between minimum jerk trajectories at any moment, allowing the agent to smoothly change its motion. Adaptability is achieved by defining trajectory goals within a goal coordinate system relative to the target object or person. This dynamic goal ensures that motions remain contextually relevant as targets move. By representing motion as a sequence of minimum jerk trajectory goals, our approach shares similarities with keyframe animation, offering potential for directability. We demonstrate the efficacy of our method through various motion examples, evaluating its adaptability and switchability. Results indicate that our approach holds promise for generating natural and interactive agent behavior, albeit with some limitations in switchability requiring further investigation.

With a spread of inexpensive and easy-to-use motion tracking methods such as cameras and trackers for virtual realities (VRs), real-time motion-tracked avatars are becoming increasingly common in virtual environments, particularly social VRs, virtual performers (e.g., virtual YouTubers), and VR games. These applications frequently involve interactions among multiple avatars or between avatars and objects for communication or gameplay. However, at present, most applications do not sufficiently consider the effects of contact for avatars, thus leading to penetration or unnatural behavior. To achieve natural avatar motion in such scenarios, the player must perform as if contact has occurred, even though, in reality, there is no contact with the player’s body. While physics simulation can solve the contact issue, the basic use of physics simulation causes tracking delay. We therefore solve this tracking delay problem by employing Featherstone’s algorithm, a reduced-coordinate method for forward dynamics, and by utilizing the Newton-Euler formulation for inverse dynamics to compute tracking forces and torques. To evaluate the delay, we measured the difference between the joint rotations of the simulated model and the input joint rotations. The evaluation with our method indicates the difference is less than 1.4 micro radians for all joints in real time with a small computational cost. Moreover, accurate tracking enables the fixation of both feet. The proposed method provides accurate tracking and soft reactions to contact.