Geometry-aware online motion retargeting is crucial for real-time character animation in gaming and virtual reality. However, existing methods often rely on complex optimization procedures or deep neural networks, which constrain their applicability in real-time scenarios. Moreover, they offer limited control over fine-grained motion details involved in character interactions, resulting in less realistic outcomes. To overcome these limitations, we propose a novel optimization framework for ultrafast, lightweight motion retargeting with joint-level control (i.e., controls over joint position, bone orientation, etc,). Our approach introduces a semantic-aware objective grounded in a spherical geometry representation, coupled with a bone-length-preserving algorithm that iteratively solves this objective. This formulation preserves spatial relationships among spheres, thereby maintaining motion semantics, mitigating interpenetration, and ensuring contact. It is lightweight and computationally efficient, making it particularly suitable for time-critical real-time deployment scenarios. Additionally, we incorporate a heuristic optimization strategy that enables rapid convergence and precise joint-level control. We evaluate our method against state-of-the-art approaches on the Mixamo dataset, and experimental results demonstrate that it achieves comparable performance while delivering an order-of-magnitude speedup.

Abstract Character hit reaction is an inherent component in game development. Natural hit reactions in games are typically achieved through the use of artist‐created hit animations and motion capture. To improve the realism of impact reactions, game developers combine physics simulation with distinct hit animations based on character statuses. However, there is currently no method that can automatically produce hit reactions based on hit information in game development. To this end, we propose a physics‐driven inverse kinematic method for generating character reaction animations. We postulate that a character's hit reactions are the result of an assault impulse spreading throughout the body and forcing the body to move. Five IK (inverse kinematics) solvers are used to control character poses. Each IK solver is used to control the movement of a different part of the body. The IK solvers, which are used to determine the positions of various bodily parts, are driven by unconstrained physics simulation. Furthermore, physics simulation with constraints is used to fine‐tune the character's movements. Experiment results show that our method outperforms Unreal Engine‐based hit animation and physics simulation.