Animating virtual characters using physics-based simulation

Over the past decades, physics-based simulation has become an established method for the animation of passive phenomena, such as cloth, water and rag-doll characters. The conception that physics-based simulation can also be used for animating actively controlled characters dates back to the early stages of computer animation, and has incited many research papers since. However, commercial animation frameworks still resort to kinematics-based approaches when it comes to animating active virtual characters. To understand this reservation, it is important to recognize the scope and complexity of controlling simulated characters. Just like with real-world characters, the pose of a physics-based character is controlled indirectly, through forces and torques generated by actuators that reside inside the body. As a result, the global position and orientation of a physics-based character cannot be controlled directly, but only through deliberate manipulation of external contacts. This poses a direct challenge to basic tasks such as balance and locomotion – a challenge that has no equivalent in traditional kinematics-based animation. Instead, it bears a much closer relation to humanoid robotics and control theory. In addition to these control challenges, physics-based virtual characters should also behave in a way that is visually appealing. In order to look natural, the motion of a character must respect the same biological constraints that are present in natural beings. Recently, researchers have begun incorporating such constraints into their character models, based on results from biomechanics research. This trend is gradually changing the direction of physics-based character animation research, and is further broadening its scope. To implement all these aspects into a robust and flexible framework has proven to be a daunting task. However, recent trends show a renewed interest in physics-based character animation. After decades of floundering, the field is maturing, with many recent publications demonstrating tremendous progress in both robustness and visual quality. As such, physics-based character animation remains an exciting research topic that is likely to play an increasingly important role in computer animation in the years to come. This thesis describes various elements involved in animating virtual characters using physics-based simulation. The first part contains a thorough review of several fundamental aspects of physics-based character animation, including modeling, simulation and control. The second part describes the results of research on how simulation and modeling techniques can be used for the assessment of different aspects of motion, without the use of a control strategy. The third part of this thesis describes two novel control methods for physics-based characters. The first method controls physics-based bipeds in a simulated environment, based on a variety of reference motions. The second method is a muscle-based control strategy for simulated bipeds, in which both the muscle routing and control parameters are optimized. This thesis concludes with a summary and an overview of possible directions for future research