Hybrid Data- And Model-driven Techniques For Virtual Character Motion Generation And Control

Currently the motion generation of virtual characters is encumbered by the difficulties such as tedious and time-consuming labor, poor controllability, bad adaptability etc.Based on the hybrid data-and model-driven strategy, this thesis studies the motion generation and control techniques of virtual characters deeply by focusing on the human motion system modeling, quick generation of grasping or operating motions with changes of the whole body posture considered, automatic and interactive whole body postures generation and motion editing and interactive control of virtual characters with multi-priorities. In the dissertation, corresponding control strategies and models are proposed and the traditional algorithms are improved at the same time. The main contents of the dissertation can be summarized as follows:(1)Firstly, in chapter one, the actual research states of the virtual character motion generation and control technology are summarized and the advantages and disadvantages of different methods are compared. In order to get the best trade-off between the lifelikeness, the controllability and the adaptability, the hybrid data-and model-driven strategy is adopted to start the research. Afterwards, in chapter two, the modeling methods of the human motion system are studied deeply so that the mathematical foundation of further researches can be established.(2) On the quick generation of grasping or operating motion with changes of the whole body posture considered, the combination of posture samples and the analytical inverse kinematics algorithm is proposed to ensure the real-time interactivity of the whole algorithm. The upper limb joint angles are solved using the analytical inverse kinematics algorithm on-line and the remaining joint angles are directly generated by posture samples which are constructed off-line. For the redundancy problem in the 7 DoFs (Degree of Freedoms) upper limb, a new objective function to minimize the wrist joint angles is presented and the optimized swivel angel is calculated efficiently by combining the optimization with the directed searching method. Furthermore, the collision detection and joint limit constraints are considered in the calculation to ensure the physical correctness of the upper limb posture. In the off-line design of the posture library, a new posture sampling and design principle based on the vertical height of the shoulder joint is proposed. Once the reaching target is given, the corresponding posture sample can be determined efficiently from the posture library according to the shoulder joint height values of different posture samples.The main advantage of the whole process is its high speed and therefore suitable for interactive applications such as games, operation trainings and so on.(3) On the automatic and interactive whole-body key posture generation, the combination of posture samples and the PIK (Prioritized Inverse Kinematics) algorithm is proposed. Firstly, the guess of the final desired body posture is generated by posture samples according to constraints specified by the user and used as the initial starting posture needed by the PIK algorithm.Then, starting from this initial guess and with all constraints considered at different priority levels, the PIK algorithm can be initialized with a bias defined by this particularly initial guess and iterated step by step to get a final refined posture. In this way, the limitation of traditional example-based IK (Inverse Kinematics) methods that they can only generate postures similar with examples can be overcome and simultaneously the naturalness of final posture results can be ensured to a certain extent. The parametric posture splicing technique is proposed to generate the guess of the desired whole body posture. In the first step, according to the set of the high level constraints, the whole body is divided into some partial joint groups, whose postures are then generated from the same posture database by using the parametric interpolation of the nearest neighbors and consequently the spliced whole body posture has a certain global similarity with the finally desired posture.Afterwards, starting from this initial guess,the PIK algorithm is used to refine the body posture step by step.To ensure the static balance of the final posture result, during the iterative loop of the PIK algorithm, a new balance calculation strategy that solves the static balance constraint and other task constraints individually rather than in the same priority loop is proposed. By this way, the problem in the determination of the priority level of the static balance constraint is solved and the balance requirement of the final posture result is satisfied simultaneously. The combination of the parametric posture splicing and the PIK algorithm can not only overcome the restriction that all posture samples must have the same kind of constraints in parametric interpolation methods and reduce the required number of posture samples but also can solve the initial posture selection problem in the PIK algorithm and thus improve the running efficiency of the PIK algorithm by reducing the number of needed iterative steps.In the summary, a pragmatic and efficient algorithmic framework is presented for the automatic and interactive generation of the whole body posture.(4) In order to make virtual characters driven by motion capture data response adaptively and lifelikely to different interactive control inputs from users or unpredicted small-scale dynamic disturbances from external environments,an interactive motion editing and control algorithm with multi-priority is presented under the acceleration level within the redundancy space of different task constraints.After a single motion sequence is given, the desired joint accelerations can be generated according to control inputs and their relative priorities set by the user and simultaneously the dynamic balance of motion adaption results can be corrected on-line by directly adjusting the desired joint accelerations.During the process of the multi-priority motion adaption of the original reference motion, the definition of the selection matrix is proposed so that the task-level constraints defined no matter in the Cartesian space or joint space can be processed in a unified way. When small-scale dynamic interactions are detected, the system is switched to the dynamic simulation immediately to generate reactive motions. Using desired joint accelerations generated by the multi-priority motion adaption algorithm, the dynamic tracking controller required for the forward dynamic simulation can be constructed conveniently by the model-based inverse dynamics with contact constraints considered. During the dynamic simulation, the combination of the optimization and the approximation method of contact forces are proposed to compute active control torques. In this way, the running efficiency of the dynamic simulation can be improved and the accuracy of the dynamic controller can be ensured simultaneously. When the dynamic perturbations are removed, a convenient transition method is proposed so that the system can switch back to the kinematics motion editing process to improve the running efficiency. Under the acceleration level, a seamless transition between the kinematics and dynamic motion editing is realized and thus a unified motion control algorithm is provided.(5) At the end of the dissertation, based on the currently available virtual character software, the prototype of the integrated software platform for the virtual character motion generation and control is established according to their corresponding advantages and disadvantages.The prototype of the whole software platform can be divided into the foreground interface layer, the middle data layer and the background calculation layer. Particularly, the commonly-used commercial software Poser and DI_Guy are used respectively as the interactive interface and the rendering engine and thus the development of corresponding applications can be facilitated. Finally, the module of virtual characters in the training simulator of a certain kind of missile equipment is developed to evaluate different motion generation and control algorithms and the prototype of the integrated software-environment proposed in the dissertation.