Interactive semi-qualitative simulation for virtual environments

3D virtual environments are being used in an increasing array of applications ranging from training systems, to video games and social communities. The steady growth of computing power and rendering capabilities makes them even more compelling as each day passes. These visually rich worlds are inhabited by intelligent agents and contain physics-based simulated artifacts ranging from a light switch to a complete aircraft. However, a whole category of applications, such as maintenance simulations and virtual construction sets, based on the ability to alter on-the-fly the structure of these devices has yet to be developed. The main reason is that for performance purposes physics-based models must be pre-compiled at the time the application is developed, and thus they cannot be altered at run time. We present the development of an interactive semi-qualitative simulation framework to support assembly-centric virtual environments. The framework's purpose is to simulate the physics-based behavior of complex devices and their interaction with virtual humans. In particular, we use automated model building methods to enable users and participating agents to alter the structure of a device while its behavior is kept physically consistent without suspending the simulation. For example disconnecting a pipe in a virtual hydraulic system may cause the fluid it carries to spill in the environment. The framework consists of a simulation engine and its object-oriented modeling language. The language captures the hybrid behavior of complex devices with hierarchical finite state machines. It encodes their continuous operation modes as differential algebraic equation systems. It also has features for semi-qualitative modeling to enable on-the-fly model composition. We showed the feasibility of our approach by using our framework to implement a virtual environment where an agent uses a construction set to assemble and modify hydraulic networks. Our uniform interfacing approach enables users and participating agents to assemble functional hydraulic networks by simply manipulating the 3D components of the construction set. We also experimented with on demand just-in-time C++ compilation of numerical models to improve the performance of the engine. In our experiments our parallel compilation technique yielded a speedup of 2.96 over interpreted arithmetic evaluation.