James K. Hahn

Realistic animation of rigid bodies

The theoretical background and implementation for a computer animation system to model a general class of three dimensional dynamic processes for arbitrary rigid bodies is presented. The simulation of the dynamic interaction among rigid bodies takes into account various physical characteristics such as elasticity, friction, mass, and moment of inertia to produce rolling and sliding contacts. If a set of bodies is statically unstable, the system dynamically drives it toward a stable configuration while obeying the geometric constraints of the system including general non-holonomic constraints. The system also provides a physical environment with which objects animated using more traditional techniques can interact. The degree of interaction is easily controlled by the animator. A computationally efficient method to merge kinematics and dynamics for articulated rigid bodies to produce realistic motion is presented.

Sound rendering

We present a general methodology to produce synchronized soundtracks for animations. A sound world is modeled by associating a characteristic sound for each object in a scene. These sounds can be generated from a behavioral or physically-based simulation. Collision sounds can be computed from vibrational response of elastic bodies to the collision impulse. Alternatively, stereotypic recorded sound effects can be associated with each interaction of objects. Sounds may also be generated procedurally, The sound world is described with a sound event file, and is rendered in two passes. First the propagation paths from 3D objects to each microphone are analyzed and used to calculate sound transformations according to the acoustic environment. These effects are convolutions, encoded into two essential parameters, delay and attenuation of each sound. Timeciependency of these two parameters is represented with key frames, thus being completely independent of the original 3D animation script. In the second pass the sounds associated with objects are instantiated, modulated by interpolated key parameters, and summed up to the final soundtrack. The advantage of a modular architecture is that the same methods can be used for all types of animations, keyframed, physically-based and behavioral. We also discuss the differences of sound and light, and the remarkable similarities in their rendering processes. CR

Towards usable VR: an empirical study of user interfaces for immersive virtual environments

This paper reports empirical results from a study into the useof 2D widgets in 3D immersive virtual environments. Severalresearchers have proposed the use of 2D interaction techniques in3D environments, however little empirical work has been done totest the usability of such approaches. We present the results oftwo experiments conducted on low-level 2D manipulation tasks withinan immersive virtual environment. We empirically show that theaddition of passive-haptic feedback for use in precise UImanipulation tasks can significantly increase user performance.Furthermore, users prefer interfaces that provide a physicalsurface, and that allow them to work with interface widgets in thesame visual field of view as the objects they are modifying.

Image Morphing Using Deformation Techniques

This paper presents a new image morphing method using a two-dimensional deformation technique which provides an intuitive model for a warp. The deformation technique derives aC1-continuous and one-to-one warp from a set of point pairs overlaid on two images. The resulting in-between image precisely reflects the correspondence of features specified by an animator. We also control the transition behaviour in a metamorphosis sequence by taking another deformable surface model, which is simpler and thus more efficient than the deformation technique for a warp. The proposed method separates transition control from feature interpolation and is easier to use than the previous techniques. The multigrid relaxation method is employed to solve a linear system in deriving a warp or transition rates. This method makes our image morphing technique fast enough for an interactive environment.

Hand-held windows: towards effective 2D interaction in immersive virtual environments

The study of human-computer interaction within immersive virtual environments requires us to balance what we have learned from the design and use of desktop interfaces with novel approaches to allow us to work effectively in three dimensions. While some researchers have called for revolutionary interfaces for these new environments, devoid of two-dimensional (2D) desktop widgets, others have taken a more evolutionary approach. Windowing within immersive virtual environments is an attempt to apply 2D interface techniques to three-dimensional (3D) worlds. 2D techniques are attractive because of their proven acceptance and widespread use on the desktop. With current methods environments, however, it is difficult for users of 3D worlds to perform precise manipulations, such as dragging sliders, or precisely positioning or orienting objects. We have developed a testbed designed to take advantage of bimanual interaction, proprioception, and passive-haptic feedback. We present preliminary results from an empirical study of 2D interaction in 3D environments using this system. We use a window registered with a tracked, physical surface, to provide support for precise manipulation of interface widgets displayed in the virtual environment.

Interpolation synthesis for articulated figure motion

Realistic real time articulated figure motion is achieved by reprocessing a stored database of motions. Motions are created to exact specification by interpolation from a set of example motions, effectively forming a parameterized motion model. A pre-processing step involving iterative calculations is used to allow efficient direct computations at run time. An inverse kinematics capability is shown that is based on interpolation. This method preserves the underlying qualities of the data, such as dynamical realism of motion capture, while generating a continuous range of required motions. Relevant applications include networked virtual reality and interactive entertainment.

BMRT: a global illumination implementation of the RenderMan standard

The RenderMan Interface specification proposed by Pixar is a standard for communication between modeling software and rendering software or devices. This standard has proven very powerful and is extremely popular in production work. Although the standard itself claims not to specify a rendering algorithm, people have speculated RenderMan and global illumination are mutually incompatible.We have implemented a rendering system which fully adheres to the RenderMan Interface and uses global illumination algorithms. Specifically, this implementation supports progressive refinement radiosity and distribution ray tracing in a two-pass approach. This rendering system is widely distributed, very popular, and has been used in production (three properties usually not found in global illumination renderers). We discuss how we overcame problems in mating global illumination algorithms with the RenderMan standard and make recommendations for future versions of the standard to better accommodate such algorithms. We also present a summary of important lessons we learned by creating and distributing this tool.

Genetic programming for articulated figure motion

SUMMARY We present an approach to articulated figure motion in which motion tasks are defined in terms of goals and ratings. The agents are dynamically-controlled robots whose behavior is determined by robotic controller programs. The controller programs for the robots are evaluated at each time step to yield torque values which drive the dynamic simulation of the motion. We use the AI technique of Genetic Programming (GP) to automatically derive control programs for the agents which achieve the goals. This type of motion specification is an alternative to key framing which allows a highly automated, learning-based approach to generation of motion. This method of motion control is very general (it can be applied to any type of motion), yet it allows for specifications of the types of specific motion which are desired for a high quality animation. We show that complex, specific, physically plausible, and aesthetically appealing motion can be generated using these methods.

Animation of human walking in virtual environments

This paper presents an interactive hierarchical motion control system dedicated to the animation of human figure locomotion in virtual environments. As observed in gait experiments, controlling the trajectories of the feet during gait is a precise end-point control task. Inverse kinematics with optimal approaches are used to control the complex relationships between the motion of the body and the coordination of its legs. For each step, the simulation of the support leg is executed first, followed by the swing leg, which incorporates the position of the pelvis from the support leg. That is, the foot placement of the support leg serves as the kinematics constraint while the position of the pelvis is defined through the evaluation of a control criteria optimization. Then, the swing leg movement is defined to satisfy two criteria in order: collision avoidance and control criteria optimization. Finally, animation attributes, such as controlling parameters and pre-processed motion modules, are applied to achieve a variety of personalities and walking styles.

Image morphing using deformable surfaces

This paper presents a new image morphing technique using deformable surfaces. Drawbacks of previous techniques are overcome by a physically-based approach which provides an intuitive model for a warp. A warp is derived by two deformable surfaces which specify horizontal and vertical displacements of points on an image. This paper also considers the control of transition behavior in a metamorphosis sequence. The presented technique separates the transition control from interpolating features making it much easier to use than the previous techniques. The multigrid relaxation method is used to compute a deformable surface for a warp or transition rates. This method makes the presented image morphing technique fast enough for an interactive environment.<<ETX>>