Focuses on the simulation of mechanical contact between nonlinearly elastic objects such as the components of the human body. The computation of the reaction forces that act on the contact surfaces (contact forces) is the key for designing a reliable contact handling algorithm. In traditional methods, contact forces are often defined as discontinuous functions of deformation, which leads to poor convergence characteristics. This problem becomes especially serious in areas with complicated self contact such as skin folds. We introduce a novel penalty finite element formulation based on the concept of material depth, the distance between a particle inside an object and the object's boundary. By linearly interpolating pre-computed material depths at node points, contact forces can be analytically integrated over contact surfaces without increasing the computational cost. The continuity achieved by this formulation supports an efficient and reliable solution of the nonlinear system. This algorithm is implemented as part of our implicit finite element program for static, quasistatic and dynamic analysis of nonlinear viscoelastic solids. We demonstrate its effectiveness on an animation showing realistic effects such as folding skin and sliding contacts of the tissues involved in knee flexion. The finite element model of the leg and its internal structures was derived from the Visible Human data set.

Accurate registration between real and virtual objects is crucial for augmented reality applications.Existing tracking methods are individually inadequate: magnetic trackers are inaccurate, mechanical trackers are cumbersome, and vision-based trackers are computationally problematic.We present a hybrid tracking method that combines the accuracy of vision-based tracking with the robustness of magnetic tracking without compromising real-time performance or usability.We demonstrate excellent registration in three sample applications.

We present a real-time stereoscopic video-see-through augmented reality (AR) system applied to the medical procedure known as ultrasound-guided needle biopsy of the breast.The AR system was used by a physician during procedures on breast models and during non-invasive examinations of human subjects.The system merges rendered live ultrasound data and geometric elements with stereo images of the patient acquired through head-mounted video cameras and presents these merged images to the physician in a head-mounted display.The physician sees a volume visualization of the ultrasound data directly under the ultrasound probe, properly registered within the patient and with the biopsy needle.Using this system, a physician successfully guided a needle into an artificial tumor within a training phantom of a human breast.We discuss the construction of the AR system and the issues and decisions which led to the system architecture and the design of the video see-through head-mounted display.We designed methods to properly resolve occlusion of the real and synthetic image elements.We developed techniques for realtime volume visualization of time-and position-varying ultrasound data.We devised a hybrid tracking system which achieves improved registration of synthetic and real imagery and we improved on previous techniques for calibration of a magnetic tracker.