Physical realism plays an important role in the simulation of the medical corpsmen procedures. The quality of the motion animation can be enhanced by adding dynamical properties to the humans and objects present in the scenes. After assigning realistic mass and inertia to both the body parts of the injured humans and the medical instruments and tools, natural looking motion can be generated through a dynamic simulator. The corpsman can interact with the environment and perform the specified operations by applying (mainly through his hands) external forces directly to the injured person. The effect of these forces will automatically be computed by the dynamic simulator and the resulting motion of the parts and objects involved will be both physically correct and convincing. The advantages offered from dynamic simulation are not limited only to the visual realism of the animation. Given accurate strength limits for the human body muscles and joints, the feasibility of certain operations that the corpsman performs can be judged. If more force to achieve a certain result (such as moving the injured body) is needed than the corpsman can apply then a request for extra help can be issued.
A fast dynamic simulator that generates physically correct motion of articulated figures has been built, based on the work of Featherstone [12,13]. Featherstone's Articulated-Body method is a recursive algorithm with complexity growing linearly with the number of links of the articulated figure. It has been proven to be one of the fastest numerical algorithms available and is versatile enough to be used in our system. The dynamic simulation procedures are embedded into Jack and can be used to simulate motion under the influence of gravity and external forces of any previously defined figure. The total mass and moments of inertia of any segment can automatically be computed given its density.
One of the most important issues that a dynamic simulator has to handle is collisions between objects. After a collision between two geometries is detected, the velocities of the corresponding objects need to be adjusted to prevent penetration. Stiff springs with dampers and analytic constraint equations have been used in the past to prevent further motion at the contact points. However, since the momentary forces during an impact are large, the simulation needs to be slowed down substantially to prevent numerical errors. We have used an alternate approach that enables us to maintain near real time simulation even during the collisions. When two objects first come in contact, a linear system of momentum balance equations is solved to compute the instantaneous change in the velocities of the objects due to the impact. If the objects remain in contact after the collision (i.e. the collision was inelastic), zero relative velocity at the contact points is maintained by solving a system of non-linear equations for the required contact force between the objects. Since the contact force will be slowly varying over time in the absence of new impacts, the solution of the system in each time step will be very close to the solution at the previous time step. Hence, the system can be solved quickly using a Newton-Raphson based approach. The method used to handle collisions in our system yields very satisfactory simulation times.
We are currently in the process of using the dynamic simulator to generate realistic looking motion for certain passive motions of the casualty. For example, we are interested in generating the motion of the body when it is being log rolled to get it in the correct position for a medical procedure, or the motion of body parts when the casualty is transported on a stretcher. A robust dynamic simulator is the key to generating physically correct animations for our purposes.
Patient response will conform dynamically and probabilistically to a set of implemented injury types. Using Metaxas' techniques of physics-based modeling and dynamic deformations [23], we will be able to do some dynamic computation of the motion of unsupported body parts and restraint of distended soft organs/tissues.