Computer Graphics International - CGI'98, Proceedings (IEEE), p. 156-165, Hannover, Germany, June 22-26, 1998. Real Time Muscle Deformations Using Mass-Spring Systems Luciana Porcher Nedel Daniel Thalmann EPFL - Swiss Federal Institute of Technology LIG - Computer Graphics Lab CH 1015 - Lausanne - VD - Switzerland (nedel,thalmann)@lig.di.epfl.ch Abstract In this paper we propose a method to simulate muscle deformation in real-time, still aiming at satisfying visual results; that is, we are not attempting perfect simulation, but building a useful tool for interactive applications. Muscles are represented at 2 levels: the action lines and the muscle shape. The action line represents the force produced by a muscle on the bones, while the muscle shapes used in the simulation consist of a surface based model fitted to the boundary of medical image data. The algorithm to model muscle shapes is described. To physically simulate deformations, we used a mass-spring system with a new kind of springs called “angular springs” which were developed to control the muscle volume during simulation. Results are presented as examples at the end of the paper. 1. Introduction It’s inherent to the human being trying to simulate real scenes as best as possible. History already reveals a great amount of interest on this subject. Earlier, traditional techniques like clay sculpturing, hand drawing and painting were used for this purposes. It is exactly the case for the representation of human appearance. With the advent of technology, still photographs and movies came, which are more like copying mechanisms limited to two dimensional reproduction. More recently, with computers, we can perform the tasks of synthesis and animation with greater creativity and precision both in two and three dimensions. For years, modeling and animation of the human figure has been an important research goal in computer graphics. At first, humans have been represented as simple articulated bodies made of segments and joints with a kinematics based model to simulate them. More recently, dynamic models have been used to improve the continuity and fluidity of the movement. However, simple skeleton-based models are limited for human picture simulation. In fact, a human body is a collection of complex rigid and non-rigid components very difficult to model. Considering the complexity of the human structure and the fact that our eyes are specially sensitive to familiar things (e.g., our own image), people tend to be very exacting about human simulation. Consequently, researchers began to use human anatomy know-how to produce human models with more realistic behavior. On the other hand, computer graphics now has a strong trend to develop real-time applications and there is a need for virtual human actors in many applications (e.g., the simulation of virtual environments, teleconference, multimedia, and interaction between real and virtual actors). Initially, the research on a realistic human model seems to be incompatible with real-time goals. In fact, it is still quite impossible today to have an application with a very accurate human simulation and high quality images, running in real time. Our goal is to achieve a realistic simulation of human movements using an anatomical approach and executing in an acceptable time for interactive applications. The work described here was developed within the European ESPRIT project CHARM and presents a model to simulate muscle deformation in real-time, producing visually acceptable results. We have chosen a physically- based model because we believe we can obtain more fluid and realistic movements using the same principles used in nature to accomplish it. Concerning the CHARM project (A Comprehensive Human Animation Resource Model), its goal was to develop a 3D human body model (more specifically the upper limb), allowing the physically-based movement and finite elements deformation. The model and simulation are validated with medical data. To simulate muscle motion, a biomechanical model that uses a finite elements