The art to keep in touch: The ''good use'' of Lagrange multipliers Jonquet Antoine Nocent Olivier Remion Yannick Physically-based modeling for computer animation allows to produce more realistic motions in less time without requiring the expertise of skilled animators. But, a computer animation is not only a numerical simulation based on classical mechanics since it follows a precise story-line. One common way to define aims in an animation is to add geometric constraints. There are several methods to manage these constraints within a physically-based framework. In this paper, we present an algorithm for constraints handling based on Lagrange multipliers. After few remarks on the equations of motion that we use, we present a first algorithm proposed by Platt. We show with a simple example that this method is not reliable. Our contribution consists in improving this algorithm to provide an efficient and robust method to handle simultaneous active constraints. Contact Simulation Physically-based animation constraints 004 periodical academic journal JVRB - Journal of Virtual Reality and Broadcasting 4(2007) 15 2008 1860-2037 urn:nbn:de:0009-6-12767 10.20385/1860-2037/4.2007.15 http://nbn-resolving.de/urn:nbn:de:0009-6-12767 jonquet2008