We present an interactive, physically-based, elastically deformable model using a particle system to model surfaces with interior volumes that can be haptically felt. Oriented particles used in existing surface-only models, and unoriented particles used in volume-only simulations are combined to form a bag-of-particles. Multiple species of surface and volume particles, coupled with prede£ned interspecies parameters, determine the elastic properties of a bag. Starting with an object represented

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... s a 3D voxel bitmap of connected components, the gradient of its distance map gives a vector £eld, or gradient map, that captures the static shape of an object and provides shape-maintaining forces. The gradient map enables the user to de£ne the geometry of the simulated objects, and provides feedback reaction forces allowing a user to feel the model. A bag-of-particles model can simulate several objects in the same scene, as well as objects composed of different materials, such as organs with multiple tissue types. We demonstrate the bag-of-particles approach using a number of different data sources, and apply it to modeling myocardium dynamics.