  This article introduces a representation of dynamic meshes, adapted to some
numerical simulations that require controlling the volume of objects with free
boundaries, such as incompressible fluid simulation, some astrophysical
simulations at cosmological scale, and shape/topology optimization. The
algorithm decomposes the simulated object into a set of convex cells called a
Laguerre diagram, parameterized by the position of $N$ points in 3D and $N$
additional parameters that control the volumes of the cells. These parameters
are found as the (unique) solution of a convex optimization problem --
semi-discrete Monge-Amp\`ere equation -- stemming from optimal transport
theory. In this article, this setting is extended to objects with free
boundaries and arbitrary topology, evolving in a domain of arbitrary shape, by
solving a partial optimal transport problem. The resulting Lagrangian scheme
makes it possible to accurately control the volume of the object, while
precisely tracking interfaces, interactions, collisions, and topology changes.
