  In this paper we define the continuous Level Lines Shortening evolution of a
two-dimensional image as the Curve Shortening operator acting simultaneously
and independently on all the level lines of the initial data, and show that it
computes a viscosity solution for the mean curvature motion. This provides an
exact analytical framework for its numerical implementation, which runs online
on any image at http://www.ipol.im/. Analogous results hold for its affine
variant version, the Level Lines Affine Shortening.
