layerlab is a Python-based toolbox for computations involving layered materials that implements a model described in the paper A Comprehensive Framework for Rendering Layered Materials by Wenzel Jakob, Eugene D'Eon, Otto Jakob and Steve Marschner.

A layered BSDF model describes the directional reflectance properties of a material whose internal structure consists of a stack of scattering and/or absorbing layers separated by smooth or rough interfaces. The bottom of the stack could be an opaque interface (such as a metal) or a transparent one. Such structural decompositions into layers and interfaces dramatically enlarge the size of the “language” that is available to describe materials, and for this reason they have been the focus of considerable interest in computer graphics in the last years.

See http://www.mitsuba-renderer.org/~wenzel/papers/layerlab-sg15.pdf for a tutorial on using layerlab.