Layering Displaced Materials with Thickness, Accumulation, and Size✱

This paper contains four contributions: (1) a physically plausible displacement [Cook et al. 1987; Cook 1984] layering operation based on the intuitive notions of material Thickness and Accumulation; (2) a universal pattern variation metric and control parameter called Size that simultaneously modifies the displaced pattern’s variation and magnitude, and which is used by the displacement layering operation for specifying and tracking the accumulated "bulk" produced by layered displacements; (3) an encapsulated Material object definition that specifies its BxDF1 response(s), displacement, Thickness of application, and desired level of displacement bulk Accumulation; (4) a Material Layer node definition that allows encapsulated Material objects to be layered over one another in an easy to specify, intuitively controlled, yet physically plausible way simply by connecting them in the desired layering order.

Layering the BxDFs of encapsulated Material objects is made possible by the layering capabilities presently defined in MaterialX [Stone et al. 2012]. However, the layering of displacements and their effects on the Material’s optical properties are not currently defined in a robust manner. The displacement layering operations as described herein remedy this current deficiency. The operations require no physical simulation, as all the necessary data and displacement layering operations are point processes. These characteristics allow the implementation of this system within any shader execution environment.

The lack of non-local interactions between complex Materials cannot account for some types of physical effects in the displacement layering (See Section 7.6). However, this limitation has proven to be of little consequence to our use of this system in practice, and any such limitations are more than offset by the production efficiencies gained from the ability to pre-define a library of modular Materials that can easily be combined with each other as needed to create an essentially infinite number of physically plausible, displaced Material composites.