SpongeCake: A Layered Microflake Surface Appearance Model

TL;DR

SpongeCake introduces a layered volumetric scattering BSDF model that efficiently simulates complex surface appearances using an analytical solution and neural network-based parameter fitting, enabling realistic rendering of diverse materials.

Contribution

The paper presents a novel layered BSDF model with an exact analytical solution for single scattering and a neural network approach for approximating multiple scattering effects.

Findings

Achieves realistic material effects with layered microflake volumes.

Provides a fast, analytical evaluation and sampling method.

Can simulate various materials like plastics, wood, and cloth.

Abstract

In this paper, we propose SpongeCake: a layered BSDF model where each layer is a volumetric scattering medium, defined using microflake or other phase functions. We omit any reflecting and refracting interfaces between the layers. The first advantage of this formulation is that an exact and analytic solution for single scattering, regardless of the number of volumetric layers, can be derived. We propose to approximate multiple scattering by an additional single-scattering lobe with modified parameters and a Lambertian lobe. We use a parameter mapping neural network to find the parameters of the newly added lobes to closely approximate the multiple scattering effect. Despite the absence of layer interfaces, we demonstrate that many common material effects can be achieved with layers of SGGX microflake and other volumes with appropriate parameters. A normal mapping effect can also be achieved through mapping of microflake orientations, which avoids artifacts common in standard normal maps. Thanks to the analytical formulation, our model is very fast to evaluate and sample. Through various parameter settings, our model is able to handle many types of materials, like plastics, wood, cloth, etc., opening a number of practical applications.