SpongeCake: A Layered Microflake Surface Appearance Model

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.

VMF Diffuse: A unified rough diffuse BRDF

“…The SGGX distribution [HDCD15] is a convenient parametric NDF for describing two-sided flake orientations in such media. Layering slabs of twosided SGGX flakes has been shown to produce a wide variety of useful behaviours [WJHY22], including approximating heightfield BRDFs. However, the microfacet theory at the heart of most popular BDRFs in graphics fundamentally pertains to heightfields and is therefore best described by a volumetric model with one-sided flakes [DHd16], which has been shown to be accurate for both homogeneous and layered configurations [dBWZ23].…”

Practical Appearance Model for Foundation Cosmetics

“…This work was later extended to support more advanced sampling estimates [GGN20, XWHM20]. Previous work [WJHY22] proposed a different formulation for multilayered materials, by proposing index‐matched layers. This significantly simplified the layer stacking, allowing for closed‐form solutions for the single‐scattering BRDF, and allowing simple learning‐based multiple scattering approximations.…”

“…In this work, we leverage multilayer material formulations for the modeling of our makeup BSDF. Specifically, we implement our model on top of position‐free Monte Carlo [GHZ18], following the index‐matched simplifications that previous work proposed [WJHY22] in the context of layered materials.…”

A Micrograin BSDF Model for the Rendering of Porous Layers

“…In contrast, our approach is built upon physically-based considerations. Compared to other representations based on microfacets or microflakes (e.g., [Barla et al 2018;d'Eon et al 2023;Dupuy et al 2016;Wang et al 2022]), it explicitly models a distribution of micrograins, which has the effect of decoupling structural and reflectance properties. This has a number of advantages, as we will show in the following.…”

“…Our approach has several advantages compared to previous work [d'Eon et al 2023;Merillou et al 2000;Wang et al 2022]. First, it decouples structural and reflectance parameters, leading to an analytical single-scattering formula regardless of the choice of micrograin reflectance.…”