OpenPBR: Novel Features and Implementation Details

TL;DR

OpenPBR addresses the need for a standardized, open physically based shading model suitable for both production and real-time rendering. It introduces a layered slab formalism with vertical layering and horizontal mixing, anchored by microfacet BRDFs (GGX) and volumetric components for subsurface, translucent, and coat interactions. The paper provides detailed guidance on implementation, practical approximations, and parameterization to enable artists to control complex materials while maintaining energy conservation and plausible appearances. It also outlines future extensions such as hazy specular, retro-reflection, decoupled metallic parameterization, fuzz enhancements, and generalized thin-walled models to broaden OpenPBR's applicability and interoperability across renderers.

Abstract

OpenPBR is a physically based, standardized uber-shader developed for interoperable material authoring and rendering across VFX, animation, and design visualization workflows. This document serves as a companion to the official specification, offering deeper insight into the model's development and more detailed implementation guidance, including code examples and mathematical derivations. We begin with a description of the model's formal structure and theoretical foundations - covering slab-based layering, statistical mixing, and microfacet theory - before turning to its physical components. These include metallic, dielectric, subsurface, and glossy-diffuse base substrates, followed by thin-film iridescence, coat, and fuzz layers. A special-case mode for rendering thin-walled objects is also described. Additional sections explore technical topics in greater depth, such as the decoupling of specular reflectivity from transmission, the choice of parameterization for subsurface scattering, and the detailed physics of coat darkening and thin-film interference. We also discuss planned extensions, including hazy specular reflection and retroreflection.

Figures (47)

• Figure 1: OpenPBR materials rendered in two separate systems (Arnold left, and Adobe's proprietary renderer right).
• Figure 2: The lineage of shading models leading to OpenPBR Surface.
• Figure 3: Schematic of the layer structure.
• Figure 4: Basic operations on slabs in the OpenPBR slab formalism.
• Figure 5: A simple example of layering: adding a coat layer to skin in order to model wet or bloody skin.