Openpbr Achieves Interoperable Material Rendering across VFX, Animation and Design Workflows

The creation of realistic digital imagery relies on accurately simulating how light interacts with surfaces, and a new physically based rendering system, OpenPBR, advances this field by offering a standardised approach to material definition. Jamie Portsmouth from Autodesk, Peter Kutz from Adobe, and Stephen Hill from Lucasfilm, spearheaded the development of this ‘uber-shader’, designed to ensure consistent and interoperable material authoring and rendering across diverse visual effects, animation, and design workflows. OpenPBR achieves this through a robust theoretical foundation incorporating layering techniques and microfacet theory, alongside detailed models for a wide range of material properties, from metallic surfaces to subsurface scattering. This work represents a significant step towards simplifying complex rendering pipelines and enabling seamless collaboration between artists and technical directors, ultimately enhancing the quality and efficiency of digital content creation.

The research provides insight into the model’s development and more detailed implementation guidance. It begins 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, including 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, with additional sections exploring technical topics such as the decoupling of specular reflectivity from transmission.

Coated Surface Rendering and Approximations

This document is a detailed set of course notes or a technical report for a SIGGRAPH 2025 course on physically-based rendering (PBR), delving into the theoretical underpinnings and practical approximations of light interaction with coated surfaces. It focuses on clear and absorbing coatings over various base materials, such as Lambertian and specular/metallic surfaces, offering an advanced treatment beyond typical PBR approaches. The document explores the derivations of the equations used in these calculations. The core topic is modeling the appearance of surfaces with a thin coating layer on top of a base material, deriving and approximating the bidirectional reflectance distribution function (BRDF) for these coated surfaces, which describes how light is reflected.

The document distinguishes between clear coats, which affect color and specular reflection, and absorbing coats, which darken and change color. Different base materials are considered, including Lambertian (diffuse), specular (metallic/smooth), and textured/rough surfaces. A significant portion of the document details deriving the BRDF equations for coated surfaces, based on geometric optics and accounting for multiple reflections and refractions within the coating layer using techniques like Airy summation. The authors present approximations to improve rendering efficiency, analyzing their accuracy.

A key concept is the darkening factor, quantifying the reduction in reflected light due to the coating, and the document explores approximating the BRDF using albedo scaling, modifying the base material’s reflectance by the coating’s properties. The research highlights the importance of the Fresnel effect, the change in reflectivity with viewing angle, which appears frequently in the equations. The document includes a revision history, indicating ongoing refinement. Accurate coating modeling is complex, requiring consideration of multiple reflections, refractions, and the properties of both the coating and the base material, and approximations are necessary for real-time rendering, balancing accuracy and performance. Base material roughness significantly affects the coating’s appearance, with rough surfaces leading to more diffuse reflections and a different darkening effect compared to smooth surfaces.

Layered Material Representation for Visual Effects

Scientists have developed OpenPBR, a physically based shader designed to standardize material authoring and rendering for visual effects, animation, and design visualization, achieving interoperability across diverse workflows. The work centers on a layered approach to material representation, meticulously defining layer structures rather than focusing on complex light transport solutions, and experiments reveal that even simplified approximations of light transport can yield plausible visual results, offering a practical balance between accuracy and computational speed. The team implemented a layering system based on combining bidirectional scattering distribution functions (BSDFs), utilizing an albedo-scaling approximation to ensure energy conservation, with the total BSDF calculated by summing the coat and substrate contributions. Measurements confirm that this method maintains energy conservation even with complex layered configurations, ensuring physically plausible results.

Tests prove the formulation accurately accounts for directional reflectance, weighting the substrate lobe based on the coat’s directional reflectance. Further refinements involved modeling the effects of volumetric transmittance through the coat layer, with the resulting BSDF adjusted by a factor accounting for total volumetric absorption. Results demonstrate that varying the coat mix weight smoothly transitions the surface appearance from uncoated to fully coated, with a darkening effect becoming more prominent as the coat’s presence increases. The research also addresses the differing physical effects of layering for incoming and outgoing rays, accurately simulating how light interacts with layered materials like glass coated with fuzz, achieving realistic rendering of complex surface interactions.

Physically Based Rendering Standard For All Software

OpenPBR represents a significant step towards standardization in material authoring and rendering, born from a collaboration to consolidate existing shading models. The project defines a single, physically based “uber-shader” intended for broad use across visual effects, animation, and design visualization, providing a common framework for describing material properties and easing the exchange of digital assets between different software packages. This offers artists a familiar interface for creating realistic surfaces. The development involved a careful balance between physical accuracy and practical usability, building upon the foundations of previous models like Disney’s “Principled” shader.

OpenPBR incorporates features such as metallic, dielectric, subsurface, and glossy base substrates, alongside more complex layers for iridescence, coating, and fuzz, even offering a specialized mode for rendering thin-walled objects. While comprehensive for many typical applications, the model is not intended as a fully general material system and may require specialized shaders for highly complex materials. The team identifies potential extensions, including hazy specular reflection and retroreflection, suggesting ongoing development and refinement.