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.

翻译：OpenPBR是一种基于物理的标准化超级着色器，专为视觉特效、动画和设计可视化工作流中可互操作的材料创作与渲染而开发。本文档作为官方规范的补充，旨在提供对模型开发的更深入见解以及更详细的实现指导，包括代码示例和数学推导。我们首先描述模型的形式化结构与理论基础——涵盖基于平板的层叠、统计混合及微表面理论——然后转向其物理组件。这些组件包括金属、电介质、次表面及光泽-漫反射基底材质，随后是薄膜虹彩、涂层和绒毛层。本文还描述了用于渲染薄壁物体的特殊模式。其他章节更深入地探讨了技术主题，例如镜面反射率与透射的解耦、次表面散射参数化的选择，以及涂层变暗和薄膜干涉的详细物理原理。我们还讨论了计划中的扩展功能，包括模糊镜面反射和回射。