基於USD檔案格式之通用PBR材質規範設計與開發

Abstract

現今許多 3D 渲染軟體如 Blender、Maya、Unreal Engine 等皆採用 PBR（Physically Based Rendering）工作流程。然而，當使用者需要跨平台交換素材時，雖然可透過 OBJ、FBX 等格式傳遞，但這些格式在各樣定義上仍缺乏一致性。Universal Scene Description（USD）是由 Pixar 開發的 3D 資料交換格式，能有效支援場景圖層、幾何與動畫等資訊的跨平台互通，然而其內建的材質系統(UsdPreviewSurface)與現代主流 PBR 模型之間仍存在落差。本研究提出 CrossPBR，一種設計用於統一各種渲染平台的材質定義。透過分析與整合 Blender、NVIDIA MDL、OpenPBR、Autodesk Standard Surface 等常見材質模型，CrossPBR 提供一套泛用的PBR材質描述。該格式可儲存於 USD 檔案中，並可透過 Omniverse Kit 進行編輯；配合經修改的 Blender 匯入程式，使其可正確載入並渲染包含 CrossPBR 材質的 USD 檔案。CrossPBR 提供一種符合現代 PBR 渲染需求的解決方案，有助於提升材質在不同 3D 平台間的相容性與再利用性，並可以透過擴充USD匯入程式，應用於各種3D渲染軟體中。Many modern 3D rendering tools such as Blender, Maya, and Unreal Engine adopt Physically Based Rendering (PBR) workflows. While formats like OBJ and FBX are commonly used for asset exchange, they lack consistency in material definitions across platforms. Universal Scene Description (USD), developed by Pixar, serves as a robust format for cross-platform scene, geometry, and animation data exchange. However, its built-in material system (UsdPreviewSurface) falls short of representing the complexity and flexibility of modern PBR models. This research introduces CrossPBR, a general-purpose PBR material definition designed to unify material representation across rendering platforms. By analyzing and integrating commonly used models such as Blender, NVIDIA MDL, OpenPBR, and Autodesk Standard Surface, CrossPBR offers a standardized and broadly applicable PBR material format. It can be embedded directly in USD files, edited with tools like Omniverse Kit, and rendered in Blender using a customized USD importer. CrossPBR addresses the gap between USD's material description and real-world rendering practices, improving material compatibility, reusability, and consistency across various 3D software environments. With importer extensibility, CrossPBR can be adapted to a wide range of rendering applications.