X3D and glTF Model Differencing for Conversions, Comparison and Conformance Testing

Abstract

The Extensible 3D (X3D) Graphics Architecture ISO/IEC 19775-1 International Standard version 4.0 is a major release that includes extensive support for version 2.0 of glTF (glTF2), a standard file format for 3D scenes, and models supporting geometry, appearance, a scene-graph hierarchical structure and model animation. X3D version 4 (X3D4) authors have the option to Inline (i.e. load) glTF models directly or else utilize native X3D nodes to create corresponding 3D models. Physically-based rendering (PBR) and non-photorealistic rendering (NPR), with corresponding lighting techniques, are each important additions to the X3D4 rendering model. These lighting models are compatible and can coexist in real-time with legacy X3D3 and VRML97 models (which utilize classic Phong shading and texturing) either independently or composed together into a single scene. The X3D4 approach to representing glTF2 characteristics is defined in complete detail in order to be functionally identical, thus having the potential to achieve the greatest possible interoperability of 3D models across the World Wide Web. Nevertheless, a persistent problem remains in that glTF renderers do not always appear to produce visually identical results. Best practices for mapping glTF structures to X3D4 nodes are emerging to facilitate consistent conversion. This paper describes a variety of techniques to help confirm rendering consistency of X3D4 players, both when loading glTF assets and when representing native X3D4 conversions. Development of an X3D4 conformance archive corresponding to the publicly available glTF examples archive is expected to reinforce the development of visually correct software renderers capable of identical X3D4 and glTF presentation.