Add KHR_materials_retroreflection extension

[mklefrancois]

Overview

This PR proposes a new Khronos extension, KHR_materials_retroreflection, which adds a physically-plausible retroreflective response to any glTF material model.

It is based on the Minimal Retroreflective Microfacet (MRM) model of Portsmouth et al., JCGT 15(1), 2026: a BSDF is made retroreflective by substituting the outgoing view direction V with its reflection about the surface normal before evaluation and sampling, redirecting the specular peak into the back-scatter direction. This produces the bright highlight seen on high-visibility clothing, glass-bead road markings, and safety signs. The model preserves reciprocity and energy conservation under reflection-symmetric NDFs (GGX, Beckmann, Phong).

A retroreflectionFactor linearly blends between the standard material (0.0) and the retroreflective material (1.0), matching OpenPBR's specular_retroreflectivity parameter. An optional retroreflectionTexture provides a per-texel multiplier.

Contents

• Extension specification (README.md)
• JSON schema (material.KHR_materials_retroreflection.schema.json)

Status

Draft

Relationship to other work

Aligns with the retroreflection models in OpenPBR 1.1 and MaterialX.

Known implementations

• NVIDIA nvpro-samples / vk_gltf_renderer

Contributors

• Martin-Karl Lefrançois, NVIDIA (@mklefrancois)
• Nia Bickford, NVIDIA (@NBickford-NV)

[CLAassistant]

All committers have signed the CLA.