Add support for KHR_materials_retroreflection

source/GltfState/gltf_state.js

@@ -95,6 +95,8 @@ class GltfState {
                 KHR_materials_dispersion: true,
                 /** KHR_materials_emissive_strength enables emissive factors larger than 1.0 */
                 KHR_materials_emissive_strength: true,
+                /** KHR_materials_retroreflection adds a retroreflective response to the metallic and dielectric BRDFs */
+                KHR_materials_retroreflection: true,
                 /** KHR_interactivity enables execution of a behavior graph */
                 KHR_interactivity: true,
                 /** KHR_node_hoverability enables hovering over nodes */
@@ -257,6 +259,12 @@ GltfState.DebugOutput = {
         IRIDESCENCE_THICKNESS: "Iridescence Thickness"
     },
 
+    /** KHR_materials_retroreflection */
+    retroreflection: {
+        /** output the retroreflection strength*/
+        RETROREFLECTION_FACTOR: "Retroreflection Strength"
+    },
+
     /** KHR_materials_anisotropy */
     anisotropy: {
         /** output the anisotropic strength*/

source/Renderer/renderer.js

@@ -1882,6 +1882,9 @@ class gltfRenderer {
         this.shader.updateUniform("u_IridescenceThicknessUVSet", material.extensions?.KHR_materials_iridescence?.iridescenceThicknessTexture?.texCoord);
         this.shader.updateUniform("u_IridescenceThicknessMinimum", material.extensions?.KHR_materials_iridescence?.iridescenceThicknessMinimum);
 
+        this.shader.updateUniform("u_RetroreflectionFactor", material.extensions?.KHR_materials_retroreflection?.retroreflectionFactor);
+        this.shader.updateUniform("u_RetroreflectionUVSet", material.extensions?.KHR_materials_retroreflection?.retroreflectionTexture?.texCoord);
+
         this.shader.updateUniform("u_SheenRoughnessFactor", material.extensions?.KHR_materials_sheen?.sheenRoughnessFactor);
         this.shader.updateUniform("u_SheenColorFactor", jsToGl(material.extensions?.KHR_materials_sheen?.sheenColorFactor));
         this.shader.updateUniform("u_SheenRoughnessUVSet", material.extensions?.KHR_materials_sheen?.sheenRoughnessTexture?.texCoord);
@@ -2222,6 +2225,11 @@ class gltfRenderer {
                 shaderDefine: "DEBUG_IRIDESCENCE_THICKNESS"
             },
 
+            {
+                debugOutput: GltfState.DebugOutput.retroreflection.RETROREFLECTION_FACTOR,
+                shaderDefine: "DEBUG_RETROREFLECTION_FACTOR"
+            },
+
             {
                 debugOutput: GltfState.DebugOutput.anisotropy.ANISOTROPIC_STRENGTH,
                 shaderDefine: "DEBUG_ANISOTROPIC_STRENGTH"

source/Renderer/shaders/ibl.glsl

@@ -139,6 +139,23 @@ vec3 getIBLRadianceAnisotropy(vec3 n, vec3 v, float roughness, float anisotropy,
 #endif
 
 
+#ifdef MATERIAL_RETROREFLECTION
+// MRM (Portsmouth et al., JCGT 15(1), 2026): substitute the view direction with
+// V_retro = reflect(-v, n) before evaluating the specular lobe. Since the regular
+// mirror lookup already reflects the view about n (`reflect(-v, n)`), and that
+// reflection is idempotent, sampling the environment at V_retro reflects again and
+// simplifies to sampling directly along v - i.e. looking back toward the viewer.
+vec3 getIBLRadianceGGXRetroreflection(vec3 n, vec3 v, float roughness)
+{
+    float lod = roughness * float(u_MipCount - 1);
+    vec3 reflection = normalize(v);
+    vec4 specularSample = getSpecularSample(reflection, lod);
+
+    return specularSample.rgb;
+}
+#endif
+
+
 vec3 getIBLRadianceCharlie(vec3 n, vec3 v, float sheenRoughness, vec3 sheenColor)
 {
     float NdotV = clampedDot(n, v);

source/Renderer/shaders/material_info.glsl

@@ -29,6 +29,9 @@ uniform float u_IridescenceIor;
 uniform float u_IridescenceThicknessMinimum;
 uniform float u_IridescenceThicknessMaximum;
 
+// Retroreflection
+uniform float u_RetroreflectionFactor;
+
 // Diffuse Transmission
 uniform float u_DiffuseTransmissionFactor;
 uniform vec3 u_DiffuseTransmissionColorFactor;
@@ -102,6 +105,9 @@ struct MaterialInfo
     float iridescenceIor;
     float iridescenceThickness;
 
+    // KHR_materials_retroreflection
+    float retroreflectionFactor;
+
     float diffuseTransmissionFactor;
     vec3 diffuseTransmissionColorFactor;
 
@@ -329,6 +335,20 @@ MaterialInfo getIridescenceInfo(MaterialInfo info)
 #endif
 
 
+#ifdef MATERIAL_RETROREFLECTION
+MaterialInfo getRetroreflectionInfo(MaterialInfo info)
+{
+    info.retroreflectionFactor = u_RetroreflectionFactor;
+
+    #ifdef HAS_RETROREFLECTION_MAP
+        info.retroreflectionFactor *= texture(u_RetroreflectionSampler, getRetroreflectionUV()).r;
+    #endif
+
+    return info;
+}
+#endif
+
+
 #ifdef MATERIAL_DIFFUSE_TRANSMISSION
 MaterialInfo getDiffuseTransmissionInfo(MaterialInfo info)
 {

source/Renderer/shaders/pbr.frag

@@ -115,6 +115,10 @@ void main()
     materialInfo = getIridescenceInfo(materialInfo);
 #endif
 
+#ifdef MATERIAL_RETROREFLECTION
+    materialInfo = getRetroreflectionInfo(materialInfo);
+#endif
+
 #ifdef MATERIAL_DIFFUSE_TRANSMISSION
     materialInfo = getDiffuseTransmissionInfo(materialInfo);
 #endif
@@ -224,6 +228,17 @@ void main()
     f_dielectric_brdf_ibl = mix(f_dielectric_brdf_ibl, rgb_mix(f_diffuse, f_specular_dielectric, iridescenceFresnel_dielectric), materialInfo.iridescenceFactor);
 #endif
 
+#ifdef MATERIAL_RETROREFLECTION
+    // Retroreflective variant of the metallic and dielectric BRDFs (MRM model): only the
+    // specular lobe's view direction is substituted with V_retro; the Fresnel mix weight is
+    // unchanged because NdotV is invariant under the V -> V_retro substitution.
+    vec3 f_specular_retro = getIBLRadianceGGXRetroreflection(n, v, materialInfo.perceptualRoughness);
+    vec3 f_metal_brdf_retro = f_metal_fresnel_ibl * f_specular_retro;
+    vec3 f_dielectric_brdf_retro = mix(f_diffuse, f_specular_retro, f_dielectric_fresnel_ibl);
+    f_metal_brdf_ibl = mix(f_metal_brdf_ibl, f_metal_brdf_retro, materialInfo.retroreflectionFactor);
+    f_dielectric_brdf_ibl = mix(f_dielectric_brdf_ibl, f_dielectric_brdf_retro, materialInfo.retroreflectionFactor);
+#endif
+
 
 #ifdef MATERIAL_CLEARCOAT
     clearcoat_brdf = getIBLRadianceGGX(materialInfo.clearcoatNormal, v, materialInfo.clearcoatRoughness);
@@ -350,6 +365,25 @@ void main()
         l_dielectric_brdf = mix(l_dielectric_brdf, rgb_mix(l_diffuse, l_specular_dielectric, iridescenceFresnel_dielectric), materialInfo.iridescenceFactor);
 #endif
 
+#ifdef MATERIAL_RETROREFLECTION
+        // V_retro = reflect(-v, n); NdotV_retro == NdotV, so only the half vector (and thus
+        // NdotH/VdotH) needs recomputing for the retroreflective specular lobe.
+        vec3 v_retro = normalize(reflect(-v, n));
+        vec3 h_retro = normalize(l + v_retro);
+        float NdotH_retro = clampedDot(n, h_retro);
+        float VdotH_retro = clampedDot(v_retro, h_retro);
+
+        vec3 dielectric_fresnel_retro = F_Schlick(materialInfo.f0_dielectric * materialInfo.specularWeight, materialInfo.f90_dielectric, abs(VdotH_retro));
+        vec3 metal_fresnel_retro = F_Schlick(baseColor.rgb, vec3(1.0), abs(VdotH_retro));
+
+        vec3 l_specular_retro = intensity * NdotL * BRDF_specularGGX(materialInfo.alphaRoughness, NdotL, NdotV, NdotH_retro);
+        vec3 l_metal_brdf_retro = metal_fresnel_retro * l_specular_retro;
+        vec3 l_dielectric_brdf_retro = mix(l_diffuse, l_specular_retro, dielectric_fresnel_retro);
+
+        l_metal_brdf = mix(l_metal_brdf, l_metal_brdf_retro, materialInfo.retroreflectionFactor);
+        l_dielectric_brdf = mix(l_dielectric_brdf, l_dielectric_brdf_retro, materialInfo.retroreflectionFactor);
+#endif
+
 #ifdef MATERIAL_CLEARCOAT
         l_clearcoat_brdf = intensity * getPunctualRadianceClearCoat(materialInfo.clearcoatNormal, v, l, h, VdotH,
             materialInfo.clearcoatF0, materialInfo.clearcoatF90, materialInfo.clearcoatRoughness);
@@ -525,6 +559,13 @@ vec3 specularTexture = vec3(1.0);
 #if DEBUG == DEBUG_IRIDESCENCE_THICKNESS
     g_finalColor.rgb = vec3(materialInfo.iridescenceThickness / 1200.0);
 #endif
+#endif
+
+    // Retroreflection:
+#ifdef MATERIAL_RETROREFLECTION
+#if DEBUG == DEBUG_RETROREFLECTION_FACTOR
+    g_finalColor.rgb = vec3(materialInfo.retroreflectionFactor);
+#endif
 #endif
 
     // Anisotropy:

source/Renderer/shaders/textures.glsl

@@ -353,6 +353,28 @@ vec2 getIridescenceThicknessUV()
 #endif
 
 
+// Retroreflection
+
+
+#ifdef MATERIAL_RETROREFLECTION
+
+uniform sampler2D u_RetroreflectionSampler;
+uniform int u_RetroreflectionUVSet;
+uniform mat3 u_RetroreflectionUVTransform;
+
+
+vec2 getRetroreflectionUV()
+{
+    vec3 uv = vec3(u_RetroreflectionUVSet < 1 ? v_texcoord_0 : v_texcoord_1, 1.0);
+#ifdef HAS_RETROREFLECTION_UV_TRANSFORM
+    uv = u_RetroreflectionUVTransform * uv;
+#endif
+    return uv.xy;
+}
+
+#endif
+
+
 // Diffuse Transmission
 
 #ifdef MATERIAL_DIFFUSE_TRANSMISSION

source/gltf/gltf.js

@@ -39,6 +39,7 @@ const allowedExtensions = [
     "KHR_materials_ior",
     "KHR_materials_iridescence",
     "KHR_materials_pbrSpecularGlossiness",
+    "KHR_materials_retroreflection",
     "KHR_materials_sheen",
     "KHR_materials_specular",
     "KHR_materials_transmission",

source/gltf/material.js

@@ -33,6 +33,7 @@ class gltfMaterial extends GltfObject {
         this.hasIridescence = false;
         this.hasAnisotropy = false;
         this.hasDispersion = false;
+        this.hasRetroreflection = false;
 
         // non gltf properties
         this.type = "unlit";
@@ -106,6 +107,12 @@ class gltfMaterial extends GltfObject {
         if (this.hasDispersion && renderingParameters.enabledExtensions.KHR_materials_dispersion) {
             defines.push("MATERIAL_DISPERSION 1");
         }
+        if (
+            this.hasRetroreflection &&
+            renderingParameters.enabledExtensions.KHR_materials_retroreflection
+        ) {
+            defines.push("MATERIAL_RETROREFLECTION 1");
+        }
 
         return defines;
     }
@@ -482,6 +489,22 @@ class gltfMaterial extends GltfObject {
             if (this.extensions.KHR_materials_dispersion !== undefined) {
                 this.hasDispersion = true;
             }
+
+            // KHR Extension: Retroreflection
+            // See https://github.com/KhronosGroup/glTF/tree/main/extensions/2.0/Khronos/KHR_materials_retroreflection
+            if (this.extensions.KHR_materials_retroreflection !== undefined) {
+                this.hasRetroreflection = true;
+
+                const retroreflectionTexture =
+                    this.extensions.KHR_materials_retroreflection.retroreflectionTexture;
+
+                if (retroreflectionTexture !== undefined) {
+                    retroreflectionTexture.samplerName = "u_RetroreflectionSampler";
+                    this.parseTextureInfoExtensions(retroreflectionTexture, "Retroreflection");
+                    this.textures.push(retroreflectionTexture);
+                    this.defines.push("HAS_RETROREFLECTION_MAP 1");
+                }
+            }
         }
 
         initGlForMembers(this, gltf, webGlContext);
@@ -686,6 +709,13 @@ class gltfMaterial extends GltfObject {
             this.extensions.KHR_materials_ior = new KHR_materials_ior();
             this.extensions.KHR_materials_ior.fromJson(jsonExtensions.KHR_materials_ior);
         }
+
+        if (jsonExtensions.KHR_materials_retroreflection !== undefined) {
+            this.extensions.KHR_materials_retroreflection = new KHR_materials_retroreflection();
+            this.extensions.KHR_materials_retroreflection.fromJson(
+                jsonExtensions.KHR_materials_retroreflection
+            );
+        }
     }
 }
 
@@ -825,6 +855,24 @@ class KHR_materials_iridescence extends GltfObject {
     }
 }
 
+class KHR_materials_retroreflection extends GltfObject {
+    static animatedProperties = ["retroreflectionFactor"];
+    constructor() {
+        super();
+        this.retroreflectionFactor = 0;
+        this.retroreflectionTexture = undefined;
+    }
+
+    fromJson(jsonRetroreflection) {
+        super.fromJson(jsonRetroreflection);
+        if (jsonRetroreflection.retroreflectionTexture !== undefined) {
+            const retroreflectionTexture = new gltfTextureInfo();
+            retroreflectionTexture.fromJson(jsonRetroreflection.retroreflectionTexture);
+            this.retroreflectionTexture = retroreflectionTexture;
+        }
+    }
+}
+
 class KHR_materials_sheen extends GltfObject {
     static animatedProperties = ["sheenRoughnessFactor", "sheenColorFactor"];
     constructor() {