Iridescent fresnel, bxdfs #918

keptsecret · 2025-08-27T09:58:12Z

No description provided.

devshgraphicsprogramming · 2025-08-28T13:34:11Z

include/nbl/builtin/hlsl/bxdf/fresnel.hlsl

+    // TODO: will probably merge with __call at some point
+    static void __polarized(const T orientedEta, const T orientedEtak, const T cosTheta, NBL_REF_ARG(T) Rp, NBL_REF_ARG(T) Rs)
+    {
+        T cosTheta_2 = cosTheta * cosTheta;
+        T sinTheta2 = hlsl::promote<T>(1.0) - cosTheta_2;
+        const T eta = orientedEta;
+        const T eta2 = eta*eta;
+        const T etak = orientedEtak;
+        const T etak2 = etak*etak;
+
+        const T etaLen2 = eta2 + etak2;
+        assert(hlsl::all(etaLen2 > hlsl::promote<T>(hlsl::exp2<scalar_type>(-numeric_limits<scalar_type>::digits))));
+        T t1 = etaLen2 * cosTheta_2;
+        const T etaCosTwice = eta * cosTheta * scalar_type(2.0);
+
+        const T rs_common = etaLen2 + cosTheta_2;
+        Rs = (rs_common - etaCosTwice) / (rs_common + etaCosTwice);
+        const T rp_common = t1 + hlsl::promote<T>(1.0);
+        Rp = (rp_common - etaCosTwice) / (rp_common + etaCosTwice);
+    }


make a new struct for this e.g. ThinFilmConductor, its separate functionality, same for the dielectric

devshgraphicsprogramming · 2025-08-28T13:35:10Z

include/nbl/builtin/hlsl/bxdf/reflection/iridescent.hlsl

+            {
+                using scalar_type = scalar_type;
+
+                scalar_type getDevshV() NBL_CONST_MEMBER_FUNC { return devsh_v; }
+                scalar_type getDevshL() NBL_CONST_MEMBER_FUNC { return devsh_l; }
+                BxDFClampMode getClampMode() NBL_CONST_MEMBER_FUNC { return _clamp; }
+
+                scalar_type devsh_v;
+                scalar_type devsh_l;
+                BxDFClampMode _clamp;
+            };
+
+            SGGXG2XQuery g2_query;
+            g2_query.devsh_v = query.getDevshV();
+            g2_query.devsh_l = query.getDevshL();
+            g2_query._clamp = _clamp;
+
+            measure_transform_type dualMeasure = __base.template __DG<SGGXG2XQuery>(g2_query, _sample, interaction, cache);
+            dualMeasure.maxNdotL = _sample.getNdotL(_clamp);
+            scalar_type DG = dualMeasure.getProjectedLightMeasure();
+            fresnel_type f = __base.getFresnel();
+            f.absCosTheta = cache.getLdotH();
+            return f() * DG;
+        }
+        else
+            return hlsl::promote<spectral_type>(0.0);
+    }
+
+    sample_type generate(NBL_CONST_REF_ARG(isotropic_interaction_type) interaction, const vector2_type u, NBL_REF_ARG(isocache_type) cache)
+    {
+        SGGXAnisotropic<Config> ggx_aniso = SGGXAnisotropic<Config>::create(__base.ndf.A.x, __base.ndf.A.y, __base.fresnel.ior3/__base.fresnel.ior2, __base.fresnel.iork3/__base.fresnel.ior2);
+        anisocache_type anisocache;
+        sample_type s = ggx_aniso.generate(anisotropic_interaction_type::create(interaction), u, anisocache);
+        cache = anisocache.iso_cache;
+        return s;
+    }
+
+    scalar_type pdf(NBL_CONST_REF_ARG(query_type) query, NBL_CONST_REF_ARG(isotropic_interaction_type) interaction, NBL_CONST_REF_ARG(isocache_type) cache)
+    {
+        struct SGGXDG1Query
+        {
+            using scalar_type = scalar_type;
+
+            scalar_type getNdf() NBL_CONST_MEMBER_FUNC { return ndf; }
+            scalar_type getG1over2NdotV() NBL_CONST_MEMBER_FUNC { return G1_over_2NdotV; }
+
+            scalar_type ndf;
+            scalar_type G1_over_2NdotV;
+        };
+
+        SGGXDG1Query dg1_query;
+        ndf_type ggx_ndf = __base.getNDF();
+        dg1_query.ndf = __base.__D(cache);
+
+        const scalar_type devsh_v = query.getDevshV();
+        dg1_query.G1_over_2NdotV = ggx_ndf.G1_wo_numerator_devsh_part(interaction.getNdotV(_clamp), devsh_v);
+
+        measure_transform_type dualMeasure = __base.template __DG1<SGGXDG1Query>(dg1_query);
+        return dualMeasure.getMicrofacetMeasure();
+    }
+
+    quotient_pdf_type quotient_and_pdf(NBL_CONST_REF_ARG(query_type) query, NBL_CONST_REF_ARG(sample_type) _sample, NBL_CONST_REF_ARG(isotropic_interaction_type) interaction, NBL_CONST_REF_ARG(isocache_type) cache)
+    {
+        scalar_type _pdf = pdf(query, interaction, cache);
+
+        spectral_type quo = hlsl::promote<spectral_type>(0.0);
+        if (_sample.getNdotL() > numeric_limits<scalar_type>::min && interaction.getNdotV() > numeric_limits<scalar_type>::min)
+        {
+            struct SGGXG2XQuery
+            {
+                using scalar_type = scalar_type;
+
+                scalar_type getDevshV() NBL_CONST_MEMBER_FUNC { return devsh_v; }
+                scalar_type getDevshL() NBL_CONST_MEMBER_FUNC { return devsh_l; }
+                BxDFClampMode getClampMode() NBL_CONST_MEMBER_FUNC { return _clamp; }
+
+                scalar_type devsh_v;
+                scalar_type devsh_l;
+                BxDFClampMode _clamp;
+            };
+
+            ndf_type ggx_ndf = __base.getNDF();
+
+            SGGXG2XQuery g2_query;
+            g2_query.devsh_v = query.getDevshV();
+            g2_query.devsh_l = query.getDevshL();
+            g2_query._clamp = _clamp;
+            const scalar_type G2_over_G1 = ggx_ndf.template G2_over_G1<SGGXG2XQuery, sample_type, isotropic_interaction_type, isocache_type>(g2_query, _sample, interaction, cache);
+
+            fresnel_type f = __base.getFresnel();
+            f.absCosTheta = cache.getLdotH();
+            const spectral_type reflectance = f();
+            quo = reflectance * G2_over_G1;
+        }
+
+        return quotient_pdf_type::create(quo, _pdf);
+    }
+
+    SCookTorrance<Config, ndf_type, fresnel_type, measure_transform_type> __base;
+};


you could have just added a Delta NDF, and then used a CookTorrance with Iridescent Fresnel

devshgraphicsprogramming · 2025-08-28T13:36:07Z

include/nbl/builtin/hlsl/colorspace.hlsl

+    NBL_CONSTEXPR_STATIC_INLINE float32_t wavelength_R = 580.0f;
+    NBL_CONSTEXPR_STATIC_INLINE float32_t wavelength_G = 550.0f;
+    NBL_CONSTEXPR_STATIC_INLINE float32_t wavelength_B = 450.0f;


noooo, different colorspaces have different wavelengths of the primaries

devshgraphicsprogramming · 2025-08-28T13:36:22Z

include/nbl/builtin/hlsl/colorspace.hlsl

+        return float32_t3x3(
+            float32_t3( 3.240970f, -1.537383f, -0.498611f),
+            float32_t3(-0.969244f,  1.875968f,  0.041555f),
+            float32_t3( 0.055630f, -0.203977f,  1.056972f)
+        );
+    }
+    static float32_t3 FromXYZ(float32_t3 val) { return hlsl::mul(FromXYZ(), val); }
+
+    static float32_t3x3 ToXYZ()
+    {
+        return float32_t3x3(
+            float32_t3(0.412391f, 0.357584f, 0.180481f),
+            float32_t3(0.212639f, 0.715169f, 0.072192f),
+            float32_t3(0.019331f, 0.119195f, 0.950532f)
+        );


why are you copying literals ? return the encode/decode matrices we already have!

devshgraphicsprogramming · 2025-08-28T13:36:59Z

include/nbl/builtin/hlsl/colorspace/decodeCIEXYZ.hlsl

-NBL_CONSTEXPR float32_t3x3 XYZtoscRGB = float32_t3x3(
-    float32_t3( 3.240970f, -1.537383f, -0.498611f),
-    float32_t3(-0.969244f,  1.875968f,  0.041555f),
-    float32_t3( 0.055630f, -0.203977f,  1.056972f)
-);


don't delete these, they were useful as static const globals

devshgraphicsprogramming · 2025-08-28T13:38:27Z

include/nbl/builtin/hlsl/bxdf/fresnel.hlsl

+        vector_type eta12 = ior2/ior1;
+        vector_type eta23 = ior3/ior2;
+        vector_type etak23 = iork3/ior2;


can you precompute those and store as members?

keptsecret added 10 commits August 26, 2025 11:50

initial iridescent fresnel, only single channel ior

4b3224a

iridescent fresnel does rgb IOR, use in brdf

9c71906

some bug fixes

0f51306

replace loop for vector operations in fresnel

f205d4e

added iridescent btdf

f6daa6f

added unit tests

09402ea

merge bxdfs, fix conflicts

a77c337

latest example

aa0f6e8

moved colorspace transform mats into struct functions

d2cb193

some more colorspace utility

33fc955

devshgraphicsprogramming reviewed Aug 28, 2025

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Base automatically changed from hlsl_bxdfs to master August 29, 2025 13:04

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Iridescent fresnel, bxdfs #918

Iridescent fresnel, bxdfs #918

keptsecret commented Aug 27, 2025

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devshgraphicsprogramming Aug 28, 2025

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devshgraphicsprogramming Aug 28, 2025

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Iridescent fresnel, bxdfs #918

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Iridescent fresnel, bxdfs #918

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keptsecret commented Aug 27, 2025

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