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

I will note that it's the same calculation all the way until the end where the regular fresnel just returns (Rs + Rp) * 0.5 instead of inout Rs, Rp

then return complex_t<T> and treat Rs as Real and Rp as imag, or make a struct to hold Polarized Reflectance and DRY the code

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

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

+template<class Config NBL_PRIMARY_REQUIRES(config_concepts::MicrofacetConfiguration<Config>)
+struct SIridescent
+{
+    using this_t = SIridescent<Config>;
+    NBL_BXDF_CONFIG_ALIAS(scalar_type, Config);
+    NBL_BXDF_CONFIG_ALIAS(vector2_type, Config);
+    NBL_BXDF_CONFIG_ALIAS(vector3_type, Config);
+    NBL_BXDF_CONFIG_ALIAS(ray_dir_info_type, Config);
+
+    NBL_BXDF_CONFIG_ALIAS(isotropic_interaction_type, Config);
+    NBL_BXDF_CONFIG_ALIAS(anisotropic_interaction_type, Config);
+    NBL_BXDF_CONFIG_ALIAS(sample_type, Config);
+    NBL_BXDF_CONFIG_ALIAS(spectral_type, Config);
+    NBL_BXDF_CONFIG_ALIAS(quotient_pdf_type, Config);
+    NBL_BXDF_CONFIG_ALIAS(isocache_type, Config);
+    NBL_BXDF_CONFIG_ALIAS(anisocache_type, Config);
+
+    using ndf_type = ndf::GGX<scalar_type, false>;
+    using fresnel_type = fresnel::Iridescent<spectral_type>;
+    using measure_transform_type = ndf::SDualMeasureQuant<scalar_type,true,ndf::MTT_REFLECT>;
+
+    NBL_CONSTEXPR_STATIC_INLINE BxDFClampMode _clamp = BxDFClampMode::BCM_MAX;
+
+    struct SCreationParams
+    {
+        scalar_type A;
+        scalar_type thickness;  // thin-film thickness in nm
+        spectral_type ior0;
+        spectral_type ior1;
+        spectral_type ior2;
+        spectral_type iork2;
+    };
+    using creation_type = SCreationParams;
+
+    struct SIridQuery
+    {
+        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; }
+
+        scalar_type devsh_v;
+        scalar_type devsh_l;
+    };
+    using query_type = SIridQuery;
+
+    static this_t create(scalar_type A, scalar_type thickness, NBL_CONST_REF_ARG(spectral_type) ior0, NBL_CONST_REF_ARG(spectral_type) ior1, NBL_CONST_REF_ARG(spectral_type) ior2, NBL_CONST_REF_ARG(spectral_type) iork2)
+    {
+        this_t retval;
+        retval.__base.ndf.A = vector2_type(A, A);
+        retval.__base.ndf.a2 = A*A;
+        retval.__base.ndf.one_minus_a2 = scalar_type(1.0) - A*A;
+        retval.__base.fresnel.Dinc = thickness;
+        retval.__base.fresnel.ior1 = ior0;
+        retval.__base.fresnel.ior2 = ior1;
+        retval.__base.fresnel.ior3 = ior2;
+        retval.__base.fresnel.iork3 = iork2;
+        return retval;
+    }
+    static this_t create(NBL_CONST_REF_ARG(creation_type) params)
+    {
+        return create(params.A, params.thickness, params.ior0, params.ior1, params.ior2, params.iork2);
+    }
+
+    query_type createQuery(NBL_CONST_REF_ARG(sample_type) _sample, NBL_CONST_REF_ARG(isotropic_interaction_type) interaction)
+    {
+        query_type query;
+        ndf_type ggx_ndf = __base.getNDF();
+        query.devsh_v = ggx_ndf.devsh_part(interaction.getNdotV2());
+        query.devsh_l = ggx_ndf.devsh_part(_sample.getNdotL2());
+        return query;
+    }
+
+    spectral_type eval(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)
+    {
+        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;
+            };
+
+            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

include/nbl/builtin/hlsl/colorspace.hlsl

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

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

…obal conversion matrices

devshgraphicsprogramming · 2025-11-03T13:34:17Z

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

-    vector_type iork3;
+    vector_type eta12;      // outside (usually air 1.0) -> thin-film IOR
+    vector_type eta23;      // thin-film -> base material IOR
+    vector_type etak23;     // thin-film -> complex component, k==0 makes dielectric


what are the pros and const of specializing and not having this member when you don't support Transmission

devshgraphicsprogramming · 2025-11-03T13:35:13Z

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

    }

-    T operator()()
+    T operator()(const scalar_type clampedCosTheta /* LdotH */)


I'd remove the LdotH comment, fresnels are always evaluated with VdotH

devshgraphicsprogramming · 2025-11-03T13:35:39Z

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

    using monochrome_type = vector<scalar_type, 1>;
    using vector_type = T;  // assert dim==3?

+    static this_t create(scalar_type Dinc, vector_type ior1, vector_type ior2, vector_type ior3, vector_type iork3)


make a creation struct, too many parameters to a function with same type, easy to mess up

devshgraphicsprogramming · 2025-11-03T13:36:23Z

include/nbl/builtin/hlsl/colorspace.hlsl

 struct colorspace_base
 {
-    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;
+    // default CIE RGB primaries wavelengths
+    NBL_CONSTEXPR_STATIC_INLINE float32_t wavelength_R = 700.0f;
+    NBL_CONSTEXPR_STATIC_INLINE float32_t wavelength_G = 546.1f;
+    NBL_CONSTEXPR_STATIC_INLINE float32_t wavelength_B = 435.8f;
 };


don't even, each colorspace just needs to provide them, end of

devshgraphicsprogramming · 2025-11-03T13:37:47Z

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

+// This file is part of the "Nabla Engine".
+// For conditions of distribution and use, see copyright notice in nabla.h
+#ifndef _NBL_BUILTIN_HLSL_BXDF_TRANSMISSION_IRIDESCENT_INCLUDED_
+#define _NBL_BUILTIN_HLSL_BXDF_TRANSMISSION_IRIDESCENT_INCLUDED_
+
+#include "nbl/builtin/hlsl/bxdf/transmission/ggx.hlsl"
+
+namespace nbl
+{
+namespace hlsl
+{
+namespace bxdf
+{
+namespace transmission
+{
+
+template<class Config NBL_PRIMARY_REQUIRES(config_concepts::MicrofacetConfiguration<Config>)
+struct SIridescent
+{
+    using this_t = SIridescent<Config>;
+    NBL_BXDF_CONFIG_ALIAS(scalar_type, Config);
+    NBL_BXDF_CONFIG_ALIAS(vector2_type, Config);
+    NBL_BXDF_CONFIG_ALIAS(vector3_type, Config);
+    NBL_BXDF_CONFIG_ALIAS(matrix3x3_type, Config);
+    NBL_BXDF_CONFIG_ALIAS(monochrome_type, Config);
+    NBL_BXDF_CONFIG_ALIAS(ray_dir_info_type, Config);
+
+    NBL_BXDF_CONFIG_ALIAS(isotropic_interaction_type, Config);
+    NBL_BXDF_CONFIG_ALIAS(anisotropic_interaction_type, Config);
+    NBL_BXDF_CONFIG_ALIAS(sample_type, Config);
+    NBL_BXDF_CONFIG_ALIAS(spectral_type, Config);
+    NBL_BXDF_CONFIG_ALIAS(quotient_pdf_type, Config);
+    NBL_BXDF_CONFIG_ALIAS(isocache_type, Config);
+    NBL_BXDF_CONFIG_ALIAS(anisocache_type, Config);
+    using brdf_type = reflection::SGGXIsotropic<Config>;
+
+    using ndf_type = ndf::GGX<scalar_type, false>;
+    using fresnel_type = fresnel::Iridescent<spectral_type>;
+    using measure_transform_type = ndf::SDualMeasureQuant<scalar_type,true,ndf::MTT_REFLECT_REFRACT>;
+
+    NBL_CONSTEXPR_STATIC_INLINE BxDFClampMode _clamp = BxDFClampMode::BCM_ABS;
+
+    struct SCreationParams


yeah special BSDF and BRDF not even needed for this

devshgraphicsprogramming · 2025-11-03T13:38:35Z

In this PR also resolve all left over comments from #930

…class

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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devshgraphicsprogramming reviewed Aug 28, 2025

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include/nbl/builtin/hlsl/colorspace.hlsl Outdated Show resolved Hide resolved

devshgraphicsprogramming reviewed Aug 28, 2025

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include/nbl/builtin/hlsl/colorspace/decodeCIEXYZ.hlsl Show resolved Hide resolved

devshgraphicsprogramming reviewed Aug 28, 2025

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include/nbl/builtin/hlsl/bxdf/fresnel.hlsl Outdated Show resolved Hide resolved

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

merge latest bxdf_fixes, fix conflicts

0f6c7a2

keptsecret changed the base branch from master to bxdf_fixes_cook_torrance November 3, 2025 04:05

keptsecret added 3 commits November 3, 2025 11:13

restore encode/decode matrices

d8b6773

colorspace specific dominant wavelengths (still missing ACES), use gl…

65239d9

…obal conversion matrices

update iridescent fresnel to match fresnel concept

9a8b77e

Base automatically changed from bxdf_fixes_cook_torrance to master November 3, 2025 13:24

devshgraphicsprogramming reviewed Nov 3, 2025

View reviewed changes

Merge branch 'master' into iridescence_bxdf

702ec8d

keptsecret added 8 commits November 4, 2025 11:16

minor typo fixes

76ad0ed

minor fixes to cook torrance, ndf infinity, move util functions into …

53c930f

…class

changes to angle adding usage

23de8ed

better orthonormality for H

ff9e03f

minor change to ggx clamp

18b3922

Merge branch 'master' into iridescence_bxdf

a5d8f5c

minor changes to create complex_t in angle_adding

f3dd05c

refactor out common section of cook torrance generate

68d3614

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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devshgraphicsprogramming commented Nov 3, 2025

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