UnityCACAO-HDRP17/com.unity.render-pipelines.high-definition/Runtime/Material/AxF/PreIntegratedFGD_CookTorrance.shader

Shader "Hidden/HDRP/PreIntegratedFGD_CookTorrance"
{
    SubShader
    {
        Tags{ "RenderPipeline" = "HDRenderPipeline" }
        Pass
        {
            ZTest Always Cull Off ZWrite Off

            HLSLPROGRAM

            #pragma editor_sync_compilation

            #pragma vertex Vert
            #pragma fragment Frag
            #pragma target 4.5
            #pragma only_renderers d3d11 playstation xboxone xboxseries vulkan metal switch
            #define PREFER_HALF 0
            #include "Packages/com.unity.render-pipelines.core/ShaderLibrary/Common.hlsl"
            #include "Packages/com.unity.render-pipelines.core/ShaderLibrary/ImageBasedLighting.hlsl"
            #include "Packages/com.unity.render-pipelines.high-definition/Runtime/ShaderLibrary/ShaderVariables.hlsl"

            // ----------------------------------------------------------------------------
            // Importance Sampling
            // ----------------------------------------------------------------------------

            // Formulas come from https://agraphicsguy.wordpress.com/2015/11/01/sampling-microfacet-brdf/ for the Beckmann normal distribution
            void SampleCookTorranceDir( real2   u,
                                        real3   V,
                                        real3x3 localToWorld,
                                        real    roughness,
                                    out real3   L,
                                    out real    NdotL,
                                    out real    NdotH,
                                    out real    VdotH )
            {
                // Cook-Torrance NDF sampling
                real cosTheta = sqrt(SafeDiv(1.0, 1.0 - (roughness * roughness) * log(1.0 - u.x)));
                real phi = TWO_PI * u.y;

                real3 localH = SphericalToCartesian(phi, cosTheta);

                NdotH = cosTheta;

                real3 localV;

                localV = mul(V, transpose(localToWorld));
                VdotH  = saturate(dot(localV, localH));

                // Compute { localL = reflect(-localV, localH) }
                real3   localL = -localV + 2.0 * VdotH * localH;
                NdotL = localL.z;

                L = mul(localL, localToWorld);
            }

            // weightOverPdf returns the weight (without the Fresnel term) over pdf. Fresnel term must be applied by the caller.
            void ImportanceSampleCookTorrance(  real2   u,
                                                real3   V,
                                                real3x3 localToWorld,
                                                real    roughness,
                                                real    NdotV,
                                            out real3   L,
                                            out real    VdotH,
                                            out real    NdotL,
                                            out real    weightOverPdf)
            {
                real    NdotH;
                SampleCookTorranceDir(u, V, localToWorld, roughness, L, NdotL, NdotH, VdotH);

                // Importance sampling weight for each sample
                // pdf = D(H) * (N.H) / (4 * (L.H))
                //   Note: the first N.H converts D() to a true PDF, but the PDF is wrt to solid
                //   angle dH. Since we integrate over dL, we change the PDF to one over the
                //   solid angle measure dL with the Jacobian dH/dL = 1/(4*L.H)
                // weight = fr * (N.L) with fr = F(H) * G(V, L) * D(H) / (4 * (N.L) * (N.V))
                // weight over pdf is:
                // weightOverPdf = F(H) * G(V, L) * (L.H) / ((N.H) * (N.V))
                // weightOverPdf = F(H) * 4 * (N.L) * V(V, L) * (L.H) / (N.H)
                //   with V(V, L) = G(V, L) / (4 * (N.L) * (N.V))
                // Reminder: (L.H) == (V.H)
                // F is applied outside the function

                weightOverPdf = G_CookTorrance(NdotH, NdotV, NdotL, VdotH) * VdotH / (NdotH * NdotV);
            }

            float4  IntegrateCookTorranceFGD(float3 V, float3 N, float roughness, uint sampleCount = 8192)
            {
                float   NdotV    = ClampNdotV( dot(N, V) );
                float4  acc      = float4(0.0, 0.0, 0.0, 0.0);

                float3x3    localToWorld = GetLocalFrame(N); //TODO: N not needed, we use a frame aligned to N, should use k_identity3x3

                for (uint i = 0; i < sampleCount; ++i)
                {
                    float2  u = Hammersley2d(i, sampleCount);

                    float   VdotH;
                    float   NdotL;
                    float   weightOverPdf;

                    float3  L; // Unused
                    ImportanceSampleCookTorrance(   u, V, localToWorld, roughness, NdotV,
                                                    L, VdotH, NdotL, weightOverPdf);

                    if (NdotL > 0.0)
                    {
                        // Integral{BSDF * <N,L> dw} =
                        // Integral{(F0 + (1 - F0) * (1 - <V,H>)^5) * (BSDF / F) * <N,L> dw} =
                        // (1 - F0) * Integral{(1 - <V,H>)^5 * (BSDF / F) * <N,L> dw} + F0 * Integral{(BSDF / F) * <N,L> dw}=
                        // (1 - F0) * x + F0 * y = lerp(x, y, F0)
                        acc.x += weightOverPdf * pow( 1 - VdotH, 5 );
                        acc.y += weightOverPdf;
                    }
                }

                acc /= sampleCount;

                return float4(acc.xy, 1.0, 0.0);
            }

            // ----------------------------------------------------------------------------
            // Pre-Integration
            // ----------------------------------------------------------------------------

            struct Attributes
            {
                uint vertexID : SV_VertexID;
            };

            struct Varyings
            {
                float4 positionCS : SV_POSITION;
                float2 texCoord   : TEXCOORD0;
            };

            Varyings Vert(Attributes input)
            {
                Varyings output;

                output.positionCS = GetFullScreenTriangleVertexPosition(input.vertexID);
                output.texCoord   = GetFullScreenTriangleTexCoord(input.vertexID);

                return output;
            }

            float4 Frag(Varyings input) : SV_Target
            {
                // These coordinate sampling must match the decoding in GetPreIntegratedDFG in lit.hlsl, i.e here we use perceptualRoughness, must be the same in shader
                float   NdotV               = input.texCoord.x;
                float   perceptualRoughness = input.texCoord.y;
                float3  V                   = float3(sqrt(1 - NdotV * NdotV), 0, NdotV);
                float3  N                   = float3(0.0, 0.0, 1.0);

                float4 preFGD = IntegrateCookTorranceFGD(V, N, PerceptualRoughnessToRoughness(perceptualRoughness));

                return float4(preFGD.xyz, 1.0);
            }

            ENDHLSL
        }
    }
    Fallback Off
}