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首先看一下实际上真实的效果
再来一张
这是专门去找的。
可以看到丝绸渲染使用了各向异性的GGX去实现，有点仿头发的感觉，接下来看一下怎么实现的。
首先，准备实现双向反射率分布函数（BRDF）的DVF项。
D项使用UE里面的各项异性GGX：
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// [Burley 2012, "Physically-Based Shading at Disney"]float D_GGXaniso(float ax, float ay, float NoH, float XoH, float YoH){ float a2 = ax * ay; float3 V = float3(ay * XoH, ax * YoH, a2 * NoH); float S = dot(V, V); return(1.0f / PI) * a2 * Square(a2 / S);}1.2.3.4.5.6.7.8.9.V项使用配合D项的Vis_SmithJointAniso
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// [Heitz 2014, "Understanding the Masking-Shadowing Function in Microfacet-Based BRDFs"]float Vis_SmithJointAniso(float ax, float ay, float NoV, float NoL, float XoV, float XoL, float YoV, float YoL){ float Vis_SmithV = NoL * length(float3(ax * XoV, ay * YoV, NoV)); float Vis_SmithL = NoV * length(float3(ax * XoL, ay * YoL, NoL)); return 0.5 * rcp(Vis_SmithV + Vis_SmithL);}这个函数里面的x就是tangent y就是bitangent 中间的o就是dot计算结果1.2.3.4.5.6.7.8.F项使用UE里面的F项：
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// [Schlick 1994, "An Inexpensive BRDF Model for Physically-Based Rendering"]float3 F_Schlick_UE4(float3 SpecularColor, float VoH){ float Fc = Pow5(1 - VoH); // 1 sub, 3 mul //return Fc + (1 - Fc) * SpecularColor; // 1 add, 3 mad // Anything less than 2% is physically impossible and is instead considered to be shadowing return saturate(50.0 * SpecularColor.g) * Fc + (1 - Fc) * SpecularColor;}1.2.3.4.5.6.7.8.9.使用DVF项实现双向反射率分布函数：
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float3 SlikBRDF(float3 DiffuseColor, float3 SpecularColor, float Roughness, float Anisotropy,float3 N, float3 T, float3 B, float3 V, float3 L, float3 LightColor, float Shadow){ float Alpha = Roughness * Roughness; float a2 = Alpha * Alpha; // Anisotropic parameters: ax and ay are the Roughness along the tangent and bitangent // Kulla 2017, "Revisiting Physically Based Shading at Imageworks" float ax = max(Alpha * (1.0 + Anisotropy), 0.001f); float ay = max(Alpha * (1.0 - Anisotropy), 0.001f); float3 H = normalize(L + V); float NoH = saturate(dot(N, H)); float NoV = saturate(abs(dot(N, V)) + 1e-5); float NoL = saturate(dot(N, L)); float VoH = saturate(dot(V, H)); float XoV = dot(T, V); float XoL = dot(T, L); float XoH = dot(T, H); float YoV = dot(B, V); float YoL = dot(B, L); float YoH = dot(B, H); float3 Radiance = NoL * LightColor * Shadow * PI; //直接光漫反射 float3 DiffuseTerm = Diffuse_Lambert(DiffuseColor) * Radiance; //直接光镜面反射 float D = D_GGXaniso(ax, ay, NoH, XoH, YoH); float Vis = Vis_SmithJointAniso(ax, ay, NoV, NoL, XoV, XoL, YoV, YoL); float3 F = F_Schlick_UE4(SpecularColor, VoH); float3 SpecularTerm = ((D * Vis) * F) * Radiance; return DiffuseTerm + SpecularTerm;}1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.20.21.22.23.24.25.26.27.28.29.30.31.32.33.34.35.其它代码，我们可以直接魔改unity的urp内置的lit函数，用于获取相关的参数，这里就不再聊了。
那么，你会发现一个问题，因为GGXaniso 主要使用的是tangent和bitangent的朝向做高光的，我们平时做高光的时候都是使用法向的， 法向贴图不会影响这两个朝向啊，那么怎么办，这么办：
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//设置T和B也受法线贴图的影响 half3 tangentTS = normalize(surfaceData.normalTS.x * half3(0, 0, 1) * _NormalAniso + half3(1, 0, 0)); half3 T = TransformTangentToWorld(tangentTS, tangentToWorld); T = NormalizeNormalPerPixel(T); half3 bitangentTS = normalize(surfaceData.normalTS.y * half3(0, 0, 1) * _NormalAniso + half3(0, 1, 0)); half3 B = TransformTangentToWorld(bitangentTS, tangentToWorld); B = NormalizeNormalPerPixel(B);1.2.3.4.5.6.7.8.我们用法向的x轴的量去影响切线空间下的切线的向量值，然后再将其转换到世界空间即可。
直接光的函数运算如下：
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float3 DirectLighting(float3 DiffuseColor, float3 SpecularColor, float Roughness, float3 WorldPos, float Anisotropy, float3 N, float3 T, float3 B, float3 V, float4 shadowCoord, float4 shadowMask){ //主光源 half3 DirectLighting_MainLight = half3(0, 0, 0); { Light light = GetMainLight(shadowCoord, WorldPos, shadowMask); half3 L = light.direction; half3 LightColor = light.color; half Shadow = light.shadowAttenuation; DirectLighting_MainLight = SlikBRDF(DiffuseColor, SpecularColor, Roughness, Anisotropy, N, T, B, V, L, LightColor, Shadow); } //附加光源 half3 DirectLighting_AddLight = half3(0, 0, 0); #ifdef _ADDITIONAL_LIGHTS uint pixelLightCount = GetAdditionalLightsCount(); for (uint lightIndex = 0; lightIndex < pixelLightCount; ++lightIndex) { Light light = GetAdditionalLight(lightIndex, WorldPos, shadowMask); half3 L = light.direction; half3 LightColor = light.color; half Shadow = light.shadowAttenuation * light.distanceAttenuation; DirectLighting_AddLight += SlikBRDF(DiffuseColor, SpecularColor, Roughness, Anisotropy, N, T, B, V, L, LightColor, Shadow); } #endif return DirectLighting_MainLight + DirectLighting_AddLight;}1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.20.21.22.23.24.25.26.27.28.计算主光源和附加光源，即可实现直接光的漫反射和镜面反射。
这是-0.9的Anisotropy的效果
这是Anisotropy的值为0.9的效果，当前的双向反射率分布函数，如果把Anisotropy设置为1或者-1，效果上表现会很差。
环境光
首先，如果在环境光镜面反射上面实现aniso的效果呢，这里有段代码：
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//根据设置的各项异性的强度，对法向进行扭曲，实现各项异性的环境高光 float3 anisotropicDirection = Anisotropy >= 0.0 ? B : T; float3 anisotropicTangent = cross(anisotropicDirection, V); float3 anisotropicNormal = cross(anisotropicTangent, anisotropicDirection); float3 bentNormal = normalize(lerp(N, anisotropicNormal, abs(Anisotropy)));1.2.3.4.5.用这个最终生成的bentNormal去替换之前的计算反射角度的N，就可以实现环境光镜面反射，效果是这样的：
为了看效果，我故意把光滑度调高了。
间接光的实现函数：
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float3 IndirectLighting(float3 DiffuseColor, float3 SpecularColor, float Roughness, float3 WorldPos, float Anisotropy, float3 N, float3 T, float3 B, float3 V, float Occlusion, float EnvRotation){ float NoV = saturate(abs(dot(N, V)) + 1e-5); //SH float3 DiffuseAO = AOMultiBounce(DiffuseColor, Occlusion); float3 RadianceSH = SampleSH(N); float3 IndirectDiffuse = RadianceSH * DiffuseColor * DiffuseAO; //根据设置的各项异性的强度，对法向进行扭曲，实现各项异性的环境高光 float3 anisotropicDirection = Anisotropy >= 0.0 ? B : T; float3 anisotropicTangent = cross(anisotropicDirection, V); float3 anisotropicNormal = cross(anisotropicTangent, anisotropicDirection); float3 bentNormal = normalize(lerp(N, anisotropicNormal, abs(Anisotropy))); //IBL half3 R = reflect(-V, bentNormal); R = RotateDirection(R, EnvRotation); half3 SpeucularLD = GlossyEnvironmentReflection(R, WorldPos, Roughness, Occlusion); half3 SpecularDFG = EnvBRDFApprox(SpecularColor, Roughness, NoV); float SpecularOcclusion = GetSpecularOcclusion(NoV, Pow2(Roughness), Occlusion); float3 SpecularAO = AOMultiBounce(SpecularColor, SpecularOcclusion); float3 IndirectSpecular = SpeucularLD * SpecularDFG * SpecularAO; return IndirectDiffuse + IndirectSpecular;}1.2.3.4.5.6.7.8.9.10.11.12.13.14.15.16.17.18.19.20.21.22.23.24.25.26.整体的效果：
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