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：
// [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);
}
V项使用配合D项的Vis_SmithJointAniso
// [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
F项使用UE里面的F项：
// [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;
}
使用DVF项实现双向反射率分布函数：
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;
}
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：
//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);
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：
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;
}
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：
//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)));
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：
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;
}
整体的效果：
