unity urp 实现丝绸渲染

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：