技术实现
Unity 自带的Lit 采用了通用的microfacet Cook-Torrance BRDF着色模型，对于各向异性，清漆，布料等支持并没有实现。这里希望搭建一个Disney Principled BRDF 来学习源码并且扩充这部分shader便于以后风格化定制。
Diffuse项实现
half3 Disney_Diffuse(half3 diffuseColor, half roughness, half ndotV, half ndotL, half LdotH)
{
half FD90 = 0.5 + 2 * LdotH * LdotH * roughness;
half FnV = SchlickFresnel(ndotV);
half FnL = SchlickFresnel(ndotL);
return lerp(1.0, FD90, FnV) * lerp(1.0, FD90, FnL);
}
unity实现如下，但是UE把LdotH修改成VdotH
half3 Diffuse_Burley_Disney(float3 diffuseColor, float roughness, float ndotV, float ndotL, float LdotH)
{
float FD90 = 0.5 + 2 * LdotH * LdotH * roughness;
float FdV = 1 + (FD90 - 1) * Pow5(1 - ndotV);
float FdL = 1 + (FD90 - 1) * Pow5(1 - ndotL);
return diffuseColor * ((1 / PI) * FdV * FdL);
}
UE的修改会造成远距离看diffuse模型变成一个没有明暗区分的模型。
这与Disney的效果十分接近
完整代码
偷懒没有逐一贴过来，完整写完直接代码在这里。
Shader "Custom/Disney PBR"
{
Properties
{
//Disney coefficients
[MainTexture] _BaseMap ("Base Color", 2D) = "white" {}
_MainColor ("Color Tint", Color) = (1.0, 1.0, 1.0, 1.0)
_Metallic ("Metallic", Range(0, 1)) = 0.0
_Smoothness ("Smoothness", Range(0, 1)) = 0.5
_Subsurface ("Subsurface", Range(0, 1)) = 0.0
_Specular ("Specular", Range(0, 1)) = 0.5
_SpecularTint ("SpecularTint", Range(0, 1)) = 0.0
_Anisotropic ("Anisotropic", Range(0, 1)) = 0.0
_Sheen ("Sheen", Range(0, 1)) = 0.0
_SheenTint ("SheenTint", Range(0, 1)) = 0.5
_Clearcoat ("Clearcoat", Range(0, 1)) = 0.0
_ClearcoatGloss ("ClearcoatGloss", Range(0, 1)) = 0.0
//Normal control
_BumpScale ("Normal Scale", Range(0, 1)) = 1.0
_BumpMap ("Normal Map", 2D) = "bump" {}
//GI control
// _IrradianceMap ("Irradiance Map", CUBE) = "white" {}
_LUT ("LUT", 2D) = "white" {}
// _PrefilterMap ("Prefilter Map", CUBE) = "white" {}
// _AO ("AO", 2D) = "white" {}
}
SubShader
{
Tags { "RenderType"="Opaque" "RenderPipeLine"="UniversalRenderPipeline" "IgnoreProjector" = "True" }
HLSLINCLUDE
#include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Core.hlsl"
#include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Lighting.hlsl"
TEXTURE2D(_BaseMap); SAMPLER(sample_BaseMap);
TEXTURE2D(_BumpMap); SAMPLER(sampler_BumpMap);
TEXTURE2D(_LUT); SAMPLER(sampler_LUT);
CBUFFER_START(UnityPerMaterial)
half4 _BaseMap_ST;
half4 _MainColor;
half _Metallic;
half _Smoothness;
half _Subsurface;
half _Specular;
half _SpecularTint;
half _Anisotropic;
half _Sheen;
half _SheenTint;
half _Clearcoat;
half _ClearcoatGloss;
half4 _BumpMap_ST;
half _BumpScale;
half4 _LUT_ST;
CBUFFER_END
struct Attributes
{
half4 positionOS : POSITION;
half3 normalOS : NORMAL;
half4 tangetOS : TANGENT;
half2 uv : TEXCOORD0;
half2 lightmapUV : TEXCOORD1;
UNITY_VERTEX_INPUT_INSTANCE_ID
};
struct Varyings
{
half4 positionCS : SV_POSITION;
half3 positionWS : TEXCOORD0;
half3 normalWS : TEXCOORD1;
half2 uv : TEXCOORD2;
half3 tangentWS : TEXCOORD3;
half2 lightmapUV : TEXCOORD4;
half3 bitangentWS : TEXCOORD5;
half4 viewDirWS : TEXCOORD6;
half fogCoord : TEXCOORD7;
UNITY_VERTEX_INPUT_INSTANCE_ID
};
ENDHLSL
Pass
{
Name "DisneyForward"
Tags{ "LightMode"="UniversalForward" }
CULL OFF
HLSLPROGRAM
#pragma vertex vert
#pragma fragment frag
#pragma multi_compile_fog
#pragma shader_feature_local_fragement _EMISSION
#define _NORMALMAP
Varyings vert(Attributes IN)
{
Varyings OUT = (Varyings)0;
UNITY_SETUP_INSTANCE_ID(IN);
UNITY_TRANSFER_INSTANCE_ID(IN, OUT);
const VertexPositionInputs vpi = GetVertexPositionInputs(IN.positionOS);
const VertexNormalInputs vni = GetVertexNormalInputs(IN.normalOS, IN.tangetOS);
// get positive or negative normal signal (should be either 1 or -1)
half sign = IN.tangetOS.w * GetOddNegativeScale();
OUT.positionWS = vpi.positionWS;
OUT.positionCS = vpi.positionCS;
OUT.uv = TRANSFORM_TEX(IN.uv, _BaseMap);
OUT.normalWS = vni.normalWS;
OUT.tangentWS = vni.tangentWS;
OUT.bitangentWS = vni.bitangentWS;
OUT.lightmapUV = IN.lightmapUV;
OUT.fogCoord = ComputeFogFactor(OUT.positionCS.z);
return OUT;
}
half Pow2(half v)
{
return v * v;
}
half Pow5(half v)
{
return v * v * v * v * v;
}
half SchlickFresnel(half v) {
v = clamp(1 - v, 0, 1);
return Pow5(v);
}
half3 Disney_Diffuse_Kfd(half roughness, half ndotV, half ndotL, half LdotH)
{
half FD90 = 0.5 + 2 * Pow2(LdotH) * roughness;
half FnV = SchlickFresnel(ndotV);
half FnL = SchlickFresnel(ndotL);
return lerp(1.0, FD90, FnV) * lerp(1.0, FD90, FnL);
}
half Disney_Specular_GTR2_iso(half NdotH, half roughness) {
half a2 = Pow2(roughness);
half den = 1.0 + (a2 - 1.0) * Pow2(NdotH);
return a2 / (Pow2(den) * PI);
}
half SmithsG_GGX_aniso(half NdotV, half XdotV, half YdotV, half ax, half ay) {
return 1 / (NdotV + sqrt(Pow2(XdotV * ax) + Pow2(YdotV * ay) + Pow2(NdotV)));
}
half3 Specular_Fresnel(half3 Ctint, half3 Cdlin, half LdotH, half Metallic) {
half FlH = SchlickFresnel(LdotH);
half3 F0 = lerp(_Specular * 0.08 * lerp(half3(1, 1, 1), Ctint, _SpecularTint), Cdlin, Metallic);
half3 F = lerp(F0, half3(1, 1, 1), FlH);
return F;
}
half Disney_Clear_GTR1(half NdotH, half a) {
if (a >= 1) {
return 1 / PI;
}
float a2 = Pow2(a);
return (a2 - 1) / (log(a2) * (1 + (a2 - 1) * Pow2(NdotH)) * PI);
}
half Disney_Clear_GGX(half NdotV, half NdotL, half a) {
half a2 = Pow2(a);
half GGXnv = 1 / (NdotV + sqrt(a2 + (1 - a2) * Pow2(NdotV)));
half GGXnl = 1 / (NdotL + sqrt(a2 + (1 - a2) * Pow2(NdotL)));
return GGXnv * GGXnl;
}
half Disney_Subsurface_ss(half roughness, half LdotH, half NdotL, half NdotV) {
half FnL = SchlickFresnel(NdotL);
half Fnv = SchlickFresnel(NdotV);
half Fss90 = Pow2(LdotH) * roughness;
half Fss = lerp(1.0, Fss90, FnL) * lerp(1.0, Fss90, Fnv);
return 1.25 * (Fss * (1 / (NdotV + NdotL) - 0.5) + 0.5);
}
half3 fresnelSchlickRoughness(half cosTheta, half3 F0, half roughness) {
return F0 + (max(half3(1.0 - roughness, 1.0 - roughness, 1.0 - roughness), F0) - F0) * Pow5(1.0 - cosTheta);
}
half4 frag(Varyings IN) : SV_Target
{
//albedo
half4 albedoAlpha = SAMPLE_TEXTURE2D(_BaseMap, sampler_BumpMap, IN.uv);
half3 albedo = albedoAlpha.rgb * _MainColor.rgb;
half alpha = albedoAlpha.a * _MainColor.a;
//rip off the energy from albedo
float Cdlum = 0.3 * albedo.r + 0.6 * albedo.g + 0.1 * albedo.b;
float3 Ctint = Cdlum > 0 ? (albedo / Cdlum) : float3(1, 1, 1);
float3 Csheen = lerp(float3(1, 1, 1), Ctint, _SheenTint);
//normal
#ifdef _NORMALMAP
half3 normalTS = UnpackNormalScale(SAMPLE_TEXTURE2D(_BumpMap, sampler_BumpMap, IN.uv), _BumpScale);
IN.normalWS = TransformTangentToWorld(normalTS, half3x3(IN.tangentWS.xyz, IN.bitangentWS.xyz, IN.normalWS.xyz));
#endif
//roughness
float perceptualRoughness = 1.0 - _Smoothness;
float roughness = perceptualRoughness * perceptualRoughness;
float squareRoughness = roughness * roughness;
// Direction Function
half3 viewDirWS = GetWorldSpaceViewDir(IN.positionWS);
// Lighting Calculation
half4 shadowCoord = TransformWorldToShadowCoord(IN.positionWS);
Light mainLight = GetMainLight(shadowCoord);
half3 mainLightDir = normalize(TransformObjectToWorldDir(mainLight.direction));
//anisotropic
half3 halfVector = normalize(viewDirWS + mainLightDir);
half aspect = sqrt(1.0 - _Anisotropic * 0.9);
half ax = max(0.001, squareRoughness / aspect);
half ay = max(0.001, squareRoughness * aspect);
half hx = max(saturate(dot(halfVector, IN.tangentWS)), 0.000001);
half hy = max(saturate(dot(halfVector, IN.bitangentWS)), 0.000001);
half XdotV = dot(hx, viewDirWS);
half YdotV = dot(hy, viewDirWS);
half XdotL = dot(hx, mainLightDir);
half YdotL = dot(hy, mainLightDir);
// Dot Product Function
half NdotL = max(0.000001, saturate(dot(IN.normalWS, mainLightDir)));
half NdotV = max(0.000001, saturate(dot(IN.normalWS, viewDirWS)));
half NdotH = max(0.000001, saturate(dot(IN.normalWS, halfVector)));
half VdotH = max(0.000001, saturate(dot(viewDirWS, halfVector)));
half LdotH = max(0.000001, saturate(dot(mainLightDir, halfVector)));
//directLight Specular
//D term
half Ds = Disney_Specular_GTR2_iso(NdotH, roughness);
//G term
half GnV = SmithsG_GGX_aniso(NdotV, XdotV, YdotV, ax, ay);
half GnL = SmithsG_GGX_aniso(NdotL, XdotL, YdotL, ax, ay);
half Gs = GnV * GnL;
//F term
half3 Fs = Specular_Fresnel(Ctint, albedo, LdotH, _Metallic);
//directLight clearcoat D F G
half Dr = Disney_Clear_GTR1(NdotH, lerp(0.1, 0.001, _ClearcoatGloss));
half3 F0 = float3(0.04, 0.04, 0.04);
half3 Fr = lerp(F0, float3(1, 1, 1), SchlickFresnel(LdotH));
half Gr = Disney_Clear_GGX(NdotV, NdotL, 0.25);
//conbine specular part
half3 specular = Gs * Fs * Ds + Dr * Gr * Fr * 0.25 * _Clearcoat;
// diffuse
half3 Kd = Disney_Diffuse_Kfd(roughness, NdotV, NdotL, VdotH);
half ss = Disney_Subsurface_ss(roughness, LdotH, NdotL, NdotV);
half3 Fsheen = SchlickFresnel(LdotH) * _Sheen * Csheen;
half3 diffuse = (albedo * lerp(Kd, ss, _Subsurface) /PI + Fsheen) * (1.0 - _Metallic);
// direct Lighting
half3 directLight = (diffuse + specular) * mainLight.color * NdotL;
directLight *= mainLight.distanceAttenuation;
// indirect Lighting
//GI Diffuse
F0 = lerp(kDieletricSpec.rgb, albedo, _Metallic);
half3 F_ibl = fresnelSchlickRoughness(NdotV, F0, roughness);
half kd_ibl = (1 - F_ibl.r) * (1 - _Metallic);
half3 irradiance = SampleSH(IN.normalWS);
half3 inDiffuse = kd_ibl * albedo * irradiance;
//GI Specular
half3 reflectVector = reflect(-viewDirWS, IN.normalWS);
half mip = roughness * (1.7 - 0.7 * roughness) * UNITY_SPECCUBE_LOD_STEPS;
half3 prefilter_Specular = SAMPLE_TEXTURECUBE_LOD(unity_SpecCube0, samplerunity_SpecCube0, reflectVector, mip);
half2 envBRDF = SAMPLE_TEXTURE2D(_LUT, sampler_LUT, float2(lerp(0, 0.99, NdotV), lerp(0, 0.99, roughness))).rg;
half3 inSpecular = prefilter_Specular * (envBRDF.r * F_ibl + envBRDF.g);
half3 indirectLight = inDiffuse + inSpecular;
half4 color = half4(directLight + indirectLight , 1);
// apply fog
color.xyz = MixFog(color.xyz, IN.fogCoord);
return color;
}
ENDHLSL
}
}
}
间接光漫反射的部分偷懒直接使用Unity URP内置的 sampleSH函数。而间接光镜面反射部分也直接用reflection probe作为prefileter 采样。envBRDF的lut贴图直接采样预计算好的。其实如果仔细看源码的部分URP在间接光照的部分也对clearcoat进行处理
最终效果
我实现的效果在左边，对比右边URP下默认的Lit
