UE4 BasePassPixelShader.usf学习笔记

BasePassPixelShader.usf中的FPixelShaderInOut_MainPS()，为BasePass阶段Shader的主要逻辑。会被PixelShaderOutputCommon.usf的MainPS(),即PixelShader入口函数中被调用。
该阶段会取得材质编辑器各个引脚的计算结果，在一些计算下最终输出GBuffer，以备后续光照计算。可以认为是“紧接”材质编辑器的下一步工作。相关的c++逻辑位于FDeferredShadingSceneRenderer::Render()的RenderBasePass()中。

但因为个人能力与时间所限，只能写一篇杂乱笔记作为记录，以供后续使用。

780~889：计算变量并且填充FMaterialPixelParameters MaterialParameters。BaseColor、Metallic、Specular就位于867~877。

915~1072：计算GBuffer或者DBuffer

915~942:计算贴花相关的DBuffer
954~1028：按照ShaderModel来填充GBuffer。（983~1008 Velocity、1013~1022 使用法线来调整粗糙度，在皮肤以及车漆ShaderModel中有用到）
1041：GBuffer.DiffuseColor = BaseColor - BaseColor * Metallic;
1059~1072：使用法线（清漆ShaderModel还会计算的底层法线）计算BentNormal以及GBufferAO。（使用SphericalGaussian）

1081~1146：

1086~1116：计算DiffuseColorForIndirect

DiffuseColorForIndirect（DiffuseDir只在Hair中计算）

次表面与皮肤：DiffuseColorForIndirect += SubsurfaceColor;
布料：DiffuseColorForIndirect += SubsurfaceColor * saturate(GetMaterialCustomData0(MaterialParameters));
头发：DiffuseColorForIndirect = 2PI HairShading( GBuffer, L, V, N, 1, TransmittanceData, 0, 0.2, uint2(0,0);

1118~1120：计算预间接光照结果

GetPrecomputedIndirectLightingAndSkyLight：
采样对应的预结算缓存：

PRECOMPUTED_IRRADIANCE_VOLUME_LIGHTING：根据TRANSLUCENCY_LIGHTING_VOLUMETRIC_PERVERTEX_NONDIRECTIONAL来判断是进行读取顶点AO值还是采样体积关照贴图来作为IrradianceSH的值。最后累加到OutDiffuseLighting上。
CACHED_VOLUME_INDIRECT_LIGHTING：采样IndirectLightingCache，最后累加到最后累加到OutDiffuseLighting上。
采样HQ_TEXTURE_LIGHTMAP或者LQ_TEXTURE_LIGHTMAP，最后累加到OutDiffuseLighting上。

调用GetSkyLighting()取得天光值并累加到OutDiffuseLighting上。最后计算OutDiffuseLighting的亮度值最后作为OutIndirectIrradiance输出。

1138：计算DiffuseColor

DiffuseColor=Diffuse间接照明 Diffse颜色 + 次表面间接光照 次表面颜色+AO

DiffuseColor += (DiffuseIndirectLighting * DiffuseColorForIndirect + SubsurfaceIndirectLighting * SubsurfaceColor) * AOMultiBounce( GBuffer.BaseColor, DiffOcclusion );

1140~1146：SingleLayerWater 覆盖颜色操作

GBuffer.DiffuseColor *= BaseMaterialCoverageOverWater;
DiffuseColor *= BaseMaterialCoverageOverWater;

1148~1211

使用ForwardDirectLighting的DiffuseLighting与SpecularLighting累加，Color，THIN_TRANSLUCENT Model则为 DiffuseColor与ColorSeparateSpecular。
SIMPLE_FORWARD_DIRECTIONAL_LIGHT：调用GetSimpleForwardLightingDirectionalLight()计算方向光结果。

根据光照模式累加，最后累加到Color上：

#if STATICLIGHTING_SIGNEDDISTANCEFIELD
    DirectionalLighting *= GBuffer.PrecomputedShadowFactors.x;
#elif PRECOMPUTED_IRRADIANCE_VOLUME_LIGHTING
    DirectionalLighting *= GetVolumetricLightmapDirectionalLightShadowing(VolumetricLightmapBrickTextureUVs);
#elif CACHED_POINT_INDIRECT_LIGHTING
    DirectionalLighting *= IndirectLightingCache.DirectionalLightShadowing;
#endif

    Color += DirectionalLighting;

float3 GetSimpleForwardLightingDirectionalLight(FGBufferData GBuffer, float3 DiffuseColor, float3 SpecularColor, float Roughness, float3 WorldNormal, float3 CameraVector)
{
    float3 V = -CameraVector;
    float3 N = WorldNormal;
    float3 L = ResolvedView.DirectionalLightDirection;
    float NoL = saturate( dot( N, L ) );

    float3 LightColor = ResolvedView.DirectionalLightColor.rgb * PI;

    FShadowTerms Shadow = { 1, 1, 1, InitHairTransmittanceData() };
    FDirectLighting Lighting = EvaluateBxDF( GBuffer, N, V, L, NoL, Shadow );

    // Not computing specular, material was forced fully rough
    return LightColor * (Lighting.Diffuse + Lighting.Transmission);
}

1213~1273：渲染雾效果

包括VertexFog、PixelFog、体积雾，以及体积光效果（lit translucency)

体积雾只要使用View.VolumetricFogGridZParams中的值计算UV，调用Texture3DSampleLevel采样FogStruct.IntegratedLightScattering，最后的值为float4(VolumetricFogLookup.rgb + GlobalFog.rgb VolumetricFogLookup.a, VolumetricFogLookup.a GlobalFog.a);。

1283~1310：取得材质中自发光值得计算结果并且累加到Color上

half3 Emissive = GetMaterialEmissive(PixelMaterialInputs);

#if !POST_PROCESS_SUBSURFACE && !MATERIAL_SHADINGMODEL_THIN_TRANSLUCENT
     // For skin we need to keep them separate. We also keep them separate for thin translucent.
    // Otherwise just add them together.
    Color += DiffuseColor;
#endif

#if !MATERIAL_SHADINGMODEL_THIN_TRANSLUCENT
    Color += Emissive;

1312~1349：SingleLayerWater光照计算

计算SunIlluminance、WaterDiffuseIndirectIlluminance、Normal、ViewVector、EnvBrdf（预积分G F * 高光颜色，位于BRDF.ush）后根据设置采用对应的方式(前向渲染与延迟渲染方式)

const float3 SunIlluminance = ResolvedView.DirectionalLightColor.rgb * PI;    // times PI because it is divided by PI on CPU (=luminance) and we want illuminance here. 
const float3 WaterDiffuseIndirectIlluminance = DiffuseIndirectLighting * PI;// DiffuseIndirectLighting is luminance. So we need to multiply by PI to get illuminance.

1352~1372：超薄透明物体光照计算

1375~1529：GBuffer相关

BlendMode处理
GBuffer.IndirectIrradiance = IndirectIrradiance;
调用LightAccumulator_Add()累加关照对BaseColor的影响。Out.MRT[0]=FLightAccumulator.TotalLight
调用EncodeGBuffer(),填充GBuffer12345数据。
Out.MRT[4] = OutVelocity;
Out.MRT[GBUFFER_HAS_VELOCITY ? 5 : 4] = OutGBufferD;
Out.MRT[GBUFFER_HAS_VELOCITY ? 6 : 5] = OutGBufferE;
Out.MRT[0].rgb *= ViewPreExposure;