BlueRoseNote/03-UnrealEngine/Rendering/RenderingPipeline/UE4 BasePassPixelShader.usf学习笔记.md

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。

780889：计算变量并且填充FMaterialPixelParameters MaterialParameters。BaseColor、Metallic、Specular就位于867877。

915~1072：计算GBuffer或者DBuffer

• 915~942:计算贴花相关的DBuffer
• 9541028：按照ShaderModel来填充GBuffer。（9831008 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 = 2*PI * 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：

1. 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。
2. CACHED_VOLUME_INDIRECT_LIGHTING：采样IndirectLightingCache，最后累加到最后累加到OutDiffuseLighting上。
3. 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

1. 使用ForwardDirectLighting的DiffuseLighting与SpecularLighting累加，Color，THIN_TRANSLUCENT Model则为 DiffuseColor与ColorSeparateSpecular。
2. 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

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

1553：FinalizeVirtualTextureFeedback

UE5

Lumen相关

• GetSkyLighting()
  • Lumen
    • GetTranslucencyGIVolumeLighting()
  • SkyLighting
    • GetEffectiveSkySHDiffuse()
      • GetVolumetricLightmapSkyBentNormal()
      • GetSkyBentNormalAndOcclusion()

GetSkyLighting() 演示了采样SkyLight与Lumen的方法。

SkyLighting

GetEffectiveSkySHDiffuse()是一个宏，会根据平台指向下面2个函数：

/**   
 * Computes sky diffuse lighting from the SH irradiance map.    
 * This has the SH basis evaluation and diffuse convolution weights combined for minimal ALU's - see "Stupid Spherical Harmonics (SH) Tricks"   
 */  
float3 GetSkySHDiffuse(float3 Normal)  
{  
    float4 NormalVector = float4(Normal, 1.0f);   
    float3 Intermediate0, Intermediate1, Intermediate2;  
    Intermediate0.x = dot(SkyIrradianceEnvironmentMap[0], NormalVector);  
    Intermediate0.y = dot(SkyIrradianceEnvironmentMap[1], NormalVector);  
    Intermediate0.z = dot(SkyIrradianceEnvironmentMap[2], NormalVector);  
  
    float4 vB = NormalVector.xyzz * NormalVector.yzzx;  
    Intermediate1.x = dot(SkyIrradianceEnvironmentMap[3], vB);  
    Intermediate1.y = dot(SkyIrradianceEnvironmentMap[4], vB);  
    Intermediate1.z = dot(SkyIrradianceEnvironmentMap[5], vB);  
  
    float vC = NormalVector.x * NormalVector.x - NormalVector.y * NormalVector.y;  
    Intermediate2 = SkyIrradianceEnvironmentMap[6].xyz * vC;  
  
    // max to not get negative colors  
    return max(0, Intermediate0 + Intermediate1 + Intermediate2);  
}  
  
/**  
* Computes sky diffuse lighting from the SH irradiance map.  
* This has the SH basis evaluation and diffuse convolution weights combined for minimal ALU's - see "Stupid Spherical Harmonics (SH) Tricks"  
* Only does the first 3 components for speed.  
*/  
float3 GetSkySHDiffuseSimple(float3 Normal)  
{  
    float4 NormalVector = float4(Normal, 1);  
  
    float3 Intermediate0;  
    Intermediate0.x = dot(SkyIrradianceEnvironmentMap[0], NormalVector);  
    Intermediate0.y = dot(SkyIrradianceEnvironmentMap[1], NormalVector);  
    Intermediate0.z = dot(SkyIrradianceEnvironmentMap[2], NormalVector);        
    // max to not get negative colors  
    return max(0, Intermediate0);  
}