---
title: Lighting阶段的ShadowMap逻辑
date: 2023-04-10 14:45:45
excerpt: 
tags: 
rating: ⭐
---

## PS Parameter
`FDeferredLightPS::FParameters GetDeferredLightPSParameters()`

- LightAttenuationTexture = ShadowMaskTexture ? ShadowMaskTexture : WhiteDummy;
- One pass projection
	- VirtualShadowMap = VirtualShadowMapUniformBuffer;
	- VirtualShadowMapId = VirtualShadowMapId;
	- ShadowMaskBits = ShadowMaskBits ? ShadowMaskBits : GSystemTextures.GetZeroUIntDummy(GraphBuilder);

最后输出
- SceneColorTexture  
- SceneDepthTexture

## Lighting阶段中VSM 渲染函数
RenderDeferredShadowProjections()，输出`ScreenShadowMaskTexture`。

## 问题所在
LightAttenuation
- x：整个场景的方向光阴影。
- y：整个场景的方向光SSS阴影。
- z：Light function + per-object shadows
- w：per-object SSS shadowing in w

zw用于非方向光或者移动端渲染路径。
```c++
if (LightData.bRadialLight || SHADING_PATH_MOBILE)  
{  
   // Remapping the light attenuation buffer (see ShadowRendering.cpp)  
  
   Shadow.SurfaceShadow = LightAttenuation.z * StaticShadowing;  
   // SSS uses a separate shadowing term that allows light to penetrate the surface  
   //@todo - how to do static shadowing of SSS correctly?   Shadow.TransmissionShadow = LightAttenuation.w * StaticShadowing;  
  
   Shadow.TransmissionThickness = LightAttenuation.w;  
}  
else  
{  
   // Remapping the light attenuation buffer (see ShadowRendering.cpp)  
   // Also fix up the fade between dynamic and static shadows   // to work with plane splits rather than spheres.  
   float DynamicShadowFraction = DistanceFromCameraFade(SceneDepth, LightData);  
   // For a directional light, fade between static shadowing and the whole scene dynamic shadowing based on distance + per object shadows  
   Shadow.SurfaceShadow = lerp(LightAttenuation.x, StaticShadowing, DynamicShadowFraction);  
   // Fade between SSS dynamic shadowing and static shadowing based on distance  
   Shadow.TransmissionShadow = min(lerp(LightAttenuation.y, StaticShadowing, DynamicShadowFraction), LightAttenuation.w);  
  
   Shadow.SurfaceShadow *= LightAttenuation.z;  
   Shadow.TransmissionShadow *= LightAttenuation.z;  
  
   // Need this min or backscattering will leak when in shadow which cast by non perobject shadow(Only for directional light)  
   Shadow.TransmissionThickness = min(LightAttenuation.y, LightAttenuation.w);  
}
```


**DeferredLightingCommon.ush**与**DeferredLightPixelShaders.usf**中的**LightAttenuation**即为ShadowMap数据。具体可以参考**GetLightAttenuationFromShadow()**，可以看得出EPIC之后打算通过抖动来消除VSM的阴影锯齿。

主要的阴影效果由**LightAttenuation**与ShadingModels.ush中的NoL提供。我们只需要调整自阴影也就是z通道，其他的整个场景的阴影。