65 lines
2.8 KiB
Markdown
65 lines
2.8 KiB
Markdown
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---
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title: Lighting阶段的ShadowMap逻辑
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date: 2023-04-10 14:45:45
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excerpt:
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tags:
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rating: ⭐
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---
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## PS Parameter
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`FDeferredLightPS::FParameters GetDeferredLightPSParameters()`
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- LightAttenuationTexture = ShadowMaskTexture ? ShadowMaskTexture : WhiteDummy;
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- One pass projection
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- VirtualShadowMap = VirtualShadowMapUniformBuffer;
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- VirtualShadowMapId = VirtualShadowMapId;
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- ShadowMaskBits = ShadowMaskBits ? ShadowMaskBits : GSystemTextures.GetZeroUIntDummy(GraphBuilder);
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最后输出
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- SceneColorTexture
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- SceneDepthTexture
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## Lighting阶段中VSM 渲染函数
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RenderDeferredShadowProjections()，输出`ScreenShadowMaskTexture`。
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## 问题所在
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LightAttenuation
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- x：整个场景的方向光阴影。
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- y：整个场景的方向光SSS阴影。
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- z：Light function + per-object shadows
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- w：per-object SSS shadowing in w
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zw用于非方向光或者移动端渲染路径。
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```c++
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if (LightData.bRadialLight || SHADING_PATH_MOBILE)
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{
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// Remapping the light attenuation buffer (see ShadowRendering.cpp)
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Shadow.SurfaceShadow = LightAttenuation.z * StaticShadowing;
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// SSS uses a separate shadowing term that allows light to penetrate the surface
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//@todo - how to do static shadowing of SSS correctly? Shadow.TransmissionShadow = LightAttenuation.w * StaticShadowing;
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Shadow.TransmissionThickness = LightAttenuation.w;
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}
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else
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{
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// Remapping the light attenuation buffer (see ShadowRendering.cpp)
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// Also fix up the fade between dynamic and static shadows // to work with plane splits rather than spheres.
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float DynamicShadowFraction = DistanceFromCameraFade(SceneDepth, LightData);
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// For a directional light, fade between static shadowing and the whole scene dynamic shadowing based on distance + per object shadows
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Shadow.SurfaceShadow = lerp(LightAttenuation.x, StaticShadowing, DynamicShadowFraction);
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// Fade between SSS dynamic shadowing and static shadowing based on distance
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Shadow.TransmissionShadow = min(lerp(LightAttenuation.y, StaticShadowing, DynamicShadowFraction), LightAttenuation.w);
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Shadow.SurfaceShadow *= LightAttenuation.z;
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Shadow.TransmissionShadow *= LightAttenuation.z;
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// Need this min or backscattering will leak when in shadow which cast by non perobject shadow(Only for directional light)
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Shadow.TransmissionThickness = min(LightAttenuation.y, LightAttenuation.w);
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}
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```
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**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。
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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。
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