【URP】[实时阴影]计算流程解析

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在Unity URP中，实时阴影的工作流程基于‌

ShadowMap技术

‌实现，主要流程如下：

【从UnityURP开始探索游戏渲染】
专栏-直达

一、阴影生成流程

‌阴影贴图（ShadowMap）生成

在光源位置设置虚拟相机，渲染场景深度到纹理：

主光源阴影由ShadowCaster Pass处理，输出深度图到_MainLightShadowmapTexture‌
使用级联阴影（Cascaded Shadow Mapping）提升精度：将ShadowMap划分为2×2图集，对应不同精度等级‌

csharp // URP核心流程if (主光源开启阴影) { MainLightShadowCasterPass.Render();// 生成_MainLightShadowmapTexture }

‌深度值比较

正常渲染时执行：

将像素坐标转换到光源空间获取深度值
采样ShadowMap比较深度：若当前深度 > ShadowMap值，则判定为阴影区域‌

二、关键Pass的作用

‌

ShadowCaster Pass

‌

‌
作用
‌：专用于生成阴影贴图
‌
输出目标
‌：_MainLightShadowmapTexture（主光源）或自定义ShadowMap
‌
Shader要求
‌：

hlsl Pass { Tags { "LightMode" = "ShadowCaster" } #pragma multi_compile_shadowcaster // 生成SHADOWS_DEPTH/SHADOW_CUBE宏‌ V2F_SHADOW_CASTER; // 声明数据结构 TRANSFER_SHADOW_CASTER_NORMALOFFSET(o) // 顶点着色器处理深度偏移‌ }

来自于Lit的阴影投射pass

// ShadowCaster 计算灯光的深度贴图，相当于以光的位置主动投射到物体上，形成的偏移阴影。用来投射阴影到其他位置的计算。 // 这里涉及到_ShadowBias在Shadow.hlsl中x分量表示DepthBias深度方向偏移，y分量表示NormalBias法线方向偏移。 Pass { Name "ShadowCaster" Tags { "LightMode" = "ShadowCaster" } // ------------------------------------- // Render State Commands ZWrite On ZTest LEqual ColorMask 0 Cull[_Cull] HLSLPROGRAM #pragma target 2.0 // ------------------------------------- // Shader Stages #pragma vertex ShadowPassVertex #pragma fragment ShadowPassFragment // ------------------------------------- // Material Keywords #pragma shader_feature_local _ALPHATEST_ON #pragma shader_feature_local_fragment _SMOOTHNESS_TEXTURE_ALBEDO_CHANNEL_A //-------------------------------------- // GPU Instancing #pragma multi_compile_instancing #include_with_pragmas "Packages/com.unity.render-pipelines.universal/ShaderLibrary/DOTS.hlsl" // ------------------------------------- // Universal Pipeline keywords // ------------------------------------- // Unity defined keywords #pragma multi_compile_fragment _ LOD_FADE_CROSSFADE // This is used during shadow map generation to differentiate between directional and punctual light shadows, as they use different formulas to apply Normal Bias #pragma multi_compile_vertex _ _CASTING_PUNCTUAL_LIGHT_SHADOW // ------------------------------------- // Includes #include "Packages/com.unity.render-pipelines.universal/Shaders/LitInput.hlsl" #include "Packages/com.unity.render-pipelines.universal/Shaders/ShadowCasterPass.hlsl" ENDHLSL }

`ShadowCasterPass.hlsl`

阴影投射主要计算在顶点计算位置，片元不需要处理纹理颜色阴影返回0给黑色。

#ifndef UNIVERSAL_SHADOW_CASTER_PASS_INCLUDED #define UNIVERSAL_SHADOW_CASTER_PASS_INCLUDED #include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Core.hlsl" #include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Shadows.hlsl" #if defined(LOD_FADE_CROSSFADE) #include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/LODCrossFade.hlsl" #endif // Shadow Casting Light geometric parameters. These variables are used when applying the shadow Normal Bias and are set by UnityEngine.Rendering.Universal.ShadowUtils.SetupShadowCasterConstantBuffer in com.unity.render-pipelines.universal/Runtime/ShadowUtils.cs // For Directional lights, _LightDirection is used when applying shadow Normal Bias. // For Spot lights and Point lights, _LightPosition is used to compute the actual light direction because it is different at each shadow caster geometry vertex. float3 _LightDirection; float3 _LightPosition; struct Attributes { float4 positionOS : POSITION; float3 normalOS : NORMAL; float2 texcoord : TEXCOORD0; UNITY_VERTEX_INPUT_INSTANCE_ID }; struct Varyings { #if defined(_ALPHATEST_ON) float2 uv : TEXCOORD0; #endif float4 positionCS : SV_POSITION; UNITY_VERTEX_INPUT_INSTANCE_ID }; float4 GetShadowPositionHClip(Attributes input) { float3 positionWS = TransformObjectToWorld(input.positionOS.xyz); float3 normalWS = TransformObjectToWorldNormal(input.normalOS); #if _CASTING_PUNCTUAL_LIGHT_SHADOW float3 lightDirectionWS = normalize(_LightPosition - positionWS); #else float3 lightDirectionWS = _LightDirection; #endif float4 positionCS = TransformWorldToHClip(ApplyShadowBias(positionWS, normalWS, lightDirectionWS)); #if UNITY_REVERSED_Z positionCS.z = min(positionCS.z, UNITY_NEAR_CLIP_VALUE); #else positionCS.z = max(positionCS.z, UNITY_NEAR_CLIP_VALUE); #endif return positionCS; } Varyings ShadowPassVertex(Attributes input) { Varyings output; UNITY_SETUP_INSTANCE_ID(input); UNITY_TRANSFER_INSTANCE_ID(input, output); #if defined(_ALPHATEST_ON) output.uv = TRANSFORM_TEX(input.texcoord, _BaseMap); #endif output.positionCS = GetShadowPositionHClip(input); return output; } half4 ShadowPassFragment(Varyings input) : SV_TARGET { UNITY_SETUP_INSTANCE_ID(input); #if defined(_ALPHATEST_ON) Alpha(SampleAlbedoAlpha(input.uv, TEXTURE2D_ARGS(_BaseMap, sampler_BaseMap)).a, _BaseColor, _Cutoff); #endif #if defined(LOD_FADE_CROSSFADE) LODFadeCrossFade(input.positionCS); #endif return 0; } #endif