【URP】[实时阴影]计算流程解析

技术分享 6个月前 (09-02) 0 999+

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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