【渲染管线】UnityURP[渲染路径]底层源码解析

技术分享 7个月前 (08-29) 0 999+

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【从UnityURP开始探索游戏渲染】专栏-直达

SRP提供的核心功能与架构‌

‌可编程管线基础‌

‌RenderPipeline基类‌

通过继承该类并重写Render()方法，开发者可自定义渲染流程调度逻辑，替代传统固定管线。（URP中UniversalRenderPipeline : RenderPipeline 继承并重写Render方法。）

‌ScriptableRenderContext‌

作为C#脚本与底层图形API的桥梁，允许通过代码调度渲染命令（如剔除、绘制）。（public struct ScriptableRenderContext : IEquatable<ScriptableRenderContext>定义自定义渲染管道使用的状态和绘图命令。）

‌管线资源分离机制‌

RenderPipelineAsset：存储配置数据（如材质、Shader参数）（URP中是
public partial class UniversalRenderPipelineAsset : RenderPipelineAsset, ISerializationCallbackReceiver）
RenderPipelineInstance：执行实际渲染逻辑的实例类。（在URP中，上面的RenderPipelineAsset资产类中重写protected override RenderPipeline CreatePipeline()方法，在其中创建渲染管线实例var pipeline = new UniversalRenderPipeline(this);）

‌关键扩展点‌

‌事件回调‌

通过RenderPipelineManager订阅渲染生命周期事件（如beginContextRendering），在特定阶段注入自定义逻辑。

‌动态渲染策略‌

支持运行时切换渲染路径（如正向/延迟渲染），适应不同硬件性能需求（这里的渲染路径是URP或HDRP自己实现的，例如Forward+也是，所以这个渲染路径只是实现管线时的自定义渲染策略，所以运行时能切换。但是一般不建议切换，因为各种shader实现时都会根据渲染路径实现相应执行的Pass，如果随便切换，会导致部分shader可能因为只适配某种渲染路径，对其他渲染路径显示渲染异常。）。

URP在SRP上的具体实现‌

‌资源与实例初始化‌

‌URP管线资源‌（`UniversalRenderPipelineAsset`）：

定义默认Shader、光照模型、后处理栈等参数。

‌实例化流程‌：

资源创建时调用CreatePipeline()生成UniversalRenderPipeline实例，接管Unity渲染循环。

其中的渲染器基类ScriptableRenderer作为渲染器可以用于所有摄像机，也可以在每个摄像机的基础上重写。它将实现光剔除和设置，并描述要在帧中执行的ScriptableRenderPass列表。渲染器可以通过额外的scriptablerendererfeature进行扩展，以支持更多的效果。渲染器的资源在ScriptableRendererData中序列化（编辑器中就在这个资源上挂载设置RendererFeature）。

UniversalRenderer 默认的3D渲染器继承自ScriptableRenderer。在其构造函数中
public UniversalRenderer(UniversalRendererData data) : base(data) 根据上述序列化的Data数据，创建默认的渲染Pass逻辑。渲染路径就是在这个类文件中一同定义的，在构造函数中根据不同路径，给出不同策略执行Pass。

定义渲染路径

namespace UnityEngine.Rendering.Universal {     /// <summary>     /// Rendering modes for Universal renderer.     /// </summary>     public enum RenderingMode     {         /// <summary>Render all objects and lighting in one pass, with a hard limit on the number of lights that can be applied on an object.</summary>         Forward = 0,         /// <summary>Render all objects and lighting in one pass using a clustered data structure to access lighting data.</summary>         [InspectorName("Forward+")]         ForwardPlus = 2,         /// <summary>Render all objects first in a g-buffer pass, then apply all lighting in a separate pass using deferred shading.</summary>         Deferred = 1     };     // 省略下面代码。。。 }

构造函数根据渲染路径给出不同Pass执行策略

/// <summary> /// Constructor for the Universal Renderer. /// </summary> /// <param name="data">The settings to create the renderer with.</param> public UniversalRenderer(UniversalRendererData data) : base(data) {       // Query and cache runtime platform info first before setting up URP.       PlatformAutoDetect.Initialize();       ...

设置各种材质和状态

// 设置各种材质和状态  #if ENABLE_VR && ENABLE_XR_MODULE             Experimental.Rendering.XRSystem.Initialize(XRPassUniversal.Create, data.xrSystemData.shaders.xrOcclusionMeshPS, data.xrSystemData.shaders.xrMirrorViewPS); #endif             m_BlitMaterial = CoreUtils.CreateEngineMaterial(data.shaders.coreBlitPS);             m_BlitHDRMaterial = CoreUtils.CreateEngineMaterial(data.shaders.blitHDROverlay);             m_CopyDepthMaterial = CoreUtils.CreateEngineMaterial(data.shaders.copyDepthPS);             m_SamplingMaterial = CoreUtils.CreateEngineMaterial(data.shaders.samplingPS);             m_StencilDeferredMaterial = CoreUtils.CreateEngineMaterial(data.shaders.stencilDeferredPS);             m_CameraMotionVecMaterial = CoreUtils.CreateEngineMaterial(data.shaders.cameraMotionVector);             m_ObjectMotionVecMaterial = CoreUtils.CreateEngineMaterial(data.shaders.objectMotionVector);              StencilStateData stencilData = data.defaultStencilState;             m_DefaultStencilState = StencilState.defaultValue;             m_DefaultStencilState.enabled = stencilData.overrideStencilState;             m_DefaultStencilState.SetCompareFunction(stencilData.stencilCompareFunction);             m_DefaultStencilState.SetPassOperation(stencilData.passOperation);             m_DefaultStencilState.SetFailOperation(stencilData.failOperation);             m_DefaultStencilState.SetZFailOperation(stencilData.zFailOperation);              m_IntermediateTextureMode = data.intermediateTextureMode;              if (UniversalRenderPipeline.asset?.supportsLightCookies ?? false)             {                 var settings = LightCookieManager.Settings.Create();                 var asset = UniversalRenderPipeline.asset;                 if (asset)                 {                     settings.atlas.format = asset.additionalLightsCookieFormat;                     settings.atlas.resolution = asset.additionalLightsCookieResolution;                 }                  m_LightCookieManager = new LightCookieManager(ref settings);             }              this.stripShadowsOffVariants = true;             this.stripAdditionalLightOffVariants = true; #if ENABLE_VR && ENABLE_VR_MODULE #if PLATFORM_WINRT || PLATFORM_ANDROID             // AdditionalLightOff variant is available on HL&Quest platform due to performance consideration.             this.stripAdditionalLightOffVariants = !PlatformAutoDetect.isXRMobile; #endif #endif

Forward和Forward+灯光准备，深度预处理、深度拷贝模式等设置。

            ForwardLights.InitParams forwardInitParams;             forwardInitParams.lightCookieManager = m_LightCookieManager;             forwardInitParams.forwardPlus = data.renderingMode == RenderingMode.ForwardPlus;             m_Clustering = data.renderingMode == RenderingMode.ForwardPlus;             m_ForwardLights = new ForwardLights(forwardInitParams);             //m_DeferredLights.LightCulling = data.lightCulling;             this.m_RenderingMode = data.renderingMode;             this.m_DepthPrimingMode = data.depthPrimingMode;             this.m_CopyDepthMode = data.copyDepthMode;  #if UNITY_ANDROID || UNITY_IOS || UNITY_TVOS             this.m_DepthPrimingRecommended = false; #else             this.m_DepthPrimingRecommended = true; #endif

关键来了！URP定制的流程在这里（从灯光阴影投射、深度和深度法线预渲染、到深度拷贝、延迟渲染中的特别执行的LightMode：”UniversalForwardOnly”、再到延迟渲染的GBuffer及其对GBuffer的屏幕空间的光照处理、不透明阶段Pass、深度拷贝、运动向量Pass、天空盒Pass、透明物体Pass、离屏UIPass、覆盖UIPass、最后的混合、深度拷贝、输出到缓冲区“_CameraColorAttachment”）

            // Note: Since all custom render passes inject first and we have stable sort,             // we inject the builtin passes in the before events.             m_MainLightShadowCasterPass = new MainLightShadowCasterPass(RenderPassEvent.BeforeRenderingShadows);             m_AdditionalLightsShadowCasterPass = new AdditionalLightsShadowCasterPass(RenderPassEvent.BeforeRenderingShadows);  #if ENABLE_VR && ENABLE_XR_MODULE             m_XROcclusionMeshPass = new XROcclusionMeshPass(RenderPassEvent.BeforeRenderingOpaques);             // Schedule XR copydepth right after m_FinalBlitPass             m_XRCopyDepthPass = new CopyDepthPass(RenderPassEvent.AfterRendering + k_AfterFinalBlitPassQueueOffset, m_CopyDepthMaterial); #endif             m_DepthPrepass = new DepthOnlyPass(RenderPassEvent.BeforeRenderingPrePasses, RenderQueueRange.opaque, data.opaqueLayerMask);             m_DepthNormalPrepass = new DepthNormalOnlyPass(RenderPassEvent.BeforeRenderingPrePasses, RenderQueueRange.opaque, data.opaqueLayerMask);              if (renderingModeRequested == RenderingMode.Forward || renderingModeRequested == RenderingMode.ForwardPlus)             {                 m_PrimedDepthCopyPass = new CopyDepthPass(RenderPassEvent.AfterRenderingPrePasses, m_CopyDepthMaterial, true);             }              if (this.renderingModeRequested == RenderingMode.Deferred)             {                 var deferredInitParams = new DeferredLights.InitParams();                 deferredInitParams.stencilDeferredMaterial = m_StencilDeferredMaterial;                 deferredInitParams.lightCookieManager = m_LightCookieManager;                 m_DeferredLights = new DeferredLights(deferredInitParams, useRenderPassEnabled);                 m_DeferredLights.AccurateGbufferNormals = data.accurateGbufferNormals;                  m_GBufferPass = new GBufferPass(RenderPassEvent.BeforeRenderingGbuffer, RenderQueueRange.opaque, data.opaqueLayerMask, m_DefaultStencilState, stencilData.stencilReference, m_DeferredLights);                 // Forward-only pass only runs if deferred renderer is enabled.                 // It allows specific materials to be rendered in a forward-like pass.                 // We render both gbuffer pass and forward-only pass before the deferred lighting pass so we can minimize copies of depth buffer and                 // benefits from some depth rejection.                 // - If a material can be rendered either forward or deferred, then it should declare a UniversalForward and a UniversalGBuffer pass.                 // - If a material cannot be lit in deferred (unlit, bakedLit, special material such as hair, skin shader), then it should declare UniversalForwardOnly pass                 // - Legacy materials have unamed pass, which is implicitely renamed as SRPDefaultUnlit. In that case, they are considered forward-only too.                 // TO declare a material with unnamed pass and UniversalForward/UniversalForwardOnly pass is an ERROR, as the material will be rendered twice.                 StencilState forwardOnlyStencilState = DeferredLights.OverwriteStencil(m_DefaultStencilState, (int)StencilUsage.MaterialMask);                 ShaderTagId[] forwardOnlyShaderTagIds = new ShaderTagId[]                 {                     new ShaderTagId("UniversalForwardOnly"),                     new ShaderTagId("SRPDefaultUnlit"), // Legacy shaders (do not have a gbuffer pass) are considered forward-only for backward compatibility                     new ShaderTagId("LightweightForward") // Legacy shaders (do not have a gbuffer pass) are considered forward-only for backward compatibility                 };                 int forwardOnlyStencilRef = stencilData.stencilReference | (int)StencilUsage.MaterialUnlit;                 m_GBufferCopyDepthPass = new CopyDepthPass(RenderPassEvent.BeforeRenderingGbuffer + 1, m_CopyDepthMaterial, true);                 m_DeferredPass = new DeferredPass(RenderPassEvent.BeforeRenderingDeferredLights, m_DeferredLights);                 m_RenderOpaqueForwardOnlyPass = new DrawObjectsPass("Render Opaques Forward Only", forwardOnlyShaderTagIds, true, RenderPassEvent.BeforeRenderingOpaques, RenderQueueRange.opaque, data.opaqueLayerMask, forwardOnlyStencilState, forwardOnlyStencilRef);             }              // Always create this pass even in deferred because we use it for wireframe rendering in the Editor or offscreen depth texture rendering.             m_RenderOpaqueForwardPass = new DrawObjectsPass(URPProfileId.DrawOpaqueObjects, true, RenderPassEvent.BeforeRenderingOpaques, RenderQueueRange.opaque, data.opaqueLayerMask, m_DefaultStencilState, stencilData.stencilReference);             m_RenderOpaqueForwardWithRenderingLayersPass = new DrawObjectsWithRenderingLayersPass(URPProfileId.DrawOpaqueObjects, true, RenderPassEvent.BeforeRenderingOpaques, RenderQueueRange.opaque, data.opaqueLayerMask, m_DefaultStencilState, stencilData.stencilReference);              bool copyDepthAfterTransparents = m_CopyDepthMode == CopyDepthMode.AfterTransparents;             RenderPassEvent copyDepthEvent = copyDepthAfterTransparents ? RenderPassEvent.AfterRenderingTransparents : RenderPassEvent.AfterRenderingSkybox;              m_CopyDepthPass = new CopyDepthPass(                 copyDepthEvent,                 m_CopyDepthMaterial,                 shouldClear: true,                 copyResolvedDepth: RenderingUtils.MultisampleDepthResolveSupported() && SystemInfo.supportsMultisampleAutoResolve && copyDepthAfterTransparents);              // Motion vectors depend on the (copy) depth texture. Depth is reprojected to calculate motion vectors.             m_MotionVectorPass = new MotionVectorRenderPass(copyDepthEvent + 1, m_CameraMotionVecMaterial, m_ObjectMotionVecMaterial, data.opaqueLayerMask);              m_DrawSkyboxPass = new DrawSkyboxPass(RenderPassEvent.BeforeRenderingSkybox);             m_CopyColorPass = new CopyColorPass(RenderPassEvent.AfterRenderingSkybox, m_SamplingMaterial, m_BlitMaterial); #if ADAPTIVE_PERFORMANCE_2_1_0_OR_NEWER             if (needTransparencyPass) #endif             {                 m_TransparentSettingsPass = new TransparentSettingsPass(RenderPassEvent.BeforeRenderingTransparents, data.shadowTransparentReceive);                 m_RenderTransparentForwardPass = new DrawObjectsPass(URPProfileId.DrawTransparentObjects, false, RenderPassEvent.BeforeRenderingTransparents, RenderQueueRange.transparent, data.transparentLayerMask, m_DefaultStencilState, stencilData.stencilReference);             }             m_OnRenderObjectCallbackPass = new InvokeOnRenderObjectCallbackPass(RenderPassEvent.BeforeRenderingPostProcessing);              m_DrawOffscreenUIPass = new DrawScreenSpaceUIPass(RenderPassEvent.BeforeRenderingPostProcessing, true);             m_DrawOverlayUIPass = new DrawScreenSpaceUIPass(RenderPassEvent.AfterRendering + k_AfterFinalBlitPassQueueOffset, false); // after m_FinalBlitPass              {                 var postProcessParams = PostProcessParams.Create();                 postProcessParams.blitMaterial = m_BlitMaterial;                 postProcessParams.requestHDRFormat = GraphicsFormat.B10G11R11_UFloatPack32;                 var asset = UniversalRenderPipeline.asset;                 if (asset)                     postProcessParams.requestHDRFormat = UniversalRenderPipeline.MakeRenderTextureGraphicsFormat(asset.supportsHDR, asset.hdrColorBufferPrecision, false);                  m_PostProcessPasses = new PostProcessPasses(data.postProcessData, ref postProcessParams);             }              m_CapturePass = new CapturePass(RenderPassEvent.AfterRendering);             m_FinalBlitPass = new FinalBlitPass(RenderPassEvent.AfterRendering + k_FinalBlitPassQueueOffset, m_BlitMaterial, m_BlitHDRMaterial);  #if UNITY_EDITOR             m_FinalDepthCopyPass = new CopyDepthPass(RenderPassEvent.AfterRendering + 9, m_CopyDepthMaterial); #endif              // RenderTexture format depends on camera and pipeline (HDR, non HDR, etc)             // Samples (MSAA) depend on camera and pipeline             m_ColorBufferSystem = new RenderTargetBufferSystem("_CameraColorAttachment");

最后最一些兼容操作，结束构造。完成URP基本管线的主体流程。

supportedRenderingFeatures = new RenderingFeatures();              if (this.renderingModeRequested == RenderingMode.Deferred)             {                 // Deferred rendering does not support MSAA.                 this.supportedRenderingFeatures.msaa = false;                  // Avoid legacy platforms: use vulkan instead.                 unsupportedGraphicsDeviceTypes = new GraphicsDeviceType[]                 {                     GraphicsDeviceType.OpenGLCore,                     GraphicsDeviceType.OpenGLES2,                     GraphicsDeviceType.OpenGLES3                 };             }              LensFlareCommonSRP.mergeNeeded = 0;             LensFlareCommonSRP.maxLensFlareWithOcclusionTemporalSample = 1;             LensFlareCommonSRP.Initialize();              m_VulkanEnablePreTransform = GraphicsSettings.HasShaderDefine(BuiltinShaderDefine.UNITY_PRETRANSFORM_TO_DISPLAY_ORIENTATION);         }

‌核心渲染流程分解‌

URP将渲染分为五个阶段，均在Render()方法中调度：

‌阶段‌	‌URP实现细节‌
‌准备阶段‌	收集场景渲染对象与光源数据，配置相机参数与目标纹理。
‌几何阶段‌	执行视锥剔除，生成GPU顶点数据；通过`ScriptableRenderContext.DrawRenderers`提交绘制命令。
‌光照阶段‌	采用简化PBR模型：计算实时光源贡献，支持烘焙光照混合；动态光源采用Tile-Based优化策略（Forward+路径时）。
‌光栅化阶段‌	执行深度预通道（Depth Prepass）减少过度绘制，结合GPU Instancing优化批次处理。
‌后处理阶段‌	在独立Pass中应用抗锯齿（FXAA/TAA）、Bloom等效果，支持自定义RendererFeature扩展。

‌性能优化关键技术‌

‌SRP Batcher‌：对相同Shader变体但不同材质的物体进行动态合批，显著降低SetPass Call。
‌光照剔除优化‌：按层级控制剔除距离（layerCullDistances），对静态物体预计算遮挡数据。

URP对SRP的扩展与简化‌

‌标准化功能封装‌
- 内置轻量级PBR光照模型，取代HDRP的复杂物理模拟以提升跨平台性能。
- 集成‌Shader Graph‌可视化工具链，降低着色器开发门槛。（后期版本内置管线也可用ShaderGraph了）
‌跨平台适配策略‌
- 动态切换渲染精度（如移动端禁用实时阴影），通过QualitySettings分级配置。
- 资源包精简：剔除HDRP的高精度贴图与计算密集型特效，缩小运行时内存占用。

对URP的扩展

URP基本管线流程在UniversalRenderer的构造函数中已经定义完整。并且在其中每个阶段都给出了插入点。那么只需要在这些插入点用创建RendererFeature的方式插入自定义的Pass来影响和扩充基本的URP管线。
还有一种方式用RenderPipelineManager 提供的点位插入自定义Pass。