Unity自定义渲染管线（Scriptable Render Pipeline）架构设计与实现指南

一、SRP技术体系概述

1. 核心设计理念

• 全托管渲染控制：通过C#脚本完全掌控渲染流程

• 模块化架构：将渲染流程拆分为可组合的RenderPass

• GPU友好设计：支持CommandBuffer与Compute Shader混合编程

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2. 与传统管线对比

二、核心架构设计

1. 分层架构图

graph TB
    A[SRP Asset] -->|配置参数| B[RenderPipeline]
    B -->|调度| C[CameraRenderer]
    C -->|组织| D[RenderPass]
    D -->|执行| E[CommandBuffer]
    E -->|提交| F[GPU]

2. 核心组件职责

• SRP Asset：管线全局配置（质量等级、Shader变体等）

• RenderPipeline：主入口类，负责每帧调度

• CameraRenderer：单相机渲染流程控制器

• RenderPass：最小渲染单元（如阴影Pass、光照Pass）

三、基础管线实现

1. SRP Asset创建

[CreateAssetMenu(menuName = "Rendering/Custom RP")]
public class CustomRenderPipelineAsset : RenderPipelineAsset {
    public bool useDynamicBatching = true;
    public bool useGPUInstancing = true;
    
    protected override RenderPipeline CreatePipeline() {
        return new CustomRenderPipeline(this);
    }
}

2. 主渲染管线类

public class CustomRenderPipeline : RenderPipeline {
    private CameraRenderer renderer = new CameraRenderer();
    private bool useDynamicBatching;
    private bool useGPUInstancing;

    public CustomRenderPipeline(CustomRenderPipelineAsset asset) {
        useDynamicBatching = asset.useDynamicBatching;
        useGPUInstancing = asset.useGPUInstancing;
    }

    protected override void Render(
        ScriptableRenderContext context, 
        Camera[] cameras
    ) {
        foreach (var camera in cameras) {
            renderer.Render(context, camera, useDynamicBatching, useGPUInstancing);
        }
    }
}

3. 相机渲染器

public class CameraRenderer {
    static ShaderTagId unlitShaderTagId = new ShaderTagId("SRPDefaultUnlit");
    
    public void Render(
        ScriptableRenderContext context, 
        Camera camera,
        bool useDynamicBatching,
        bool useGPUInstancing
    ) {
        // 设置相机矩阵
        context.SetupCameraProperties(camera);
        
        // 清除渲染目标
        var cmd = new CommandBuffer { name = camera.name };
        cmd.ClearRenderTarget(true, true, Color.clear);
        context.ExecuteCommandBuffer(cmd);
        cmd.Clear();
        
        // 物体排序与绘制
        var sortingSettings = new SortingSettings(camera) {
            criteria = SortingCriteria.CommonOpaque
        };
        var drawingSettings = new DrawingSettings(
            unlitShaderTagId, 
            sortingSettings
        ) {
            enableDynamicBatching = useDynamicBatching,
            enableInstancing = useGPUInstancing
        };
        
        // 执行绘制
        var filterSettings = new FilteringSettings(RenderQueueRange.opaque);
        context.DrawRenderers(
            cullingResults, ref drawingSettings, ref filterSettings
        );
        
        context.Submit();
    }
}

四、高级渲染功能实现

1. 多Pass渲染架构

public class LightingPass : ScriptableRenderPass {
    public override void Configure(
        CommandBuffer cmd, 
        RenderTextureDescriptor cameraTextureDescriptor
    ) {
        // 创建临时RT
        cmd.GetTemporaryRT(
            Shader.PropertyToID("_LightBuffer"),
            1024, 1024, 24, FilterMode.Bilinear
        );
    }

    public override void Execute(
        ScriptableRenderContext context, 
        ref RenderingData renderingData
    ) {
        // 绘制光照几何体
        cmd.DrawMesh(lightMesh, matrix, lightMaterial);
        context.ExecuteCommandBuffer(cmd);
    }

    public override void FrameCleanup(CommandBuffer cmd) {
        // 释放临时RT
        cmd.ReleaseTemporaryRT(Shader.PropertyToID("_LightBuffer"));
    }
}

2. 延迟渲染路径实现

G-Buffer生成Shader

struct GBufferOutput {
    float4 albedo   : SV_Target0; // RGB:BaseColor, A:Smoothness
    float4 normal   : SV_Target1; // RGB:WorldNormal, A:Metallic
    float4 emission : SV_Target2; // RGB:Emission, A:Occlusion
};

GBufferOutput FragGBuffer(Varyings input) {
    GBufferOutput output;
    // 采样材质属性...
    output.albedo = float4(baseColor, smoothness);
    output.normal = float4(normalWS * 0.5 + 0.5, metallic);
    output.emission = float4(emission, occlusion);
    return output;
}

光照计算Pass

public class DeferredLightingPass : ScriptableRenderPass {
    private Material lightingMat;
    
    public override void Execute(...) {
        // 设置G-Buffer纹理
        cmd.SetGlobalTexture("_AlbedoBuffer", albedoRT);
        cmd.SetGlobalTexture("_NormalBuffer", normalRT);
        
        // 全屏四边形绘制光照
        cmd.DrawProcedural(
            Matrix4x4.identity, lightingMat, 0,
            MeshTopology.Triangles, 3, 1
        );
    }
}

五、性能优化技术

1. SRP Batcher优化

// Shader中声明PerObject数据
CBUFFER_START(UnityPerMaterial)
float4 _BaseColor;
float _Metallic;
CBUFFER_END

// C#端启用SRP Batcher
GraphicsSettings.useScriptableRenderPipelineBatching = true;

2. GPU Driven Rendering

ComputeBuffer argsBuffer = new ComputeBuffer(
    5, sizeof(uint), ComputeBufferType.IndirectArguments
);

// 通过Compute Shader生成绘制参数
computeShader.Dispatch(kernel, groupCount, 1, 1);

// 间接绘制
cmd.DrawMeshInstancedIndirect(
    mesh, 0, material, bounds, argsBuffer
);

3. 多线程渲染支持

// 创建并行上下文
var renderThread = new RenderThread();
renderThread.Start();

// 提交渲染任务
renderThread.Schedule(() => {
    context.SetupCameraProperties(camera);
    context.DrawRenderers(...);
    context.Submit();
});

六、实战案例：卡通渲染管线

1. 架构设计

2. 关键Shader代码

// 轮廓Pass
Pass {
    Cull Front
    HLSLPROGRAM
    float _OutlineWidth;
    
    Varyings VertOutline(Varyings input) {
        float3 normal = input.normal * _OutlineWidth;
        input.positionOS += normal;
        return input;
    }
    ENDHLSL
}

// 色阶处理
float3 QuantizeColor(float3 color) {
    float levels = 3.0;
    return floor(color * levels) / levels;
}

七、调试与优化工具

1. 帧调试器扩展

[MenuItem("Custom RP/Debug/Show Light Count")]
static void ToggleLightCountDebug() {
    Shader.EnableKeyword("_DEBUG_LIGHT_COUNT");
}

2. 性能分析标记

using (new ProfilingScope(cmd, 
    new ProfilingSampler("Lighting Pass"))) 
{
    // 光照计算代码...
}

八、完整项目参考

Unlit渲染管线全流程详解

通过自定义渲染管线，开发者可突破Unity默认渲染的限制，实现从移动端到主机的全平台优化方案。核心在于：1) 合理设计RenderPass执行顺序；2) 利用CommandBuffer精细控制GPU资源；3) 结合Compute Shader实现GPU Driven架构。建议将常用渲染功能（如阴影、后处理）模块化封装，便于不同项目间复用。