【從UnityURP開始探索遊戲渲染】專欄-直達

FullScreenPassRendererFeature是Unity URP渲染管線中用於實現全屏後處理效果的核心組件，它允許開發者在渲染流程的特定階段插入自定義的全屏着色器效果。

功能與作用

• ‌核心功能‌：通過ScriptableRenderPass在URP管線中注入全屏四邊形繪製命令，應用自定義Shader實現屏幕空間特效（如模糊、色調調整等）

• ‌典型應用場景‌：

  • 屏幕後處理（如Bloom、景深模擬）
  • 全局濾鏡效果（黑白化、夜視模式）
  • 特殊視覺特效（掃描線、像素化）

發展歷史

• ‌URP初期‌：需手動編寫完整的ScriptableRendererFeature和ScriptableRenderPass類實現全屏效果
• ‌URP 7.0+ ：引入預置的FullScreenPassRendererFeature簡化開發流程
• ‌URP 17.0‌：重構API至RenderGraph系統，優化資源管理機制

原理

FullScreenPassRendererFeature是Unity URP中用於實現全屏後處理效果的核心組件，其底層原理基於URP的可編程渲染管線架構。

核心原理

• ‌繼承關係‌：繼承自ScriptableRendererFeature基類，通過Create()方法初始化自定義的ScriptableRenderPass子類實例
• ‌執行流程‌：在URP渲染管線的特定階段（如不透明物體渲染後）插入全屏繪製命令，通過CommandBuffer調用Blit或DrawProcedural實現
• ‌資源管理‌：使用RTHandle系統動態管理渲染目標，自動處理不同分辨率下的資源分配與釋放

關鍵代碼示例

以下實現一個基礎的全屏泛光效果：

• BloomFeature.cs

  using UnityEngine;
  using UnityEngine.Rendering;
  using UnityEngine.Rendering.Universal;
  
  [System.Serializable]
  public class BloomSettings {
      public float intensity = 1.0f;
      public Color tint = Color.white;
  }
  
  public class BloomFeature : ScriptableRendererFeature {
      public BloomSettings settings = new BloomSettings();
      private BloomPass m_Pass;
  
      public override void Create() {
          m_Pass = new BloomPass(settings);
          m_Pass.renderPassEvent = RenderPassEvent.AfterRenderingPostProcessing;
      }
  
      public override void AddRenderPasses(
          ScriptableRenderer renderer, 
          ref RenderingData renderingData) {
          renderer.EnqueuePass(m_Pass);
      }
  }
  
  class BloomPass : ScriptableRenderPass {
      private Material m_Material;
      private BloomSettings m_Settings;
      private RTHandle m_TempTexture;
  
      public BloomPass(BloomSettings settings) {
          m_Settings = settings;
          m_Material = CoreUtils.CreateEngineMaterial("Hidden/Blur");
      }
  
      public override void Configure(CommandBuffer cmd, 
          RenderTextureDescriptor cameraTextureDescriptor) {
          m_TempTexture = RTHandles.Alloc(
              Vector2.one * 0.5f, 
              colorFormat: cameraTextureDescriptor.colorFormat);
      }
  
      public override void Execute(ScriptableRenderContext context, 
          ref RenderingData renderingData) {
          var cmd = CommandBufferPool.Get("BloomPass");
          Blit(cmd, 
              renderingData.cameraData.renderer.cameraColorTargetHandle,
              m_TempTexture, 
              m_Material, 0);
          Blit(cmd, 
              m_TempTexture,
              renderingData.cameraData.renderer.cameraColorTargetHandle);
          context.ExecuteCommandBuffer(cmd);
          CommandBufferPool.Release(cmd);
      }
  
      public override void FrameCleanup(CommandBuffer cmd) {
          m_TempTexture.Release();
      }
  }

技術細節

• ‌材質控制‌：通過Shader的LightMode標籤匹配（如UniversalForward）確定渲染路徑
• ‌時序控制‌：RenderPassEvent枚舉精確控制執行時機（如AfterRenderingOpaques）
• ‌多Pass協作‌：支持通過多個ScriptableRenderPass實現效果疊加，如先提取亮部再模糊

性能優化

• ‌臨時紋理複用‌：通過RTHandle的縮放參數實現動態分辨率渲染
• ‌命令緩衝池‌：使用CommandBufferPool減少GC開銷
• ‌材質實例化‌：避免每幀創建新材質

該機制相比傳統OnRenderImage方案，在URP中能更好地集成到SRP批處理系統中，且支持XR多通道渲染等高級特性

實現流程示例

以下為完整實現黑白濾鏡效果的URP示例：

• GrayFeature.cs

  using UnityEngine;
  using UnityEngine.Rendering;
  using UnityEngine.Rendering.Universal;
  
  public class GrayRenderFeature : ScriptableRendererFeature {
      public RenderPassEvent renderEvent = RenderPassEvent.AfterRendering;
      public Shader shader;
      private Material grayMaterial;
      private GrayPass grayPass;
  
      public override void Create() {
          grayMaterial = CoreUtils.CreateEngineMaterial(shader);
          grayPass = new GrayPass(grayMaterial, renderEvent);
      }
  
      public override void AddRenderPasses(ScriptableRenderer renderer, ref RenderingData renderingData) {
          grayPass.SetTarget(renderer.cameraColorTargetHandle);
          renderer.EnqueuePass(grayPass);
      }
  }
  
  public class GrayPass : ScriptableRenderPass {
      private Material material;
      private RTHandle source;
  
      public GrayPass(Material material, RenderPassEvent passEvent) {
          this.material = material;
          this.renderPassEvent = passEvent;
      }
  
      public void SetTarget(RTHandle colorHandle) {
          source = colorHandle;
      }
  
      public override void Execute(ScriptableRenderContext context, ref RenderingData renderingData) {
          CommandBuffer cmd = CommandBufferPool.Get("GrayEffect");
          Blitter.BlitCameraTexture(cmd, source, source, material, 0);
          context.ExecuteCommandBuffer(cmd);
          CommandBufferPool.Release(cmd);
      }
  }

• GrayEffect.shader

  Shader "Custom/GrayEffect" {
      Properties {
          _MainTex ("Base (RGB)", 2D) = "white" {}
      }
      SubShader {
          Pass {
              CGPROGRAM
              #pragma vertex vert
              #pragma fragment frag
              #include "UnityCG.cginc"
  
              struct appdata {
                  float4 vertex : POSITION;
                  float2 uv : TEXCOORD0;
              };
  
              struct v2f {
                  float2 uv : TEXCOORD0;
                  float4 vertex : SV_POSITION;
              };
  
              sampler2D _MainTex;
  
              v2f vert (appdata v) {
                  v2f o;
                  o.vertex = UnityObjectToClipPos(v.vertex);
                  o.uv = v.uv;
                  return o;
              }
  
              fixed4 frag (v2f i) : SV_Target {
                  fixed4 col = tex2D(_MainTex, i.uv);
                  float gray = dot(col.rgb, float3(0.299, 0.587, 0.114));
                  return fixed4(gray, gray, gray, col.a);
              }
              ENDCG
          }
      }
  }

參數説明與用例

‌實際配置步驟‌：

• 創建URP Renderer Asset（若不存在）
• 通過Inspector添加FullScreenPassRendererFeature
• 指定Material並使用ShaderGraph創建效果
• 調整RenderPassEvent控制執行順序

性能優化建議

• 使用RTHandle管理渲染目標避免內存浪費
• 複雜效果建議結合Volume系統實現參數動態調整
• 移動端需注意帶寬佔用，推薦使用Half Resolution處理

【從UnityURP開始探索遊戲渲染】專欄-直達
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