ChannelMixer是Unity URP后處理系統(tǒng)中用于顏色通道混合的核心效果組件，主要用于調(diào)整RGB通道的混合比例以實(shí)現(xiàn)特定的色彩分級效果。其發(fā)展歷史可追溯至影視行業(yè)的傳統(tǒng)調(diào)色技術(shù)，后被整合到U

【從UnityURP開始探索游戲渲染】專欄-直達(dá)

ChannelMixer是Unity URP后處理系統(tǒng)中用于顏色通道混合的核心效果組件，主要用于調(diào)整RGB通道的混合比例以實(shí)現(xiàn)特定的色彩分級效果。其發(fā)展歷史可追溯至影視行業(yè)的傳統(tǒng)調(diào)色技術(shù)，后被整合到Unity的Post Processing Stack中，并隨著URP的演進(jìn)成為Volume框架下的標(biāo)準(zhǔn)化模塊

核心功能與參數(shù)

?通道混合原理?

通過修改輸入顏色通道（Red/Green/Blue）對輸出通道的貢獻(xiàn)權(quán)重，實(shí)現(xiàn)如單色保留、色調(diào)偏移等效果。例如增加綠色通道對紅色輸出的影響會(huì)使綠色區(qū)域偏紅。

ChannelMixer在Unity URP后處理中的底層原理基于顏色通道的線性代數(shù)變換，通過對RGB通道的權(quán)重矩陣運(yùn)算實(shí)現(xiàn)色彩重構(gòu).

輸出通道 = (R × R權(quán)重) + (G × G權(quán)重) + (B × B權(quán)重) + 常數(shù)偏移

該計(jì)算在片元著色器中逐像素執(zhí)行，通過矩陣乘法實(shí)現(xiàn)顏色空間轉(zhuǎn)換.

數(shù)學(xué)原理分解

• ?通道變換矩陣
  • 每個(gè)輸出通道由3×4變換矩陣控制（包含常數(shù)項(xiàng)），例如紅色輸出通道計(jì)算為：
  • $R_{out}=R_{in}\times M_{00}+G_{in}\times M_{01}+B_{in}\times M_{02}+M_{03}$
  • 其中M_03為常數(shù)偏移量。
• ?亮度保持機(jī)制?
  • 當(dāng)啟用Preserve Luminosity時(shí)，系統(tǒng)會(huì)自動(dòng)歸一化權(quán)重系數(shù)，確保滿足：
  • $M_{00}+M_{01}+M_{02}=1.0$
  • 防止畫面過曝或欠曝

URP實(shí)現(xiàn)示例 關(guān)鍵實(shí)現(xiàn)解析

• ?矩陣運(yùn)算階段?

  片元著色器通過dot(col.rgb, _RedOut.xyz)實(shí)現(xiàn)向量點(diǎn)乘，等效于矩陣乘法。例如設(shè)置_RedOut=(0.5,0.3,0.2)時(shí)，新紅色通道值為原RGB通道的50%+30%+20%混合。

• ?動(dòng)態(tài)參數(shù)傳遞?

  URP通過Volume組件將參數(shù)打包為Vector4（xyz為RGB權(quán)重，w為常數(shù)偏移），每幀更新至Shader。例如電影級調(diào)色常用配置：

  csharp
  _RedOut = new Vector4(0.8f, 0.1f, 0.1f, 0);// 強(qiáng)化紅色主基調(diào)
  _GreenOut = new Vector4(0f, 1.2f, -0.2f, 0);// 增強(qiáng)綠色并抑制藍(lán)色
  

• ?后處理管線集成?

  RendererFeature在BeforeRenderingPostProcessing事件點(diǎn)插入通道混合操作，通過雙次Blit避免直接修改源紋理。臨時(shí)渲染目標(biāo)_TempChannelMixerTexture確保混合過程可逆.

• ChannelMixer.shader

  Shader "PostProcessing/ChannelMixer" {
  Properties {
  _MainTex ("Texture", 2D) = "white" {}
  _RedOut ("Red Output", Vector) = (1,0,0,0)
  _GreenOut ("Green Output", Vector) = (0,1,0,0)
  _BlueOut ("Blue Output", Vector) = (0,0,1,0)
  }
  SubShader {
  HLSLPROGRAM
  #pragma only_renderers d3d11
  #pragma exclude_renderers gles
  #pragma target 5.0
  #pragma multi_compile_postprocess
      #include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Core.hlsl"
      #include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/PostProcessing.hlsl"
  
      TEXTURE2D(_MainTex);
      SAMPLER(sampler_MainTex);
      float4 _RedOut;
      float4 _GreenOut;
      float4 _BlueOut;
  
      struct v2f {
          float2 uv : TEXCOORD0;
          float4 vertex : SV_POSITION;
      };
  
      v2f vert(appdata v) {
          v2f o;
          o.vertex = TransformWorldToClipPos(v.vertex);
          o.uv = v.uv;
          return o;
      }
  
      float4 frag(v2f i) : SV_Target {
          float4 col = SAMPLE_TEXTURE2D(_MainTex, sampler_MainTex, i.uv);
          float3 result;
          result.r = dot(col.rgb, _RedOut.xyz) + _RedOut.w;
          result.g = dot(col.rgb, _GreenOut.xyz) + _GreenOut.w;
          result.b = dot(col.rgb, _BlueOut.xyz) + _BlueOut.w;
          return float4(result, col.a);
      }
      ENDHLSL
  }
  }
  

• ChannelMixerRendererFeature.cs

  using UnityEngine;
  using UnityEngine.Rendering;
  using UnityEngine.Rendering.Universal;
  
  public class ChannelMixerRendererFeature : ScriptableRendererFeature {
      class CustomRenderPass : ScriptableRenderPass {
          Material _material;
          RenderTargetHandle _tempTexture;
  
          public CustomRenderPass(Material material) {
              _material = material;
              _tempTexture.Init("_TempChannelMixerTexture");
          }
  
          public override void Execute(ScriptableRenderContext context, ref RenderingData data) {
              CommandBuffer cmd = CommandBufferPool.Get("Channel Mixer");
              RenderTextureDescriptor desc = data.cameraData.cameraTargetDescriptor;
  
              cmd.GetTemporaryRT(_tempTexture.id, desc);
              Blit(cmd, source: "_CameraColorTexture", dest: _tempTexture.id, _material);
              Blit(cmd, source: _tempTexture.id, dest: "_CameraColorTexture");
  
              cmd.ReleaseTemporaryRT(_tempTexture.id);
              context.ExecuteCommandBuffer(cmd);
              CommandBufferPool.Release(cmd);
          }
      }
  
      [System.Serializable]
      public class Settings {
          public Material material;
      }
  
      public Settings settings = new Settings();
      CustomRenderPass _scriptablePass;
  
      public override void Create() {
          _scriptablePass = new CustomRenderPass(settings.material);
          _scriptablePass.renderPassEvent = RenderPassEvent.BeforeRenderingPostProcessing;
      }
  
      public override void AddRenderPasses(ScriptableRenderer renderer, ref RenderingData data) {
          renderer.EnqueuePass(_scriptablePass);
      }
  }
  

?參數(shù)說明?

• ?Output Channel?：選擇要調(diào)整的目標(biāo)通道（Red/Green/Blue）
• ?Red/Green/Blue Contribution?：各輸入通道對當(dāng)前輸出通道的影響權(quán)重（范圍-2到2）
• ?Preserve Luminosity?：保持整體亮度不變，避免過度曝光。

實(shí)現(xiàn)流程示例

ChannelMixerExample.cs

using UnityEngine;
using UnityEngine.Rendering;
using UnityEngine.Rendering.Universal;

public class ChannelMixerExample : VolumeComponent, IPostProcessComponent {
    public Vector3Parameter redOut = new(new Vector3(1, 0, 0));
    public Vector3Parameter greenOut = new(new Vector3(0, 1, 0)); 
    public Vector3Parameter blueOut = new(new Vector3(0, 0, 1));

    public bool IsActive() => true;
    public bool IsTileCompatible() => false;
}

ChannelMixerRenderPass.cs

using UnityEngine;
using UnityEngine.Rendering;
using UnityEngine.Rendering.Universal;

public class ChannelMixerRenderPass : ScriptableRenderPass {
    private Material _material;
    private ChannelMixerExample _volume;

    public override void Execute(ScriptableRenderContext context, ref RenderingData data) {
        _volume = VolumeManager.instance.stack.GetComponent<ChannelMixerExample>();
        if (_volume == null) return;

        var cmd = CommandBufferPool.Get("ChannelMixer");
        _material.SetVector("_RedOut", _volume.redOut.value);
        _material.SetVector("_GreenOut", _volume.greenOut.value);
        _material.SetVector("_BlueOut", _volume.blueOut.value);
        Blit(cmd, ref data, _material, 0);
        context.ExecuteCommandBuffer(cmd);
    }
}

實(shí)際應(yīng)用案例

?電影感調(diào)色?

將藍(lán)色通道注入紅色輸出（如設(shè)置RedOut為(0.8, 0, 0.2)），可創(chuàng)造類似《黑客帝國》的青色陰影效果。

?風(fēng)格化渲染?

通過反轉(zhuǎn)綠色通道貢獻(xiàn)（GreenOut設(shè)為(-0.5, 1, 0)），實(shí)現(xiàn)賽博朋克風(fēng)格的色彩偏移。

?黑白濾鏡?

統(tǒng)一各通道權(quán)重（如RedOut=(0.3,0.6,0.1)）可生成高質(zhì)量灰度圖像，比直接去飽和度更可控。

膠片模擬?

• 設(shè)置BlueOut=(0,0,0.9,0.1)使藍(lán)色通道輕微壓縮，模擬柯達(dá)膠片特性

?色盲輔助?

• 將紅色通道注入綠色輸出（GreenOut=(0.5,0.5,0,0)）提升紅綠色盲辨識(shí)度

?風(fēng)格化渲染?

• 負(fù)值權(quán)重創(chuàng)造互補(bǔ)色效果，如RedOut=(1, -0.2, 0, 0)產(chǎn)生賽博朋克色調(diào)

操作步驟

• 在Volume中添加Channel Mixer效果
• 調(diào)整目標(biāo)通道的RGB權(quán)重三角滑塊
• 啟用Preserve Luminosity避免過曝

該技術(shù)現(xiàn)已成為URP標(biāo)準(zhǔn)管線的一部分，通過Volume系統(tǒng)實(shí)現(xiàn)非破壞性調(diào)整，比傳統(tǒng)Shader方案更易集成到美術(shù)工作流中

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【從UnityURP開始探索游戲渲染】專欄-直達(dá)
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