【URP】Unity[后处理]通道混合ChannelMixer
核心功能与参数
通道混合原理
通过修改输入颜色通道（Red/Green/Blue）对输出通道的贡献权重，实现如单色保留、色调偏移等效果。例如增加绿色通道对红色输出的影响会使绿色区域偏红。
ChannelMixer在Unity URP后处理中的底层原理基于颜色通道的线性代数变换，通过对RGB通道的权重矩阵运算实现色彩重构.
输出通道 = (R × R权重) + (G × G权重) + (B × B权重) + 常数偏移
该计算在片元着色器中逐像素执行，通过矩阵乘法实现颜色空间转换.
数学原理分解
通道变换矩阵
每个输出通道由3×4变换矩阵控制（包含常数项），例如红色输出通道计算为：
$R_{out}=R_{in}\times M_{00}+G_{in}\times M_{01}+B_{in}\times M_{02}+M_{03}$
其中M_03为常数偏移量。
亮度保持机制
当启用Preserve Luminosity时，系统会自动归一化权重系数，确保满足：
$M_{00}+M_{01}+M_{02}=1.0$
防止画面过曝或欠曝
URP实现示例 关键实现解析
矩阵运算阶段
片元着色器通过dot(col.rgb, _RedOut.xyz)实现向量点乘，等效于矩阵乘法。例如设置_RedOut=(0.5,0.3,0.2)时，新红色通道值为原RGB通道的50%+30%+20%混合。
动态参数传递
URP通过Volume组件将参数打包为Vector4（xyz为RGB权重，w为常数偏移），每帧更新至Shader。例如电影级调色常用配置：
csharp
_RedOut = new Vector4(0.8f, 0.1f, 0.1f, 0);// 强化红色主基调
_GreenOut = new Vector4(0f, 1.2f, -0.2f, 0);// 增强绿色并抑制蓝色
后处理管线集成
RendererFeature在BeforeRenderingPostProcessing事件点插入通道混合操作，通过双次Blit避免直接修改源纹理。临时渲染目标_TempChannelMixerTexture确保混合过程可逆.
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);
}
}
参数说明
Output Channel：选择要调整的目标通道（Red/Green/Blue）
Red/Green/Blue Contribution：各输入通道对当前输出通道的影响权重（范围-2到2）
Preserve Luminosity：保持整体亮度不变，避免过度曝光。
实现流程示例
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);
}
}
实际应用案例
电影感调色
将蓝色通道注入红色输出（如设置RedOut为(0.8, 0, 0.2)），可创造类似《黑客帝国》的青色阴影效果。
风格化渲染
通过反转绿色通道贡献（GreenOut设为(-0.5, 1, 0)），实现赛博朋克风格的色彩偏移。
黑白滤镜
统一各通道权重（如RedOut=(0.3,0.6,0.1)）可生成高质量灰度图像，比直接去饱和度更可控。
胶片模拟
设置BlueOut=(0,0,0.9,0.1)使蓝色通道轻微压缩，模拟柯达胶片特性
色盲辅助
将红色通道注入绿色输出（GreenOut=(0.5,0.5,0,0)）提升红绿色盲辨识度
风格化渲染
负值权重创造互补色效果，如RedOut=(1, -0.2, 0, 0)产生赛博朋克色调
操作步骤
在Volume中添加Channel Mixer效果
调整目标通道的RGB权重三角滑块
启用Preserve Luminosity避免过曝
该技术现已成为URP标准管线的一部分，通过Volume系统实现非破坏性调整，比传统Shader方案更易集成到美术工作流中