【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