原理

利用另一个算子，均值滤波器，这个算子的作用可以平均邻域内的像素，从而达到模糊的效果

高斯滤波：很好的模拟了邻域每个像素对当前像素的影响程度：距离越近，影响越大。高斯核的维数越高，模糊程度越大。

实现

把5x5的高斯核变成横向和纵向的两个一位高斯核，用两个pass来得到最后的图像，通过图像缩放提高性能，通过应用次数来控制模糊程度

脚本类：

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using UnityEngine;
using System.Collections;

public class GaussianBlur : PostEffectsBase {

	public Shader gaussianBlurShader;
	private Material gaussianBlurMaterial = null;

	public Material material {  
		get {
			gaussianBlurMaterial = CheckShaderAndCreateMaterial(gaussianBlurShader, gaussianBlurMaterial);
			return gaussianBlurMaterial;
		}  
	}

	// Blur iterations - larger number means more blur.
	//迭代次数，模糊范围，缩放系数
	[Range(0, 4)]
	public int iterations = 3;
	
	// Blur spread for each iteration - larger value means more blur
	[Range(0.2f, 3.0f)]
	public float blurSpread = 0.6f;
	
	[Range(1, 8)]
	public int downSample = 2;
	
	/// 1st edition: just apply blur
//	void OnRenderImage(RenderTexture src, RenderTexture dest) {
//		if (material != null) {
//			int rtW = src.width;
//			int rtH = src.height;
//通过该函数分配了一块与屏幕图像大小相同的缓冲区，用来存放第一个pass的结果
//			RenderTexture buffer = RenderTexture.GetTemporary(rtW, rtH, 0);
//
//			// Render the vertical pass
//			Graphics.Blit(src, buffer, material, 0);
//			// Render the horizontal pass
//			Graphics.Blit(buffer, dest, material, 1);
//
//			RenderTexture.ReleaseTemporary(buffer);
//		} else {
//			Graphics.Blit(src, dest);
//		}
//	} 

	/// 2nd edition: scale the render texture
//	void OnRenderImage (RenderTexture src, RenderTexture dest) {
//		if (material != null) 
//让缓冲区大小缩小，使用双线性插值，当把图片导出缓冲区的时候，多个像素归并为一个，可以减少像素数，可以提供性能的同时提供更好的模糊效果
//			int rtW = src.width/downSample;
//			int rtH = src.height/downSample;
//			RenderTexture buffer = RenderTexture.GetTemporary(rtW, rtH, 0);
//			buffer.filterMode = FilterMode.Bilinear;
//
//			// Render the vertical pass
//			Graphics.Blit(src, buffer, material, 0);
//			// Render the horizontal pass
//			Graphics.Blit(buffer, dest, material, 1);
//
//			RenderTexture.ReleaseTemporary(buffer);
//		} else {
//			Graphics.Blit(src, dest);
//		}
//	}

	/// 3rd edition: use iterations for larger blur
	void OnRenderImage (RenderTexture src, RenderTexture dest) {
		if (material != null) {
			int rtW = src.width/downSample;
			int rtH = src.height/downSample;

			RenderTexture buffer0 = RenderTexture.GetTemporary(rtW, rtH, 0);
			buffer0.filterMode = FilterMode.Bilinear;

			Graphics.Blit(src, buffer0);

			for (int i = 0; i < iterations; i++) {
				//每一次迭代的时候，都扩大模糊范围
				material.SetFloat("_BlurSize", 1.0f + i * blurSpread);

				RenderTexture buffer1 = RenderTexture.GetTemporary(rtW, rtH, 0);

				// Render the vertical pass
				Graphics.Blit(buffer0, buffer1, material, 0);

				RenderTexture.ReleaseTemporary(buffer0);
				buffer0 = buffer1;
				buffer1 = RenderTexture.GetTemporary(rtW, rtH, 0);

				// Render the horizontal pass
				Graphics.Blit(buffer0, buffer1, material, 1);

				RenderTexture.ReleaseTemporary(buffer0);
				buffer0 = buffer1;
			}

			Graphics.Blit(buffer0, dest);
			RenderTexture.ReleaseTemporary(buffer0);
		} else {
			Graphics.Blit(src, dest);
		}
	}
}

shader

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// Upgrade NOTE: replaced 'mul(UNITY_MATRIX_MVP,*)' with 'UnityObjectToClipPos(*)'

Shader "Unity Shaders Book/Chapter 12/Gaussian Blur" {
	Properties {
		_MainTex ("Base (RGB)", 2D) = "white" {}
		_BlurSize ("Blur Size", Float) = 1.0
	}
	SubShader {
		CGINCLUDE
		
		#include "UnityCG.cginc"
		
		sampler2D _MainTex;  
		half4 _MainTex_TexelSize;
		float _BlurSize;
		  
		struct v2f {
			float4 pos : SV_POSITION;
			half2 uv[5]: TEXCOORD0;
		};
		  
		v2f vertBlurVertical(appdata_img v) {
			v2f o;
			o.pos = UnityObjectToClipPos(v.vertex);
			
			half2 uv = v.texcoord;
			
			o.uv[0] = uv;
			o.uv[1] = uv + float2(0.0, _MainTex_TexelSize.y * 1.0) * _BlurSize;
			o.uv[2] = uv - float2(0.0, _MainTex_TexelSize.y * 1.0) * _BlurSize;
			o.uv[3] = uv + float2(0.0, _MainTex_TexelSize.y * 2.0) * _BlurSize;
			o.uv[4] = uv - float2(0.0, _MainTex_TexelSize.y * 2.0) * _BlurSize;
					 
			return o;
		}
		
		v2f vertBlurHorizontal(appdata_img v) {
			v2f o;
			o.pos = UnityObjectToClipPos(v.vertex);
			
			half2 uv = v.texcoord;
			
			o.uv[0] = uv;
			o.uv[1] = uv + float2(_MainTex_TexelSize.x * 1.0, 0.0) * _BlurSize;
			o.uv[2] = uv - float2(_MainTex_TexelSize.x * 1.0, 0.0) * _BlurSize;
			o.uv[3] = uv + float2(_MainTex_TexelSize.x * 2.0, 0.0) * _BlurSize;
			o.uv[4] = uv - float2(_MainTex_TexelSize.x * 2.0, 0.0) * _BlurSize;
					 
			return o;
		}
		
		fixed4 fragBlur(v2f i) : SV_Target {
		//由于5x5，其实是对称的，所以只需要知道一边的参数值就行
			float weight[3] = {0.4026, 0.2442, 0.0545};
			
			fixed3 sum = tex2D(_MainTex, i.uv[0]).rgb * weight[0];
			
			for (int it = 1; it < 3; it++) {
				sum += tex2D(_MainTex, i.uv[it*2-1]).rgb * weight[it];
				sum += tex2D(_MainTex, i.uv[it*2]).rgb * weight[it];
			}
			
			return fixed4(sum, 1.0);
		}
		    
		ENDCG
		
		ZTest Always Cull Off ZWrite Off
		
		Pass {
			NAME "GAUSSIAN_BLUR_VERTICAL"
			
			CGPROGRAM
			  
			#pragma vertex vertBlurVertical  
			#pragma fragment fragBlur
			  
			ENDCG  
		}
		
		Pass {  
			NAME "GAUSSIAN_BLUR_HORIZONTAL"
			
			CGPROGRAM  
			
			#pragma vertex vertBlurHorizontal  
			#pragma fragment fragBlur
			
			ENDCG
		}
	} 
	FallBack "Diffuse"
}