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高斯模糊

卷积的另一个常见应用就是高斯模糊。模糊的方法有很多，如均值模糊和中值模糊。均值模糊也是用了卷积操作，它使用的卷积核中的各个权重值相等，并且相加等于1，也就是卷积之后得到的像素值是其相邻像素值的平均值。中值模糊则是选择邻域内对所有像素排序后选择中间的值替换原来的颜色。而更高级的做法是高斯模糊。

一、高斯滤波

这篇文章讲的比较好，感谢作者的分享！
简单易懂的高斯滤波

高斯滤波有两个重要的步骤：
1、根据高斯分布算出高斯核
2、根据高斯核做卷积相加
高斯函数：

高斯函数

一维高斯核

二、实现

原图片

高斯模糊的图片

using System;
using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class GaussianBlur : PostEffectsBase
{
    public Shader gaussianBlurShader;

    private Material gaussianBlurMaterial = null;

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

    [Range(0, 8)] public int downSample = 1;
    [Range(0, 4)] public int iteration = 3;
    [Range(0.2f, 3.0f)] public float blurSpread = 0.6f;

    void OnRenderImage(RenderTexture src, RenderTexture dest)
    {
        if (material != null)
        {
            //对原纹理做一次缩放
            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);
            
            //每次迭代，水平竖直方向各做一次高斯模糊
            for (int i = 0; i < iteration; i++)
            {
                material.SetFloat("_BlurSize", 1.0f + i * blurSpread);
                RenderTexture buffer1 = RenderTexture.GetTemporary(rtW,rtH,0);
                Graphics.Blit(buffer0,buffer1,material,0);

                RenderTexture.ReleaseTemporary(buffer0);
                buffer0 = buffer1;
                buffer1 = RenderTexture.GetTemporary(rtW,rtH,0);
                Graphics.Blit(buffer0,buffer1,material,1);
                RenderTexture.ReleaseTemporary(buffer0);
                buffer0 = buffer1;
            }
            Graphics.Blit(buffer0,dest);
            RenderTexture.ReleaseTemporary(buffer0);
        }
        else
        {
            Graphics.Blit(src,dest);
        }
    }
}

Shader "Unlit/GaussionBlur"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
        _BlurSize("BlurSize",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 GuassionBlurVertical(appdata_img v)
        {
            v2f o;
            o.pos = UnityObjectToClipPos(v.vertex);
            
            half2 uv = v.texcoord;
            o.uv[0] = uv;
            o.uv[1] = uv + float2(0,_MainTex_TexelSize.y * 1) * _BlurSize;
            o.uv[2] = uv - float2(0,_MainTex_TexelSize.y * 1) * _BlurSize;
            o.uv[3] = uv + float2(0,_MainTex_TexelSize.y * 2) * _BlurSize;
            o.uv[4] = uv - float2(0,_MainTex_TexelSize.y * 2) * _BlurSize;

            return o;
        }

        v2f GuassionBlurHorizontal(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) * _BlurSize;
            o.uv[2] = uv - float2(_MainTex_TexelSize.x * 1,0) * _BlurSize;
            o.uv[3] = uv + float2(_MainTex_TexelSize.x * 2,0) * _BlurSize;
            o.uv[4] = uv - float2(_MainTex_TexelSize.x * 2,0) * _BlurSize;

            return o;
        }

        fixed4 fragBlur(v2f i):SV_TARGET
        {
            //本来卷积核是{0.0545,0.2442,0.4026,0.2442,0.0545};
            //去掉重复的
            float weight[3] = {0.4026, 0.2442, 0.0545};
            fixed3 sum = tex2D(_MainTex,i.uv[0]) * weight[0];
            //根据权重做卷积操作
            for(int it = 1; it < 3; it++)
            {
                sum += tex2D(_MainTex,i.uv[2*it-1]) * weight[it];
                sum += tex2D(_MainTex,i.uv[2*it]) * weight[it];
            }

            return fixed4(sum,1.0);
        }

        ENDCG

        ZTest Always Cull Off ZWrite Off

        Pass
        {
            NAME "GAUSSION_BLUR_VERTICAL"
            CGPROGRAM

            #pragma vertex GuassionBlurVertical
            #pragma fragment fragBlur

            ENDCG
        }

        Pass
        {
            NAME "GAUSSION_BLUR_HORZONTAL"
            CGPROGRAM
            #pragma vertex GuassionBlurHorizontal
            #pragma fragment fragBlur

            ENDCG
        }
    }
}