【Shader】后处理之模糊算法

原理部分详见：高品质后处理：十种图像模糊算法的总结与实现 - 知乎

这里只做一下Shader效果复现。

        Shader部分：构建卷积核，进行单次卷积运算

sampler2D _MainTex;
//Unity获取纹素大小参数
//x = 1.0/width, y = 1.0/height, z = width, w = height
float4 _MainTex_TexelSize;
float _BlurOffset;

half4 frag_BoxFilter_4Tap (v2f_img i) : SV_Target
{
    half4 sum = 0;
    //使用2×2卷积核
    sum += tex2D(_MainTex, i.uv + _MainTex_TexelSize.xy * half2(1, 1) * _BlurOffset);
    sum += tex2D(_MainTex, i.uv + _MainTex_TexelSize.xy * half2(1, -1) * _BlurOffset);
    sum += tex2D(_MainTex, i.uv + _MainTex_TexelSize.xy * half2(-1, 1) * _BlurOffset);
    sum += tex2D(_MainTex, i.uv + _MainTex_TexelSize.xy * half2(-1, -1) * _BlurOffset);
    sum *= 0.25;
    return sum;
}

half4 frag_BoxFilter_9Tap (v2f_img i) : SV_Target
{
    half4 sum = 0;
    //使用3×3卷积核
    sum = tex2D(_MainTex, i.uv);
    sum += tex2D(_MainTex, i.uv + _MainTex_TexelSize.xy * half2(1, 1) * _BlurOffset);
    sum += tex2D(_MainTex, i.uv + _MainTex_TexelSize.xy * half2(1, -1) * _BlurOffset);
    sum += tex2D(_MainTex, i.uv + _MainTex_TexelSize.xy * half2(-1, 1) * _BlurOffset);
    sum += tex2D(_MainTex, i.uv + _MainTex_TexelSize.xy * half2(-1, -1) * _BlurOffset);
    sum += tex2D(_MainTex, i.uv + _MainTex_TexelSize.xy * half2(0, -1) * _BlurOffset);
    sum += tex2D(_MainTex, i.uv + _MainTex_TexelSize.xy * half2(0, 1) * _BlurOffset);
    sum += tex2D(_MainTex, i.uv + _MainTex_TexelSize.xy * half2(-1, 0) * _BlurOffset);
    sum += tex2D(_MainTex, i.uv + _MainTex_TexelSize.xy * half2(1, 0) * _BlurOffset);
    sum /= 9.0;
    return sum;
}

        C#脚本：循环多次采样Shader

public Material mat;
[Range(0,20)] public int _Iteration = 4;
[Range(0,20)] public float _BlurRadius = 1.0f;

private void OnRenderImage(RenderTexture src, RenderTexture dest) 
{
    int width = src.width;
    int height = src.height;
    RenderTexture RT1 = RenderTexture.GetTemporary(width, height);
    RenderTexture RT2 = RenderTexture.GetTemporary(width, height);

    mat.SetFloat("_BlurOffset", _BlurRadius);
    Graphics.Blit(src, RT1);
    for (int i = 0; i < _Iteration; i++)
    {
        Graphics.Blit(RT1, RT2, mat, 0);
        Graphics.Blit(RT2, RT1, mat, 0);
    }
    Graphics.Blit(RT1, dest);

    //release：要养成用完就丢的好习惯
    RenderTexture.ReleaseTemporary(RT1);
    RenderTexture.ReleaseTemporary(RT2);
}

         Shader部分：

half4 frag_Horizontal (v2f_img i) : SV_Target
{
    half4 sum = 0;
    sum = tex2D(_MainTex, i.uv) * 0.4026;
    sum += tex2D(_MainTex, i.uv + _MainTex_TexelSize.xy * half2(1, 0) * _BlurOffset) * 0.2442;
    sum += tex2D(_MainTex, i.uv + _MainTex_TexelSize.xy * half2(2, 0) * _BlurOffset) * 0.0545;
    sum += tex2D(_MainTex, i.uv + _MainTex_TexelSize.xy * half2(-1, 0) * _BlurOffset) * 0.2442;
    sum += tex2D(_MainTex, i.uv + _MainTex_TexelSize.xy * half2(-2, 0) * _BlurOffset) * 0.0545;
    return sum;
}

half4 frag_Vertical (v2f_img i) : SV_Target
{
    half4 sum = 0;
    sum = tex2D(_MainTex, i.uv) * 0.4026;
    sum += tex2D(_MainTex, i.uv + _MainTex_TexelSize.xy * half2(0, 1) * _BlurOffset) * 0.2442;
    sum += tex2D(_MainTex, i.uv + _MainTex_TexelSize.xy * half2(0, 2) * _BlurOffset) * 0.0545;
    sum += tex2D(_MainTex, i.uv + _MainTex_TexelSize.xy * half2(0, -1) * _BlurOffset) * 0.2442;
    sum += tex2D(_MainTex, i.uv + _MainTex_TexelSize.xy * half2(0, -2) * _BlurOffset) * 0.0545;
    return sum;
}

        C#脚本： 

for (int i = 0; i < _Iteration; i++)
{
    Graphics.Blit(RT1, RT2, mat, 0);
    Graphics.Blit(RT2, RT1, mat, 1);
}

        Dual Kawase Blur的核心思路在于blit过程中进行降采样和升采样，即对RT进行了降采样以及升采样。

        这里对Box Blur进行双重模糊，Shader部分不用修改，只需对C#脚本进行一定的修改：

//降采样
for (int i = 0; i < _Iteration; i++)
{
    RenderTexture.ReleaseTemporary(RT2);
    width /= 2;
    height /= 2;
    RT2 = RenderTexture.GetTemporary(width, height);
    Graphics.Blit(RT1, RT2, mat, 1);

    RenderTexture.ReleaseTemporary(RT1);
    width /= 2;
    height /= 2;
    RT1 = RenderTexture.GetTemporary(width, height);
    Graphics.Blit(RT2, RT1, mat, 1);
}

//升采样
for (int i = 0; i < _Iteration; i++)
{
    RenderTexture.ReleaseTemporary(RT2);
    width *= 2;
    height *= 2;
    RT2 = RenderTexture.GetTemporary(width, height);
    Graphics.Blit(RT1, RT2, mat, 1);

    RenderTexture.ReleaseTemporary(RT1);
    width *= 2;
    height *= 2;
    RT1 = RenderTexture.GetTemporary(width, height);
    Graphics.Blit(RT2, RT1, mat, 1);
}