bloom是模拟真实相机，较亮的部分扩散到周围，造成一种朦胧的感觉。

• 先用第一个pass筛选出原图中较亮的部分
• 然后在第二和第三个pass中把较亮的图像进行高斯模糊

• 最后在第四个pass中把模糊后的亮部和原图进行混合得到bloom效果

    void OnRenderImage(RenderTexture src, RenderTexture dest)
    {
        if (material != null)
        {
            material.SetFloat("_LuminanceThreshold", luminanceThreshold);
            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, material, 0);
            for (int i = 0; i < iterations; i++)
            {
                material.SetFloat("_BlurSize", 1.0f + i * blurSpread);
                RenderTexture buffer1 = RenderTexture.GetTemporary(rtW, rtH, 0);
                Graphics.Blit(buffer0, buffer1, material, 1);
                RenderTexture.ReleaseTemporary(buffer0);
                buffer0 = buffer1;
            }
            material.SetTexture("_Bloom", buffer0);
            Graphics.Blit(src, dest, material, 3);
            RenderTexture.ReleaseTemporary(buffer0);
        }
        else
        {
            Graphics.Blit(src, dest);
        }
    }
  

  Shader "UnityShaderLearning/Bloom"
  {
    Properties
    {
        _MainTex ("Base (RGB)", 2D) = "white" {}
        _Bloom ("Bloom (RGB)", 2D) = "black" {}
        _LuminanceThreshold("Luminance Threshold", Float) = 0.5
        _BlurSize("Blur Size", Float) = 1.0
    }
    SubShader
    {
        CGINCLUDE
        #include "UnityCG.cginc"
        sampler2D _MainTex;
        half4 _MainTex_TexelSize;
        sampler2D _Bloom;
        float _LuminanceThreshold;
        float _BlurSize;
  
        struct v2f
        {
            float4 pos : SV_POSITION;
            half2 uv : TEXCOORD0;
        };
  
        v2f vertExtractBright(appdata_img v)
        {
            v2f o;
            o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
            o.uv = v.texcoord;
            return o;
        }
  
        fixed luminance(fixed4 color)
        {
            return 0.2125 * color.r + 0.7154 * color.g + 0.0721 * color.b;
        }
  
        fixed4 fragExtractBright(v2f i) : SV_Target
        {
            fixed4 c = tex2D(_MainTex, i.uv);
            fixed val = clamp(luminance(c) - _LuminanceThreshold, 0.0, 1.0);
            return c * val;
        }
  
        struct v2fBloom
        {
            float4 pos : SV_POSITION;
            half4 uv : TEXCOORD0;
        };
  
        v2fBloom vertBloom(appdata_img v)
        {
            v2fBloom o;
            o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
            o.uv.xy = v.texcoord;
            o.uv.zw = v.texcoord;
            #if UNITY_UV_STARTS_AT_TOP
            if (_MainTex_TexelSize.y < 0.0)
                o.uv.w = 1.0 - o.uv.w;
            #endif
            return o;
        }
  
        fixed4 fragBloom(v2fBloom i) : SV_Target
        {
            return tex2D(_MainTex, i.uv.xy) * tex2D(_Bloom, i.uv.zw);
        }
        ENDCG
        ZTest Always Cull Off ZWrite Off
        Pass
        {
            CGPROGRAM
            #pragma vertex vertBloom
            #pragma fragment fragBloom
            ENDCG
        }
  
        UsePass "UnityShaderLearning/GaussianBlurShader/GAUSSIAN_BLUR_VERTICAL"
  
        UsePass "UnityShaderLearning/GaussianBlurShader/GAUSSIAN_BLUR_HORIZONTAL"
  
        Pass
        {
            CGPROGRAM
            #pragma vertex vertBloom
            #pragma fragment fragBloom
            ENDCG
        }
    }
    Fallback Off
  }