fixed luminance(fixed4 color)
    return 0.2125 * color.r + 0.7154 * color.g + 0.0721 * color.b

fixed4 fragExractBright(v2f i) : SV_Target
{
    fixed4 c = tex2D(_MainTex,i.uv);
    fixed4 val = clamp(luminance(c) - _LuminanceThreshold , 0.0 , 1.0 );
    return c * val;
}

Shader "Unlit/Bloom"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
        //用于存储亮度纹理模糊后的结果
        _Bloom("Bloom",2D) = "" {}
        //亮度阈值 控制亮度纹理 亮度区域的
        _LuminanceThreshold("LuminanceThreshold",Float) = 0.5
        
        //模糊半径
        _BlurSpread("BlurSize",Float) = 1.0
    }
    SubShader
    {
        CGINCLUDE
        
        sampler2D _MainTex;
        half4 _MainTex_TexelSize;
        sampler2D _Bloom;
        float _LuminanceThreshold;
        float _BlurSpread;
        #include "UnityCG.cginc"
        struct v2f
            {
                float2 uv : TEXCOORD0;
                float4 vertex : SV_POSITION;
            };
        //计算颜色亮度值（灰度值）
        fixed luminance(fixed4 color)
        {
            return 0.2125 * color.r + 0.7154 * color.g + 0.0721 * color.b;
        }
        ENDCG
        //提取Pass
        Pass
        {
            //后处理标配
            ZTest Always
            Cull Off
            ZWrite Off
            
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            
            v2f vert(appdata_base v)
            {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.uv = v.texcoord;
                return o;
            }
            fixed4 frag(v2f i) :SV_Target
            {
                //采样源纹理颜色
                fixed4 color = tex2D(_MainTex,i.uv);
                //得到亮度颜色贡献值
                fixed4 value = clamp(luminance(color) - _LuminanceThreshold,0,1);
                //返回颜色 * 亮度贡献值
                return color * value;
            }   

            

            
            ENDCG
        }
        //复用高斯模糊的Pass
        UsePass "Unlit/GaussianBlur/GASSIAN_BLUR_HORIZONTAL"
        UsePass "Unlit/GaussianBlur/GASSIAN_BLUR_VERTICAL"
        //合成Pass
        Pass
        {
            //后处理标配
            ZTest Always
            Cull Off
            ZWrite Off
            CGPROGRAM
            #pragma vertex vertBloom
            #pragma fragment fragBloom

            struct v2fBloom
            {
                float4 pos:POSITION;
                //xy主要用于对主纹理采样
                //zw主要用于对亮度模糊后的纹理进行采样
                half4 uv:TEXCOORD0;
            };

            v2fBloom vertBloom(appdata_base v)
            {
                v2fBloom o;
                o.pos = UnityObjectToClipPos(v.vertex);
                //亮度纹理和主纹理 要采样相同的地方进行颜色叠加
                o.uv.xy = v.texcoord;
                o.uv.zw = v.texcoord;

                //用宏去判断uv坐标是否被翻转
                #if UNITY_UV_STARTS_AT_TOP
                //如果纹素的y小于0，表示对y需要进行翻转
                if(_MainTex_TexelSize.y < 0)
                    o.uv.w = 1 - o.uv.w;
                #endif
                return o;
            }
            fixed4 fragBloom(v2fBloom i):SV_Target
            {
                return tex2D(_MainTex,i.uv.xy) + tex2D(_Bloom,i.uv.zw);
            }
            
            ENDCG
        }
    }
    Fallback Off
}