Shader "Unlit/LightArea"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
        _Alpha ("Alpha", Range(0, 1)) = 1
    }
    SubShader
    {
        Tags { 
            "RenderType" = "Transparent"
            "RenderPipeline" = "UniversalPipeline"
        }
        LOD 100

        Pass
        {
            Name "LightArea"
            
            Tags
            {
                "LightMode" = "LightArea"
            }
            
            // 开启混合
            Blend SrcAlpha One
            ZWrite Off
            ZTest Always
            Cull Off
            
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag

            // make gpu instancing work
            #pragma multi_compile_instancing

            #include "UnityCG.cginc"

            struct appdata
            {
                float4 vertex : POSITION;
                float2 uv : TEXCOORD0;
                // gpu instance id
                UNITY_VERTEX_INPUT_INSTANCE_ID
            };

            struct v2f
            {
                float2 uv : TEXCOORD0;
                float4 vertex : SV_POSITION;
                float passAlpha : COLOR1;
            };

            sampler2D _MainTex;
            float4 _MainTex_ST;

            // gpu instance 数据
            UNITY_INSTANCING_BUFFER_START(props)
                // alpha
                UNITY_DEFINE_INSTANCED_PROP(float, _Alpha)
            UNITY_INSTANCING_BUFFER_END(props)
            
            v2f vert (appdata v)
            {
                v2f o;
                // gpu instance id
                UNITY_SETUP_INSTANCE_ID(v);
                
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.uv = TRANSFORM_TEX(v.uv, _MainTex);
                float instAlpha = UNITY_ACCESS_INSTANCED_PROP(props, _Alpha);
                o.passAlpha = instAlpha;
                return o;
            }

            fixed4 frag (v2f i) : SV_Target
            {
                // sample the texture
                fixed4 col = tex2D(_MainTex, i.uv);
                col *= i.passAlpha;
                return col;
            }
            ENDCG
        }
    }
}