Shader "UI/OutlightForImage"
{
    Properties
    {
        _MainTex("Texture", 2D) = "white" {}
        _LightColor("Light Color", Color) = (1,1,1,1)
        _Size("Glow Size", Float) = 1.0
    }

    SubShader
    {
        Tags { "Queue"="Transparent" "IgnoreProjector"="True" "RenderType"="Transparent" }
        LOD 100

        // UI-friendly settings
        Cull Off
        ZWrite Off
        ZTest Always
        Blend SrcAlpha OneMinusSrcAlpha

        Pass
        {
            CGPROGRAM
            #include "UnityCG.cginc"
            #pragma vertex vert
            #pragma fragment frag
            #pragma target 2.0

            struct appdata_t {
                float4 vertex : POSITION;
                float2 texcoord : TEXCOORD0;
            };

            struct v2f {
                float4 pos : SV_POSITION;
                float2 uv : TEXCOORD0;
            };

            sampler2D _MainTex;
            float4 _MainTex_TexelSize;
            float4 _LightColor;
            float _Size;

            v2f vert(appdata_t v)
            {
                v2f o;
                o.pos = UnityObjectToClipPos(v.vertex);
                o.uv = v.texcoord;
                return o;
            }

            fixed4 frag(v2f i) : SV_Target
            {
                fixed4 color = tex2D(_MainTex, i.uv);
                fixed4 glow = _LightColor;
                float alphaSum = color.a;

                for (float j = 1.0; j <= _Size; j += 1.0)
                {
                    alphaSum += tex2D(_MainTex, i.uv + _MainTex_TexelSize.xy * float2(j, 0)).a;
                    alphaSum += tex2D(_MainTex, i.uv + _MainTex_TexelSize.xy * float2(-j, 0)).a;
                    alphaSum += tex2D(_MainTex, i.uv + _MainTex_TexelSize.xy * float2(0, j)).a;
                    alphaSum += tex2D(_MainTex, i.uv + _MainTex_TexelSize.xy * float2(0, -j)).a;
                }

                glow.a = alphaSum / (4.0 * _Size + 1.0);

                // 如果原图有像素就显示原图，否则显示 glow
                return step(0.1, color.a) * color + step(0.1, 1.0 - color.a) * glow;
            }
            ENDCG
        }
    }
}