Shader "Custom/Outline"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
        _OutlineColor ("Outline Color", Color) = (0,0,0,1)
        _OutlineThickness ("Outline Thickness", Range(0.0, 0.03)) = 0.01
    }
    SubShader
    {
        Tags
        {
            "Queue" = "Overlay"
        }
        Pass
        {
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #include "UnityCG.cginc"

            struct appdata
            {
                float4 vertex : POSITION;
                float2 uv : TEXCOORD0;
            };

            struct v2f
            {
                float2 uv : TEXCOORD0;
                float4 vertex : SV_POSITION;
                float4 posWorld : TEXCOORD1;
            };

            sampler2D _MainTex;
            float4 _MainTex_ST;
            float4 _OutlineColor;
            float _OutlineThickness;

            v2f vert(appdata v)
            {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.uv = TRANSFORM_TEX(v.uv, _MainTex);
                o.posWorld = mul(unity_ObjectToWorld, v.vertex);
                return o;
            }

            float4 frag(v2f i) : SV_Target
            {
                float4 texCol = tex2D(_MainTex, i.uv);
                float2 pixelSize = fwidth(i.uv) * _OutlineThickness;
                float2 uvOffsets[8] = {
                    float2(1, 0), float2(-1, 0),
                    float2(0, 1), float2(0, -1),
                    float2(1, 1), float2(-1, -1),
                    float2(1, -1), float2(-1, 1)
                };
                float outline = 0.0;
                for (int j = 0; j < 8; j++)
                {
                    outline += 1.0 - tex2D(_MainTex, i.uv + uvOffsets[j] * pixelSize).a;
                }
                outline = clamp(outline, 0.0, 1.0);
                float4 outlineColor = _OutlineColor * outline;
                return texCol + outlineColor * (1 - texCol.a);
            }
            ENDCG
        }
    }
}