Shader "Custom/OutlineUnlit"
{
    Properties
    {
        _MainColor ("Main Color", Color) = (0.1, 0.3, 0.8, 0.3)
        _OutlineColor ("Outline Color", Color) = (0.2, 0.5, 1, 1)
        _OutlineWidth ("Outline Width", Range(0, 10)) = 2.0
    }
    SubShader
    {
        Tags { "RenderType"="Transparent" "Queue"="Transparent" }
        Blend SrcAlpha OneMinusSrcAlpha
        ZWrite Off

        // Pass 1: clip-space outline (screen-pixel width, scale-independent)
        Pass
        {
            Name "OUTLINE"
            Cull Front
            ZTest Always
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #include "UnityCG.cginc"

            float _OutlineWidth;
            float4 _OutlineColor;

            struct appdata { float4 vertex : POSITION; float3 normal : NORMAL; };
            struct v2f { float4 pos : SV_POSITION; };

            v2f vert(appdata v)
            {
                v2f o;
                o.pos = UnityObjectToClipPos(v.vertex);

                // transform normal to clip space
                float3 clipNormal = mul((float3x3)UNITY_MATRIX_VP, mul((float3x3)unity_ObjectToWorld, v.normal));

                // offset in clip space by a fixed pixel width
                float2 offset = normalize(clipNormal.xy) * _OutlineWidth * 0.001;

                // aspect ratio correction
                offset.x /= _ScreenParams.x / _ScreenParams.y;

                o.pos.xy += offset * o.pos.w;
                return o;
            }

            fixed4 frag(v2f i) : SV_Target { return _OutlineColor; }
            ENDCG
        }

        // Pass 2: inner fill
        Pass
        {
            Name "FILL"
            Cull Back
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #include "UnityCG.cginc"

            float4 _MainColor;

            struct appdata { float4 vertex : POSITION; };
            struct v2f { float4 pos : SV_POSITION; };

            v2f vert(appdata v) { v2f o; o.pos = UnityObjectToClipPos(v.vertex); return o; }
            fixed4 frag(v2f i) : SV_Target { return _MainColor; }
            ENDCG
        }
    }
}