Shader "Custom/FresnelOverlay_Add1"
{
    Properties
    {
        [HDR]_RimColor   ("Fresnel Color (HDR)", Color) = (3,3,3,1)
        _RimPower        ("Fresnel Power", Range(0.1, 8)) = 3.0
        _RimStrength     ("Fresnel Strength", Range(0, 10)) = 1.0
        _DoubleSided     ("Double-Sided (0=Back,1=Off,2=Front)", Range(0,2)) = 2
        _DepthBias       ("Depth Bias (to avoid misses)", Float) = 0.0
    }

    SubShader
    {
        Tags { "Queue"="Transparent+10" "RenderType"="Transparent" }

        // Additive overlay on top of what’s already rendered
        Blend One One
        ZWrite Off
        ZTest Equal          // draw only where depth already matches (on the mesh)
        Cull Back            // default: draw front faces only
        ColorMask RGB

        Pass
        {
            CGPROGRAM
            #pragma vertex   vert
            #pragma fragment frag
            #pragma target 3.0
            #pragma multi_compile_instancing

            #include "UnityCG.cginc"

            float4 _RimColor;
            float  _RimPower;
            float  _RimStrength;
            float  _DoubleSided;  // 0=Back, 1=Off (both), 2=Front
            float  _DepthBias;

            struct appdata
            {
                float4 vertex : POSITION;
                float3 normal : NORMAL;
                UNITY_VERTEX_INPUT_INSTANCE_ID
            };

            struct v2f
            {
                float4 pos      : SV_POSITION;
                float3 worldPos : TEXCOORD0;
                float3 worldNrm : TEXCOORD1;
                UNITY_VERTEX_INPUT_INSTANCE_ID
                UNITY_VERTEX_OUTPUT_STEREO
            };

            v2f vert (appdata v)
            {
                v2f o;
                UNITY_SETUP_INSTANCE_ID(v);
                UNITY_INITIALIZE_OUTPUT(v2f, o);
                UNITY_INITIALIZE_VERTEX_OUTPUT_STEREO(o);

                float3 worldPos = mul(unity_ObjectToWorld, v.vertex).xyz;
                float3 worldNrm = UnityObjectToWorldNormal(v.normal);

                // Optional tiny depth bias to make ZTest Equal more forgiving
                float4 clip = UnityObjectToClipPos(v.vertex);
                clip.z += _DepthBias * clip.w;

                o.pos      = clip;
                o.worldPos = worldPos;
                o.worldNrm = worldNrm;
                return o;
            }

            fixed4 frag (v2f i) : SV_Target
            {
                // Handle sidedness quickly
                float3 N = normalize(i.worldNrm);
                float3 V = normalize(_WorldSpaceCameraPos - i.worldPos);

                // Which faces to keep:
                //  Front: dot(N,V) > 0
                //  Back:  dot(N,V) < 0
                //  Both:  no discard
                float ndotv = dot(N, V);
                if (_DoubleSided < 0.5)        // Back faces only
                {
                    if (ndotv > 0) discard;
                }
                else if (_DoubleSided > 1.5)   // Front faces only
                {
                    if (ndotv < 0) discard;
                }
                // else: both sides

                // Fresnel
                ndotv = saturate(abs(ndotv));  // abs for double-sided consistency
                float rim = pow(1.0 - ndotv, _RimPower);

                float3 color = _RimColor.rgb * (rim * _RimStrength);
                return float4(color, 1);
            }
            ENDCG
        }
    }

    Fallback Off
}