OutlineInvertedHull.shader

Shader "Unlit/Outline"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}

        _OutlineColor("Outline Color", Color) = (0,0,0,1)
        _OutlineThickness("Outline Thickness", Range(0,1)) = 0.03
    }
    SubShader
    {
        Tags { "RenderType"="Opaque" }
        LOD 100
        
        // Regular pass
        Pass
        {
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            // make fog work
            #pragma multi_compile_fog

            #include "UnityCG.cginc"

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

            struct v2f
            {
                float2 uv : TEXCOORD0;
                UNITY_FOG_COORDS(1)
                float4 vertex : SV_POSITION;
            };

            sampler2D _MainTex;
            float4 _MainTex_ST;

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

            fixed4 frag (v2f i) : SV_Target
            {
                // sample the texture
                fixed4 col = tex2D(_MainTex, i.uv);
                // apply fog
                UNITY_APPLY_FOG(i.fogCoord, col);
                return col;
            }
            ENDCG
        }
        
        // Outline pass using inverted hull trick
        Pass
        {
            Cull front // Only show back faces

            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag

            #include "UnityCG.cginc"

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

            struct v2f
            {
                float4 position : SV_POSITION;
            };

            fixed4 _OutlineColor;
            float _OutlineThickness;

            v2f vert (appdata v)
            {
                v2f o;
                float3 n = normalize(v.normal);
                float3 p = v.vertex + n * _OutlineThickness; // Extrude mesh along local surface normal
                o.position = UnityObjectToClipPos(p);
                return o;
            }

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