Shader "TNTC/ExtendIslands"{
    Properties{
        _MainTex ("Texture", 2D) = "white" {}
        _UVIslands ("Texture UVIsalnds", 2D) = "white" {}
        _OffsetUV ("UVOffset", float) = 1
    }

    SubShader{
        Tags { "RenderType"="Opaque" }
        LOD 100

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

            #include "UnityCG.cginc"

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

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

            sampler2D _MainTex;
            float4 _MainTex_ST;
            float4 _MainTex_TexelSize;
            float _OffsetUV;
            sampler2D _UVIslands;

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

            fixed4 frag (v2f i) : SV_Target{
                float2 offsets[8] = {float2(-_OffsetUV, 0), float2(_OffsetUV, 0), float2(0, _OffsetUV), float2(0, -_OffsetUV), float2(-_OffsetUV, _OffsetUV), float2(_OffsetUV, _OffsetUV), float2(_OffsetUV, -_OffsetUV), float2(-_OffsetUV, -_OffsetUV)};
				float2 uv = i.uv;
				float4 color = tex2D(_MainTex, uv);
				float4 island = tex2D(_UVIslands, uv);

                if(island.z < 1){
                    float4 extendedColor = color;
                    for	(int i = 0; i < offsets.Length; i++){
                        float2 currentUV = uv + offsets[i] * _MainTex_TexelSize.xy;
                        float4 offsettedColor = tex2D(_MainTex, currentUV);
                        extendedColor = max(offsettedColor, extendedColor);
                    }
                    color = extendedColor;
                }
				return color;
            }
            ENDCG
        }
    }
}