// GPU edge padding / texture dilation against visible UV seams.
//
// Between the UV islands there is an unpainted border (gutter). During bilinear
// sampling at an island edge, the GPU mixes the painted edge with the unpainted
// base color -> bright seam lines. This pass "bleeds" the painted color up to
// _Pad texels beyond the island edges.
//
// The dilation source is exclusively painted texels (alpha > 0). The padding
// texels produced get alpha = 0 -> next frame they are NO LONGER a source, so the
// padding does not grow unbounded (it stays at _Pad texels). The threshold is > 0
// (not >= 0.5) so soft, partially transparent brush edges count as painted too.
Shader "Hidden/MeshPaintDilate"
{
    Properties
    {
        _MainTex ("Texture", 2D) = "white" {}
    }
    SubShader
    {
        Tags { "RenderType" = "Opaque" }
        Pass
        {
            ZTest Always
            ZWrite Off
            Cull Off
            Blend Off

            CGPROGRAM
            #pragma vertex vert_img
            #pragma fragment frag
            #include "UnityCG.cginc"

            sampler2D _MainTex;
            float4 _MainTex_TexelSize; // (1/w, 1/h, w, h)
            int _Pad;

            #define MAXPAD 16

            fixed4 frag (v2f_img i) : SV_Target
            {
                fixed4 c = tex2D(_MainTex, i.uv);
                if (c.a > 0.0) return c; // already painted (even partially) -> leave unchanged

                float2 ts = _MainTex_TexelSize.xy;
                float best = 1e20;
                fixed3 bestCol = c.rgb;
                bool found = false;

                // Search the neighborhood (up to _Pad texels) for the nearest painted texel.
                [loop] for (int y = -MAXPAD; y <= MAXPAD; y++)
                {
                    if (abs(y) > _Pad) continue;
                    [loop] for (int x = -MAXPAD; x <= MAXPAD; x++)
                    {
                        if (abs(x) > _Pad) continue;
                        fixed4 s = tex2D(_MainTex, i.uv + float2(x, y) * ts);
                        if (s.a > 0.0)
                        {
                            float dd = (float)(x * x + y * y);
                            if (dd < best) { best = dd; bestCol = s.rgb; found = true; }
                        }
                    }
                }

                // Gutter: color of the nearest painted texel, but alpha = 0 (not a source).
                if (found) return fixed4(bestCol, 0.0);
                return c; // out of range -> base color unchanged
            }
            ENDCG
        }
    }
    Fallback Off
}