Shader "Custom/SimpleGrid"
{
    Properties
    {
        _MainColor ("Sub Grid Color", Color) = (0.5, 0.5, 0.5, 0.2)
        _AxisColor ("Major Grid Color", Color) = (0.8, 0.8, 0.8, 0.5)
        _GridSpacing ("Grid Spacing", Float) = 1.0
        _LineWidth ("Line Width", Range(0.001, 0.1)) = 0.02
        _FadeDist ("Fade Distance", Float) = 100.0
    }
    SubShader
    {
        Tags { "RenderType"="Transparent" "Queue"="Transparent-1" } // -1 保证在普通物体后面
        Blend SrcAlpha OneMinusSrcAlpha
        ZWrite Off
        Cull Off

        Pass
        {
            HLSLPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Core.hlsl"

            struct Attributes { float4 vertex : POSITION; };
            struct Varyings { float4 position : SV_POSITION; float3 worldPos : TEXCOORD0; };

            CBUFFER_START(UnityPerMaterial)
                float4 _MainColor;
                float4 _AxisColor;
                float _GridSpacing;
                float _LineWidth;
                float _FadeDist;
            CBUFFER_END

            Varyings vert(Attributes IN)
            {
                Varyings OUT;
                OUT.position = TransformObjectToHClip(IN.vertex);
                OUT.worldPos = TransformObjectToWorld(IN.vertex).xyz;
                return OUT;
            }

            // 优化的网格计算
            float gridAlpha(float2 worldPos, float spacing, float width)
            {
                float2 coord = worldPos / spacing;
                // fwidth 用于保持线条在屏幕上的像素宽度一致
                float2 grid = abs(frac(coord - 0.5) - 0.5) / fwidth(coord);
                float lineVal = min(grid.x, grid.y);
                // 1.0 - smoothstep 提供抗锯齿
                return 1.0 - smoothstep(width, width + 1.0, lineVal);
            }

            half4 frag(Varyings IN) : SV_Target
            {
                // 距离裁剪：太远直接 discard，节省后续计算
                float dist = distance(IN.worldPos, _WorldSpaceCameraPos);
                if (dist > _FadeDist) discard;
                
                float alphaFade = 1.0 - (dist / _FadeDist);
                // 二次衰减让边缘更柔和
                alphaFade *= alphaFade; 

                float2 pos = IN.worldPos.xz;

                // 计算两层网格
                float baseGrid = gridAlpha(pos, _GridSpacing, _LineWidth);
                float largeGrid = gridAlpha(pos, _GridSpacing * 10.0, _LineWidth); // 10倍格

                // 混合
                float4 finalColor = _MainColor;
                finalColor.a *= baseGrid;

                // 叠加高亮网格
                // 使用 max 避免 alpha 混合错误
                finalColor = lerp(finalColor, _AxisColor, largeGrid);
                finalColor.a = max(finalColor.a, _AxisColor.a * largeGrid);
                
                finalColor.a *= alphaFade;
                
                // 极低 Alpha 剔除
                if (finalColor.a < 0.01) discard;

                return finalColor;
            }
            ENDHLSL
        }
    }
}