Shader "Custom/Grid"
{
    Properties
    {
        _LineColor("Line Color", Color) = (1,1,1,1)
        _BackgroundColor("Background Color", Color) = (0,0,0,0)
        _GridScale("Grid Scale", Float) = 1
        _LineWidth("Line Width", Float) = 0.05
        _Fade("Fade", Float) = 0.1
        _Plane("Plane (0: XZ, 1: XY, 2: YZ)", Float) = 0
        _DisappearStartDistance("Disappear Start Distance", Float) = 50
        _DisappearEndDistance("Disappear End Distance", Float) = 100
    }
    SubShader
    {
        Tags { "RenderType"="Transparent" "Queue"="Transparent" }
        LOD 100

        Pass
        {
            Name "GridPass"
            Cull Off
            Blend SrcAlpha OneMinusSrcAlpha
            ZWrite Off

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

            // 内置变量：摄像机在世界空间中的位置
            //float3 _WorldSpaceCameraPos;

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

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

            CBUFFER_START(UnityPerMaterial)
            float4 _LineColor;
            float4 _BackgroundColor;
            float _GridScale;
            float _LineWidth;
            float _Fade;
            float _Plane;
            float _DisappearStartDistance;
            float _DisappearEndDistance;
            CBUFFER_END

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

            // 根据 _Plane 选择二维坐标
            float2 GetGridCoordinates(float3 pos)
            {
                if (_Plane < 0.5)      // XZ 平面
                    return pos.xz;
                else if (_Plane < 1.5) // XY 平面
                    return pos.xy;
                else                   // YZ 平面
                    return pos.yz;
            }

            // 根据 _Plane 获取摄像机在网格平面上的位置
            float2 GetCameraPlanePosition()
            {
                if (_Plane < 0.5)      // XZ 平面：摄像机投影到 XZ
                    return _WorldSpaceCameraPos.xz;
                else if (_Plane < 1.5) // XY 平面：摄像机投影到 XY
                    return _WorldSpaceCameraPos.xy;
                else                   // YZ 平面：摄像机投影到 YZ
                    return _WorldSpaceCameraPos.yz;
            }

            half4 frag(Varyings IN) : SV_Target
            {
                // 计算二维网格坐标并乘以 _GridScale
                float2 gridCoord = GetGridCoordinates(IN.worldPos) * _GridScale;
                // 用 frac 与 fwidth 实现平滑网格线（抗锯齿）
                float2 grid = abs(frac(gridCoord - 0.5) - 0.5) / fwidth(gridCoord);
                float lineIntensity = 1.0 - smoothstep(0.0, _LineWidth, min(grid.x, grid.y));
                
                // 计算摄像机在平面上的位置与当前片元的二维距离
                float2 camPos2D = GetCameraPlanePosition();
                float2 fragPos2D = GetGridCoordinates(IN.worldPos);
                float dist = distance(fragPos2D, camPos2D);
                
                // 超过 100 米开始淡出，到 200 米完全透明
                float distanceFade = saturate((_DisappearEndDistance - dist) / _DisappearStartDistance);
                
                // 最终 alpha 是网格线强度乘以距离淡出因子
                float finalAlpha = lineIntensity * distanceFade;
                
                // 背景颜色始终透明，所以直接输出网格线颜色与 alpha
                return float4(_LineColor.rgb, finalAlpha);
            }
            ENDHLSL
        }
    }
    FallBack "Diffuse"
}