// 案例 22.8–22.9：网格贴花（Mesh Decal）——贴在表面上的一块真实网格，用深度偏移（Offset）避免 Z-Fighting
Shader "Cookbook/Ch22/MeshDecal"
{
    Properties
    {
        [MainTexture] _BaseMap ("Decal (RGBA)", 2D) = "white" {}
        [MainColor] _BaseColor ("Color", Color) = (1,1,1,1)
        [Toggle(_NORMALMAP)] _UseNormal ("Use Normal Map", Float) = 0
        [NoScaleOffset][Normal] _BumpMap ("Normal Map", 2D) = "bump" {}
        _BumpScale ("Normal Scale", Range(0, 2)) = 1
        _Smoothness ("Smoothness", Range(0, 1)) = 0.3
        _OffsetFactor ("Depth Offset Factor", Float) = -1
        _OffsetUnits ("Depth Offset Units", Float) = -1
        [Toggle] _DebugColor ("Debug: Flat Color", Float) = 0
    }
    SubShader
    {
        Tags { "RenderType"="Transparent" "RenderPipeline"="UniversalPipeline" "Queue"="Transparent-100" "IgnoreProjector"="True" }

        Pass
        {
            Name "MeshDecal"
            Tags { "LightMode"="UniversalForward" }
            Blend SrcAlpha OneMinusSrcAlpha
            ZWrite Off
            ZTest LEqual
            // 深度偏移：depth += factor × 斜率 + units × 最小可分辨深度单位
            // 负值 = 往相机方向拉一点，让与底面共面的贴花稳定地赢下深度测试
            Offset [_OffsetFactor], [_OffsetUnits]

            HLSLPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #pragma shader_feature_local_fragment _NORMALMAP
            #pragma multi_compile _ _MAIN_LIGHT_SHADOWS _MAIN_LIGHT_SHADOWS_CASCADE _MAIN_LIGHT_SHADOWS_SCREEN
            #pragma multi_compile _ _ADDITIONAL_LIGHTS_VERTEX _ADDITIONAL_LIGHTS
            #pragma multi_compile_fragment _ _ADDITIONAL_LIGHT_SHADOWS
            #pragma multi_compile_fragment _ _SHADOWS_SOFT _SHADOWS_SOFT_LOW _SHADOWS_SOFT_MEDIUM _SHADOWS_SOFT_HIGH
            #pragma multi_compile _ _FORWARD_PLUS
            #pragma multi_compile_instancing
            #include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Lighting.hlsl"

            CBUFFER_START(UnityPerMaterial)
                float4 _BaseMap_ST;
                half4 _BaseColor;
                half _BumpScale, _Smoothness;
                float _OffsetFactor, _OffsetUnits;
                half _DebugColor;
            CBUFFER_END
            TEXTURE2D(_BaseMap); SAMPLER(sampler_BaseMap);
            TEXTURE2D(_BumpMap); SAMPLER(sampler_BumpMap);

            struct Attributes
            {
                float4 positionOS : POSITION;
                float3 normalOS   : NORMAL;
                float4 tangentOS  : TANGENT;
                float2 uv         : TEXCOORD0;
                UNITY_VERTEX_INPUT_INSTANCE_ID
            };
            struct Varyings
            {
                float4 positionCS : SV_POSITION;
                float3 positionWS : TEXCOORD0;
                half3  normalWS   : TEXCOORD1;
                half4  tangentWS  : TEXCOORD2;
                float2 uv         : TEXCOORD3;
            };

            Varyings vert(Attributes v)
            {
                Varyings o;
                UNITY_SETUP_INSTANCE_ID(v);
                VertexPositionInputs p = GetVertexPositionInputs(v.positionOS.xyz);
                VertexNormalInputs n = GetVertexNormalInputs(v.normalOS, v.tangentOS);
                o.positionCS = p.positionCS;
                o.positionWS = p.positionWS;
                o.normalWS = n.normalWS;
                o.tangentWS = half4(n.tangentWS, v.tangentOS.w * GetOddNegativeScale());
                o.uv = TRANSFORM_TEX(v.uv, _BaseMap);
                return o;
            }

            half4 frag(Varyings i) : SV_Target
            {
                half4 decal = SAMPLE_TEXTURE2D(_BaseMap, sampler_BaseMap, i.uv) * _BaseColor;
                if (_DebugColor > 0.5) return half4(0.1, 0.8, 1.0, 0.55);   // 调试：显示生成的网格覆盖范围

                half3 n = normalize(i.normalWS);
            #if defined(_NORMALMAP)
                half3 nTS = UnpackNormalScale(SAMPLE_TEXTURE2D(_BumpMap, sampler_BumpMap, i.uv), _BumpScale);
                float3 bitangent = i.tangentWS.w * cross(i.normalWS, i.tangentWS.xyz);
                n = normalize(TransformTangentToWorld(nTS, half3x3(i.tangentWS.xyz, bitangent, i.normalWS)));
            #endif
                InputData inputData = (InputData)0;
                inputData.positionWS = i.positionWS;
                inputData.normalWS = n;
                inputData.viewDirectionWS = GetWorldSpaceNormalizeViewDir(i.positionWS);
                inputData.shadowCoord = TransformWorldToShadowCoord(i.positionWS);
                inputData.bakedGI = SampleSH(n);
                inputData.normalizedScreenSpaceUV = GetNormalizedScreenSpaceUV(i.positionCS);
                inputData.shadowMask = half4(1, 1, 1, 1);
                SurfaceData s = (SurfaceData)0;
                s.albedo = decal.rgb;
                s.smoothness = _Smoothness;
                s.occlusion = 1;
                s.alpha = decal.a;
                half3 color = UniversalFragmentPBR(inputData, s).rgb;
                return half4(color, decal.a);
            }
            ENDHLSL
        }
    }
}