// 第 4 步：透明度裁剪（Alpha Clip / Alpha Test）。alpha 低于阈值的像素直接丢弃。
Shader "Cookbook/Ch03/AlphaClip"
{
    Properties
    {
        [MainTexture] _BaseMap ("Base Map", 2D) = "white" {}
        [MainColor]   _BaseColor ("Base Color", Color) = (1, 1, 1, 1)
        _Cutoff ("Alpha Cutoff", Range(0, 1)) = 0.5
        // 剪掉的地方能看到背面，所以通常关掉背面剔除：0=Off 1=Front 2=Back
        [Enum(UnityEngine.Rendering.CullMode)] _Cull ("Cull", Float) = 0
    }

    SubShader
    {
        // AlphaTest 队列（2450）排在普通不透明物体之后，RenderType 让替换着色/工具识别它
        Tags { "RenderType"="TransparentCutout" "RenderPipeline"="UniversalPipeline" "Queue"="AlphaTest" }

        Pass
        {
            Name "Unlit"
            Tags { "LightMode"="UniversalForward" }
            Cull [_Cull]                         // 渲染状态可以引用属性，用方括号

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

            struct Attributes
            {
                float4 positionOS : POSITION;
                float2 uv         : TEXCOORD0;
                half4  color      : COLOR;
            };

            struct Varyings
            {
                float4 positionCS : SV_POSITION;
                float2 uv         : TEXCOORD0;
                half4  color      : COLOR;
            };

            TEXTURE2D(_BaseMap);
            SAMPLER(sampler_BaseMap);
            float4 _BaseMap_ST;
            half4  _BaseColor;
            half   _Cutoff;

            Varyings vert (Attributes input)
            {
                Varyings output;
                output.positionCS = TransformObjectToHClip(input.positionOS.xyz);
                output.uv = TRANSFORM_TEX(input.uv, _BaseMap);
                output.color = input.color;
                return output;
            }

            half4 frag (Varyings input) : SV_Target
            {
                half4 col = SAMPLE_TEXTURE2D(_BaseMap, sampler_BaseMap, input.uv) * _BaseColor * input.color;
                // clip(x)：x < 0 时丢弃当前片元，不写颜色也不写深度
                clip(col.a - _Cutoff);
                return half4(col.rgb, 1);        // 不透明物体输出 alpha=1
            }
            ENDHLSL
        }
    }
}