// 石头人砸地 · 扩散冲击波圆环 Shader
// 用法：挂在一个平躺的 Quad 上，由 C# 脚本驱动 _Progress 从 0→1
Shader "Custom/ShockwaveRing"
{
    Properties
    {
        _RingColor ("圆环颜色", Color) = (0.8, 0.75, 0.6, 1.0)
        _Progress ("扩散进度 (0~1)", Range(0, 1)) = 0
        _RingWidth ("圆环粗细", Range(0.02, 0.3)) = 0.08
        _Softness ("边缘柔化", Range(0.001, 0.1)) = 0.02
    }
    SubShader
    {
        Tags { "RenderType"="Transparent" "Queue"="Transparent" "IgnoreProjector"="True" }
        LOD 100
        ZWrite Off
        Blend SrcAlpha OneMinusSrcAlpha

        Pass
        {
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #include "UnityCG.cginc"

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

            struct v2f
            {
                float2 uv : TEXCOORD0;
                float4 vertex : SV_POSITION;
            };

            float4 _RingColor;
            float _Progress;
            float _RingWidth;
            float _Softness;

            v2f vert (appdata v)
            {
                v2f o;
                o.vertex = UnityObjectToClipPos(v.vertex);
                o.uv = v.uv;
                return o;
            }

            fixed4 frag (v2f i) : SV_Target
            {
                // 以 UV 中心(0.5,0.5)为圆心
                float2 p = i.uv - 0.5;
                float dist = length(p);

                // 当前圆环半径：从中心(0)扩散到边缘(0.5)
                float radius = _Progress * 0.5;
                float inner = radius - _RingWidth;

                // 只保留 [inner, radius] 这一圈，边缘柔化
                float ring = smoothstep(inner - _Softness, inner, dist)
                           * (1.0 - smoothstep(radius - _Softness, radius, dist));

                // 越扩散越淡
                float alpha = ring * (1.0 - _Progress) * _RingColor.a;

                return float4(_RingColor.rgb, alpha);
            }
            ENDCG
        }
    }
}