Shader "Custom/SphereGradientShader"
{
    Properties
    {
        [HDR] _BaseColor("Base Color", Color) = (1,1,1,1)
        [HDR] _HeadColor("Head Color (Facing End)", Color) = (1,1,1,1)
        [HDR] _TailColor("Tail Color (Facing Start)", Color) = (1,1,1,1)
        [Toggle(_EMISSION_ON)] _UseEmission("Enable Emission", Float) = 0
        [HDR] _EmissionColor("Emission Color", Color) = (0,0,0,0)
        _EmissionIntensity("Emission Intensity", Range(0, 10)) = 1.0
        _MoveDir("Movement Direction", Vector) = (0,0,1,0)
        _GradientScale("Gradient Scale", Range(0, 2)) = 1.0
        _GradientPower("Gradient Power", Range(0.5, 5)) = 2.0
        
        [Enum(UnityEngine.Rendering.BlendMode)] _SrcBlend("Src Blend", Float) = 1
        [Enum(UnityEngine.Rendering.BlendMode)] _DstBlend("Dst Blend", Float) = 0
        [Enum(Off, 0, On, 1)] _ZWrite("ZWrite", Float) = 1
    }
    
    SubShader
    {
        Tags { "RenderType" = "Opaque" "RenderPipeline" = "UniversalPipeline" "Queue" = "Geometry" }
        LOD 100

        Pass
        {
            Blend [_SrcBlend] [_DstBlend]
            ZWrite [_ZWrite]
            
            HLSLPROGRAM
            #pragma shader_feature _EMISSION_ON
            #pragma vertex vert
            #pragma fragment frag
            #include "Packages/com.unity.render-pipelines.universal/ShaderLibrary/Core.hlsl"

            struct Attributes
            {
                float4 positionOS : POSITION;
                float2 uv : TEXCOORD0;
                float3 normalOS : NORMAL;
            };

            struct Varyings
            {
                float4 positionCS : SV_POSITION;
                float2 uv : TEXCOORD0;
                float3 normalWS : TEXCOORD1;
                float3 positionWS : TEXCOORD2;
            };

            CBUFFER_START(UnityPerMaterial)
                float4 _BaseColor;
                float4 _HeadColor;
                float4 _TailColor;
                float4 _EmissionColor;
                float _EmissionIntensity;
                float4 _MoveDir;
                float _GradientScale;
                float _GradientPower;
            CBUFFER_END

            Varyings vert(Attributes input)
            {
                Varyings output;
                output.positionWS = TransformObjectToWorld(input.positionOS.xyz);
                output.positionCS = TransformWorldToHClip(output.positionWS);
                output.uv = input.uv;
                output.normalWS = TransformObjectToWorldNormal(input.normalOS);
                return output;
            }

            half4 frag(Varyings input) : SV_Target
            {
                float3 normal = normalize(input.normalWS);
                float3 moveDir = normalize(_MoveDir.xyz);
                
                // 计算空间渐变：-1 到 1 映射到 0 到 1
                // 面向 endPoint (moveDir) 时 dot 接近 1，使用 HeadColor
                // 面向 startPoint (-moveDir) 时 dot 接近 -1，使用 TailColor
                // 反转：使用 -dot 或者 1.0 - spatialLerp
                float spatialLerp = dot(normal, -moveDir) * 0.5 + 0.5;
                
                // 应用幂函数调整渐变分布
                spatialLerp = clamp(spatialLerp, 0.0, 1.0);
                
                half4 color = lerp(_TailColor, _HeadColor, spatialLerp);

                // 叠加一个基础的菲涅尔效果增加立体感
                float3 viewDir = normalize(GetWorldSpaceViewDir(input.positionWS));
                float fresnel = 1.0 - saturate(dot(normal, viewDir));
                fresnel = pow(fresnel, _GradientPower) * _GradientScale;
                
                color.rgb += fresnel * color.rgb * 0.5; // 边缘轻微提亮
                
                // 自发光叠加
                #if defined(_EMISSION_ON)
                color.rgb += _EmissionColor.rgb * _EmissionIntensity;
                #endif

                return color;
            }
            ENDHLSL
        }
    }
}