Shader "Custom/GradientBackground"
{
    Properties
    {
        _MainTex ("Base (RGB)", 2D) = "white" { }
        _GradientStart ("Start Color", Color) = (1, 1, 1, 1)
        _GradientEnd ("End Color", Color) = (0, 0, 0, 0)
    }
    SubShader
    {
        Tags { "RenderType"="Opaque" }
        Pass
        {
            CGPROGRAM
            #pragma vertex vert
            #pragma fragment frag
            #include "UnityCG.cginc"

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

            struct v2f
            {
                float4 pos : POSITION;
                float2 uv : TEXCOORD0;
            };

            // 材质属性
            sampler2D _MainTex;
            float4 _GradientStart;
            float4 _GradientEnd;
            
            v2f vert(appdata v)
            {
                v2f o;
                o.pos = UnityObjectToClipPos(v.vertex);
                o.uv = v.uv;
                return o;
            }

            half4 frag(v2f i) : SV_Target
            {
                // 计算从左到右的透明度渐变
                float alpha = lerp(_GradientStart.a, _GradientEnd.a, i.uv.x);

                // 获取纹理颜色
                half4 texColor = tex2D(_MainTex, i.uv);
                
                // 应用透明度渐变到纹理颜色
                texColor.a *= alpha;

                return texColor;
            }
            ENDCG
        }
    }
    FallBack "Diffuse"
}