Shader "Unlit/AlphaByDistance"
{
	Properties
	{
		_MainTex ("Texture", 2D) = "white" {}
		_Distance ("Distance", Range(1,3)) = 3		//start 的距离
		_Degree ("Degree", Range(0,1)) = 0.5		//start - end的值
	}
	SubShader
	{
		Tags { "RenderType"="Opaque" "Queue"="Transparent"}
		LOD 100
			Blend SrcAlpha OneMinusSrcAlpha

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

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

			//定点着色器
			struct v2f
			{
				float2 uv : TEXCOORD0;
				float4 vertex : SV_POSITION;
				float DisInCamera : TEXCOORD1;
			};

			sampler2D _MainTex;
			float4 _MainTex_ST;
			float _Distance;
			float _Degree;

			//片元着色器
			v2f vert (appdata v)
			{
				v2f o;
				o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
				o.uv = TRANSFORM_TEX(v.uv, _MainTex);
				//计算定点和Camera之间的距离
				o.DisInCamera = length(_WorldSpaceCameraPos - mul(unity_ObjectToWorld, v.vertex));
				return o;
			}
			
			//片段着色器
			fixed4 frag (v2f i) : SV_Target
			{
				// sample the texture
				fixed4 col = tex2D(_MainTex, i.uv);
				
				col.a = 1;
				//如果像素和Camera之间距离小于_Distance则给Alpha赋值
				if(i.DisInCamera < _Distance)
				{
					col.a = i.DisInCamera / _Degree - abs(_Distance - _Degree) / _Degree;
				}
				return col;
			}
			ENDCG
		}
	}
}