算法：
1 设置淡入的起始距离 和 结束距离
2 在vertex阶段计算物体顶点与相机的距离
3 根据距离计算alpha

源码如下：

Shader "Custom/Fade" {
	Properties{
		_MainTex("Texture", 2D) = "white" { }
		_FadeDistanceNear("Near fadeout dist (View Space)", float) = 35
		_FadeDistanceFar("Far fadeout dist (View Space)", float) = 40
	}
		SubShader
		{
			Tags { "Queue" = "Transparent" "RenderType" = "Transparent"}
			ZWrite On
			Blend SrcAlpha OneMinusSrcAlpha
			pass
			{
				CGPROGRAM
				#pragma vertex vert
				#pragma fragment frag
				#include "UnityCG.cginc"
				sampler2D _MainTex;
				float4 _MainTex_ST;
				struct v2f {
					float4  pos : SV_POSITION;
					float2  uv : TEXCOORD0;
					float fade : TEXCOORD1;
				};
				float _FadeDistanceNear;
				float _FadeDistanceFar;
				v2f vert(appdata_base v)
				{
					v2f o;
					o.pos = UnityObjectToClipPos(v.vertex);
					o.uv = TRANSFORM_TEX(v.texcoord,_MainTex);
					//相机坐标系的物体坐标
					float3 posView = mul(UNITY_MATRIX_MV,v.vertex).xyz;
					//计算与相机距离
					float dis = length(posView);
					//计算fade
					o.fade = 1 - saturate((dis - _FadeDistanceNear) / (_FadeDistanceFar - _FadeDistanceNear));
					return o;
				}
				float4 frag(v2f i) : COLOR
				{
					float4 texCol = tex2D(_MainTex,i.uv);
					float4 outp = texCol;
					//fade作为alpha
					return float4(outp.rgb,i.fade);
				}
				ENDCG
			}
		}
}