unity3d:UI shader区域mask透明

Shader "UI/MaskAreaAlpha" {
	Properties {
		_MainTex("Texture", 2D) = "white" {}
		_Mask("遮罩", 2D) = "white" {}
		_Area("区域",Int) = 0
		
		_Color("Tint", Color) = (1,1,1,1)
		_StencilComp("Stencil Comparison", Float) = 8
		_Stencil("Stencil ID", Float) = 0
		_StencilOp("Stencil Operation", Float) = 0
		_StencilWriteMask("Stencil Write Mask", Float) = 255
		_StencilReadMask("Stencil Read Mask", Float) = 255
		_ColorMask("Color Mask", Float) = 15
		[Toggle(UNITY_UI_ALPHACLIP)] _UseUIAlphaClip("Use Alpha Clip", Float) = 0

	}
	SubShader {
		Tags
			{

			"Queue" = "Transparent"
			"IgnoreProjector" = "True"
			"RenderType" = "Transparent"
			"PreviewType" = "Plane"
			"CanUseSpriteAtlas" = "True"
			}

			Stencil
			{
				Ref[_Stencil]
				Comp[_StencilComp]
				Pass[_StencilOp]
				ReadMask[_StencilReadMask]
				WriteMask[_StencilWriteMask]
			}
			ColorMask[_ColorMask]

			Cull Off
			Lighting Off
			ZWrite Off
			Blend One OneMinusSrcAlpha

			Pass
			{
				CGPROGRAM
				#pragma vertex vert
				#pragma fragment frag

				#include "UnityCG.cginc"
				#include "UnityUI.cginc"
				#pragma multi_compile __ UNITY_UI_ALPHACLIP

				sampler2D _MainTex;
				sampler2D _Mask;
				int _Area; //区域值
				float4 _ClipRect;

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

				struct v2f
				{
					float2 uv : TEXCOORD0;
					float4 color : COLOR;
					float4 vertex : SV_POSITION;
					float4 worldPosition : TEXCOORD1;
				};

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


				int SetAlpha(fixed4 mask)
				{
					int t = (int)round(mask.r*255.0) << 16 | (int)round(mask.g*255.0) << 8 | (int)round(mask.b*255.0);
					int isAplha = step(1, _Area & t);
					return isAplha;
				}
				
				fixed4 frag(v2f i) : SV_Target
				{
					half4 col = tex2D(_MainTex, i.uv) * i.color;
					half4 mask = tex2D(_Mask,i.uv);
					int isAplha = SetAlpha(mask);
					col.a = col.a  * isAplha;
					
					col.a *= UnityGet2DClipping(i.worldPosition.xy, _ClipRect);  
#ifdef UNITY_UI_ALPHACLIP
					clip(col.a - 0.001);
#endif

					return col;
				}
				ENDCG
			}
	}
}