Zoom shader

gistfile1.txt

// Upgrade NOTE: replaced 'mul(UNITY_MATRIX_MVP,*)' with 'UnityObjectToClipPos(*)'

Shader "Custom/Zoom" {  
    Properties {
		_MainTex("Base (RGB)", 2D) = "white" {}
		_LWTex("LieWen (RGB)", 2D) = "white" {}
		_ForceVector("ForceVector", Vector) = (0,0,0)
		_length("len", float) = 0
    }  
    SubShader {  
        pass{  
        CGPROGRAM  
// Upgrade NOTE: excluded shader from OpenGL ES 2.0 because it does not contain a surface program or both vertex and fragment programs.
		#pragma exclude_renderers gles
		#pragma vertex vert   
		#pragma fragment frag
		#include "UnityCG.cginc"  
		sampler2D _MainTex;  
		sampler2D _LWTex;
		float4 _MainTex_ST;  
		float _length; 
		float4  _ForceVector;

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

		struct Input
		{
			float2 uv_MainTex;
			float2 uv_Bump;
		};

		v2f vert(appdata_base v)  
		{  
			v2f o;  
			o.pos = mul(UNITY_MATRIX_MVP, v.vertex );  
			o.uv = TRANSFORM_TEX(v.texcoord,_MainTex);  
			return o;  
		}

		//输入uv返回映射之后的uv,xLength为边框的大小  
		float2 mapuv(float2 uv,float xLength)  
		{  
			float2 newuv;  
			float rate = (1 - 2 * xLength ) / 1;        
			if(uv.x > xLength && uv.x < (1-xLength) && uv.y > xLength && uv.y < (1-xLength))  
			{  
				newuv.x = (uv.x-xLength)/rate;  
				newuv.y = (uv.y-xLength)/rate;  
			}  
			else  
			{  
				newuv.x=uv.x;  
				newuv.y=uv.y;  
			}  
			return newuv;  
		}

		float4 frag(v2f i):COLOR  
		{  
			float4 outp;
			float2 newuv;

			if (distance(_ForceVector.xyz, i.pos.xyz) < 0.5)
			{
				newuv = mapuv(i.uv, _length);
			}
			else
			{
				newuv = mapuv(i.uv, 0.5);
			}
			outp = tex2D(_LWTex, newuv);


			return outp;  
		}  
        ENDCG  
        }  
    }  
}  


我想要在输入的点周围0.5的范围内 显示 _LWTex的图，
这么写没有效果，求指导

两个vec不在一个几何空间里