unity3d shader for terrain

SplattingTerrain.shader

Shader "Custom/SplattingTerrain"
{
	Properties
	{
		_R ("R channel Image", 2D) = "white" {}
		_G ("G channel Image", 2D) = "white" {}
		_B ("B channel Image", 2D) = "white" {}
		_A ("A channel Image", 2D) = "white" {}
	}
	SubShader
	{
		Tags { "RenderType"="Opaque" }
		CGPROGRAM
		#pragma surface surf Lambert
		struct Input
		{
			float2 uv_R;
			float2 uv_G;
			float2 uv_B;
			float2 uv_A;
			float4 color:COLOR;
		};
		sampler2D _R;
		sampler2D _G;
		sampler2D _B;
		sampler2D _A;
		void surf(Input IN, inout SurfaceOutput o)
		{
			o.Albedo = tex2D(_R, IN.uv_R).rgb * IN.color.r;
			o.Albedo += tex2D(_G, IN.uv_G).rgb * IN.color.g;
			o.Albedo += tex2D(_B, IN.uv_B).rgb * IN.color.b;
			o.Albedo += tex2D(_A, IN.uv_A).rgb * IN.color.a;
			o.Alpha = 1;
		}
		ENDCG
	}
	FallBack "Diffuse"
}