原理：通过Luminance宏输出_MainTex的灰度值。

Shader "Shader_tex2D" {
	Properties {
		_MainTex ("Base (RGB)", 2D) = "white" {}
	}
	SubShader {
		Tags { "RenderType" = "Opaque" }
		LOD 200

		CGPROGRAM
		//声明表面着色函数并设置光照模式为Lambert(漫反射)
		#pragma surface surf Lambert
		sampler2D _MainTex;
		struct Input {
			float2 uv_MainTex;
		};
		void surf (Input IN, inout SurfaceOutput o) {
			half4 c = tex2D(_MainTex, IN.uv_MainTex);//从纹理中采样
			c = float4(Luminance(c));//设置灰度值
			o.Albedo = c.rgb;//设置反射光颜色
			o.Alpha = c.a;
		}
		ENDCG
	}

Fallback "Diffuse"
	
} // shader