遮罩可以保护某些区域，使他们免于某些修改。例如：模型表面某些区域反光强烈一些，而某些区域弱一些，或者用于混合多张纹理

流程：通过采样得到这招纹理的纹理素值，然后使用其中某个（或某几个）通道的值来与某种表面属性进行相乘，这样，当该通道的值为0时，可以保护表面不受该属性的影响。总而言之，使用遮罩纹理可以让美术人员更加精准的控制模型表面的各种性质；

 

Shader "Custom/MaskTextureShader" {
	Properties {
		_Color ("Color", Color) = (1,1,1,1)
		_MainTex ("Albedo (RGB)", 2D) = "white" {}
		_Glossiness ("Smoothness", Range(8,255)) = 20
		_BumpMap("Normal Map",2D) = "white"{} //法线纹理
		_BumpScale("Bump Scale",Float) = 1.0 //法线纹理控制系数
		_SpecularMask("SpecularMask",2D) = "white"{} //高光反射遮罩纹理
		_SpecularScale("Specular Scale",Float) = 1.0 //控制影响系数
		_Specular("Specular",Color) = (1,1,1,1)		
	}
	SubShader {
		Pass{
			Tags { "LightMode"="ForwardBase" }

			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			#include "lighting.cginc"

			fixed4 _Color;
			sampler2D _MainTex;
			float4 _MainTex_ST;  //我们为主纹理，法线纹理，遮罩纹理定义了他们共同使用的纹理属性变量
			sampler2D _BumpMap;
			float _BumpScale;
			sampler2D _SpecularMask;
			float _SpecularScale;
			fixed4 _Specular;
			float _Glossiness;

			struct a2v {
				float4 vertex:POSITION;
				float3 normal:NORMAL;
				float4 tangent:TANGENT;
				float4 texcoord:TEXCOORD0;
			};

			struct v2f{
				float4 pos:SV_POSITION;
				float2 uv:TEXCOORD0;
				float3 lightDir:TEXCOORD1;
				float3 viewDir:TEXCOORD2;
			};

			v2f vert(a2v v)
			{
				v2f o;
				o.pos = UnityObjectToClipPos(v.vertex);
				o.uv.xy = v.texcoord.xy * _MainTex_ST.xy + _MainTex_ST.zw; //对纹理的偏移缩放
				TANGENT_SPACE_ROTATION;
				o.lightDir = mul(rotation,ObjSpaceLightDir(v.vertex)).xyz;
				o.viewDir = mul(rotation,ObjSpaceViewDir(v.vertex)).xyz;
				return o;
			}

			fixed4 frag(v2f i):SV_Target{
				fixed3 tangentLightDir = normalize(i.lightDir);
				fixed3 tangentViewDir = normalize(i.viewDir);

				fixed3 tangentNormal = UnpackNormal(tex2D(_BumpMap,i.uv));
				tangentNormal.xy *= _BumpScale;
				tangentNormal.z = sqrt(1.0 - saturate(dot(tangentNormal.xy,tangentNormal.xy)));

				fixed3 albedo = tex2D(_MainTex,i.uv).rgb * _Color.rgb;
				fixed3 ambient = UNITY_LIGHTMODEL_AMBIENT.xyz * albedo;
				
				fixed3 diffuse = _LightColor0.rgb * albedo * max(0,dot(tangentNormal,tangentViewDir));
				
				fixed3 halfDir = normalize(tangentLightDir + tangentViewDir);
				fixed specularMask = tex2D(_SpecularMask,i.uv).r * _SpecularScale;
				fixed3 Specular = _LightColor0.rgb * _Specular.rgb * pow(max(0,dot(tangentNormal,halfDir)),_Glossiness) * specularMask;
				return fixed4(ambient + diffuse + Specular,1);
			}


			ENDCG
		}				
	}
	FallBack "Diffuse"
}