Unity Outline shader for transparent object

Outline.shader

Shader "Screw/Alpha Outline" {
	Properties {
		_Color ("Main Color", Color) = (.5,.5,.5,1)
		_OutlineColor ("Outline Color", Color) = (0,0,0,1)
		_Outline ("Outline width", Range (0.0, 0.15)) = .005
		_OutlineOffset ("Outline Offset", Vector) = (0, 0, 0)
		_MainTex ("Base (RGB)", 2D) = "white" { }
		_Alpha ("Alpha", Float) = 1
	}
 
	CGINCLUDE
		#include "UnityCG.cginc"
		 
		struct appdata {
			half4 vertex : POSITION;
			half3 normal : NORMAL;
			half2 texcoord : TEXCOORD0;
		};
		 
		struct v2f {
			half4 pos : POSITION;
			half2 uv : TEXCOORD0;
			half3 normalDir : NORMAL;
		};
		 
		uniform half4 _Color;
		uniform half _Outline;
		uniform half4 _OutlineColor;
		 
	ENDCG
 
	SubShader {
		Tags { "Queue" = "Transparent" }
 
 		Pass {
			Name "STENCIL"
			ZWrite Off
			ZTest Always
			ColorMask 0

			Stencil {
                Ref 2
                Comp always
                Pass replace
                ZFail decrWrap
            }
			
			CGPROGRAM

			#pragma vertex vert2
			#pragma fragment frag
						
			v2f vert2 (appdata v)
			{
				v2f o;
				o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
				
				return o;
			}
					
			half4 frag (v2f i) : COLOR
			{
				return _Color;
			}
			
			ENDCG


		}
 		
		Pass {
			Name "OUTLINE"
			Tags { "LightMode" = "Always" }
			Cull Off
			ZWrite Off
			ColorMask RGB
 
			Blend SrcAlpha OneMinusSrcAlpha
			
			Stencil {
                Ref 2
                Comp NotEqual
                Pass replace
                ZFail decrWrap
            }

			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			
			half3 _OutlineOffset;
			
			v2f vert(appdata v) {
				v2f o;
				
				half3 vertex = v.vertex.xyz;
				vertex -= _OutlineOffset;
				vertex.x *= _Outline+1;
				vertex.y *= _Outline+1;
				vertex.z *= _Outline+1;	
				vertex += _OutlineOffset;
				o.pos = mul(UNITY_MATRIX_MVP, half4(vertex, v.vertex.w));
			 
				return o;
			}
			
			half _Alpha;
			half4 frag(v2f i) :COLOR {
				return half4(_OutlineColor.rgb, _Alpha);
			}
			ENDCG
		}
 
		Pass {
			Name "BASE"
			ZWrite On
			ZTest LEqual
			Blend SrcAlpha OneMinusSrcAlpha

			CGPROGRAM

			#pragma vertex vert2
			#pragma fragment frag
						
			v2f vert2 (appdata v)
			{
				v2f o;
				o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
				o.uv = v.texcoord;
				o.normalDir = normalize(mul(half4(v.normal, 0), _World2Object).xyz);
				
				return o;
			}
			
			uniform sampler2D _MainTex;
			uniform half4 _LightColor0;
			half _Alpha;
			
			half4 frag (v2f i) : COLOR
			{
				half4 c = tex2D(_MainTex, i.uv) * _Color;
//				c.a = _Color.a;
				c.a = _Alpha;
				return c;
				
				half3 lightDirection = normalize(_WorldSpaceLightPos0.xyz );
				half diffuse = max(0.4, dot(i.normalDir, lightDirection));
				half3 tex = tex2D(_MainTex, i.uv).rgb;
				
				half3 color = diffuse * _LightColor0.rgb * tex * _Color.rgb;
				return half4(color, _Alpha);
			}
			
			ENDCG
		}
		
	}
 
	Fallback Off
}

请教下兄弟，第一个pass是干嘛用的呢？
what's the first pass doing ? thx :)

Looks like it's just writing 2 into the stencil buffer for all the area covered by the model. The second pass is then pushing out the vertices along their normals and painting that as the outline, but not painting on the areas with stencil=2.