Alpha測試：
符合條件的 alpha 畫素顯示出來，不符合的丟棄掉。

Shader 1.0

AlphaTest  	Always  0.5

Always: 表示關閉測試。總是通過。
Never: 總是不通過。 不會渲染出來 。
變換引數：

_AlphaCut ("Alpha  cut ", float) = 0.5
AlphaTest    Greater [_AlphaCut]

Shader 2.0 :

Shader "Hidden/AlphaTest"
{
	Properties
	{
		_MainTex (
 
"Texture", 2D) = "white" {}
	}
	SubShader
	{
		// No culling or depth
		//Cull Off ZWrite Off ZTest Always
        Blend SrcAlpha OneMinusSrcAlpha // Traditional transparency
		Pass
		{
			CGPROGRAM
			#pragma vertex vert
			#pragma fragment frag
			
			#include "UnityCG.cginc"

			struct appdata
			{
				float4 vertex :
 
 POSITION;
				float2 uv : TEXCOORD0;
			};

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

			v2f vert (appdata v)
			{
				v2f o;
				o.vertex = UnityObjectToClipPos(v.vertex);
				o.uv = v.uv;
				return o;
			}
			
			sampler2D _MainTex;

			fixed4 frag (v2f i) : SV_Target
			{
				fixed4 col = tex2D(_MainTex, i.uv);
				// just invert the colors
				//col.rgb = 1 - col.rgb;
                if(col.a>0.5)
                {
                    return fixed4(0,0,0,0);
                }
                else
                {
                    return col;
                }
				return col;
			}
			ENDCG
		}
	}
}