shader make it transparent

I want to make an object invisible and learn something about tex2dProj.I dont have any good documents,but try the codes.

Code (csharp):

1.        
2.           v2f vert (appdata_t v)
3.             {
4.                 v2f o;
5.                 o.vertex = mul(UNITY_MATRIX_MVP, v.vertex);
6.                 o.color = v.color;
7.                 o.texcoord = v.texcoord;
8.                 o.screenPos = ComputeScreenPos(o.vertex);
9.                 //o.worldPos = TRANSFORM_TEX(v.vertex.xy, _MainTex);
10.                 return o;
11.             }
12.  
13.             sampler2D _GrabTexture;
14.             half4 frag (v2f IN) : COLOR
15.             {
16.                half4 originCol = tex2Dproj(_GrabTexture,IN.screenPos);
17.                half4 completedColor = originCol;
18. /[CODE]
19. these codes does not work.... anyone fixes it?