AlphaTest Questions

Hey,
I am doing some shaders using alphaTest and I must say I ran into some weird stuff.

1- Shader with no prepass in deferred rendering path
The object will have it's cutout parts rendered in black if the shader has a property _Color (even if it's not used). Note: Same effect if the _Cutoff property is named something else.

The Shader:

Code (CSharp):

1. Shader "Custom/AlphaTest" {
2.     Properties {
3.         _Color ("MainColor",Color) = (0,0,0,0)
4.         _MainTex ("Base (RGB)", 2D) = "white" {}
5.         _DisolveTex("Disolve Texture", 2D) = "white" {}
6.         _Cutoff("Disolve Power",Range(0,1)) = 0
7.        
8.     }
9.     SubShader {
10.         Tags { "Queue"="AlphaTest" "IgnoreProjector"="True" "RenderType"="TransparentCutout" }
11.         LOD 200
12.        
13.         CGPROGRAM
14.         #pragma surface surf Lambert exclude_path:prepass alphatest:Zero
15.  
16.         sampler2D _MainTex;
17.         sampler2D _DisolveTex;
18.         float _Cutoff;
19.  
20.         struct Input {
21.             float2 uv_MainTex;
22.         };
23.  
24.         void surf (Input IN, inout SurfaceOutput o) {
25.             fixed3 tex = tex2D (_MainTex, IN.uv_MainTex);
26.             fixed  alpha = tex2D (_DisolveTex, IN.uv_MainTex).r;
27.            
28.             o.Albedo = tex;
29.             o.Alpha = alpha - _Cutoff;
30.         }
31.         ENDCG
32.     }
33.     FallBack "Diffuse"
34. }
35.  

The look (a bit hard to see but the black part is not the space background, it's just black):


2- Same Shader, no _Color no prepass
the shader doesn't write to the zbuffer as soon as the _Cutoff property is not equal to 0. so post effects like DOF will break. Adding a _Color property will break alpha test but it will write in Z again.




Why isn't there any documentation on this and why do these variable have ( or not ) to be there with that exact name?!