(Offset Factor , Units) + shadows bring some ZFighting

Hi all,

In my scene, i've just 3 planes with the same position.
The first one has the standard diffuse material.
The second has the following shader :

Code (csharp):

1.  
2. Shader "_Mine/ZFightPlane1"
3. {
4.     Properties
5.     {
6.         _MainTex ("Base (RGB)", 2D) = "white" {}
7.         _Alpha ("Alpha", 2D) = "white" {}
8.     }
9.  
10.     SubShader
11.     {
12.         Tags { "RenderType"="Opaque" }
13.         Offset -1,-8
14.  
15.         Blend SrcAlpha OneMinusSrcAlpha
16.        
17.         CGPROGRAM
18.         #pragma surface surf Lambert alpha
19.  
20.         sampler2D _MainTex;
21.         sampler2D _Alpha;
22.  
23.         struct Input
24.         {
25.             float2 uv_MainTex;
26.         };
27.  
28.         void surf (Input IN, inout SurfaceOutput o)
29.         {
30.             o.Albedo = tex2D(_MainTex, IN.uv_MainTex).rgb;
31.             o.Alpha = tex2D(_Alpha, IN.uv_MainTex).r;
32.         }
33.  
34.         ENDCG
35.     }
36.    
37. }
38.  

For the third plane, the shader is the same except the value of Offset :

Code (csharp):

1. Offset -1,-15

I have one directional light in my scene.
I disabled all "cast shadows" on the three planes renderer.

When I active Hard or Soft shadows, ZFighting comes again !

I added a cube in order to see shadows on my planes, but only the first one receive them...



How can we fix that ? If you can bring me some clarifications, it'll be great

Thanks in advance.