Getting trouble with camera.RenderWithShader()

Hi there,

I am trying to use the RenderWithShader function of the camera but i had no luck untill now and i cannot find why. So i could use some help here.

The idea is to generate a mask through Render Texture in order to desaturate the whole screen except some objects (which i identify by setting their layer to "HighlightedCharacter").
I set the camera background to white and i replace the concerned objects' shader with a plain black coloring shader, finally using the output of the camera render as a mask in my post effect shader.
The problem i have is that the shader does not seem to be replaced, it renders the object normally (with its original shader).

Here is the effect script:

Code (csharp):

1. using UnityEngine;
2.  
3. [ExecuteInEditMode]
4. [AddComponentMenu("Image Effects/Color Adjustments/Desaturation")]
5. public class _DesaturationEffect : ImageEffectBase
6. {
7.     public float saturationFactor;
8.     public Shader maskShader;
9.  
10.     private RenderTexture _maskTex;
11.     private GameObject _maskCam;
12.  
13.     void CleanUpTextures()
14.     {
15.         if (_maskTex)
16.         {
17.             RenderTexture.ReleaseTemporary(_maskTex);
18.             _maskTex = null;
19.         }
20.     }
21.    
22.     void OnPreRender()
23.     {
24.         if (!enabled || !gameObject.activeInHierarchy) return;
25.        
26.         CleanUpTextures();
27.        
28.         if (!_maskCam)
29.         {
30.             _maskCam = new GameObject("MaskCamera");
31.             _maskCam.AddComponent<Camera>();
32.             _maskCam.camera.enabled = false;
33.             _maskCam.hideFlags = HideFlags.HideAndDontSave;
34.         }
35.         _maskCam.camera.CopyFrom(camera);
36.         _maskCam.camera.backgroundColor = new Color(1,1,1,1);
37.         _maskCam.camera.clearFlags = CameraClearFlags.SolidColor;
38.         _maskCam.camera.cullingMask = 1 << LayerMask.NameToLayer("HighlightedCharacter");
39.  
40.         _maskTex = RenderTexture.GetTemporary((int)camera.pixelWidth, (int)camera.pixelHeight, 16, RenderTextureFormat.ARGB32);
41.         _maskCam.camera.targetTexture = _maskTex;
42.         _maskCam.camera.RenderWithShader(maskShader, null);
43.     }
44.    
45.     void OnRenderImage (RenderTexture source, RenderTexture destination)
46.     {
47.         material.SetFloat("_Desaturate", saturationFactor);
48.         material.SetTexture("_MaskTex", _maskTex);
49.         Graphics.Blit(source, destination, material);
50.         CleanUpTextures();
51.     }
52.  
53.     new void OnDisable()
54.     {
55.         if (_maskCam)
56.         {
57.             DestroyImmediate(_maskCam);
58.         }
59.         base.OnDisable();
60.     }
61. }

And the shader code:

Code (csharp):

1. Shader "Hidden/_DesaturationEffect"
2. {
3.     Properties
4.     {
5.         _MainTex ("Base (RGB)", 2D) = "white" {}
6.     }
7.  
8.     SubShader
9.     {
10.         Pass
11.         {
12.             ZTest Always Cull Off ZWrite Off
13.             Fog { Mode off }
14.                
15.             CGPROGRAM
16.            
17.                 #pragma vertex vert_img
18.                 #pragma fragment frag
19.                 #pragma fragmentoption ARB_precision_hint_fastest
20.                 #include "UnityCG.cginc"
21.  
22.                 uniform sampler2D _MainTex;
23.                 uniform sampler2D _MaskTex;
24.                 uniform half _Desaturate;
25.  
26.                 fixed4 frag (v2f_img i) : COLOR
27.                 {
28.                     fixed4 tex = tex2D(_MainTex, i.uv);
29.                     fixed4 mask = tex2D(_MaskTex, i.uv);
30.  
31.                     half3 grayXfer = half3(0.3, 0.59, 0.11);
32.                     half grayf = dot(grayXfer, tex.rgb);
33.                     half3 gray = half3(grayf, grayf, grayf);
34.                     fixed4 col = fixed4(lerp(tex.rgb, gray, _Desaturate * mask.r), 1.0);
35.  
36.                     return col;
37.                 }
38.                
39.             ENDCG
40.  
41.         }
42.     }
43.     Fallback off
44. }

Thanks in advance for your help