Controlled aera rendering via shader

Hi,

I have started using shader in Unity but with some difficulties. I am trying to render some GameObject (i will call CubeB) in a specific aera of my screen. I want them in front of all other GameObject. I want to control the size of this aera.

But first I try to render just a Color in this aera.

For that I set a Plane in my scene with a script and just a transform. The script picks a material with the shader, transform the plane and draw a Mesh. The plane is normalized and the Mesh is set at each frame. The size, position, rotation of the plane change every frame via the following script :

     /// Origin of the player's head
     [Tooltip("Transform of the player's head")]
     public Transform HeadTransform;
 
     /// Material used to play with the Zbuffer for the RA rendering
     [Tooltip("Material used to play with the Zbuffer for the RA rendering.")]
     [SerializeField]
     private Material FadeMaterial;
     private Material FadeMaterialInstance;
 
     private Mesh PlaneMesh;
 
     private Matrix4x4 local;
     
     void Start()
     {
         PlaneMesh = new Mesh();
         Vector3[] verts = new Vector3[]
         {
             new Vector3(-1, -1, 0),
             new Vector3(-1, 1, 0),
             new Vector3(1, 1, 0),
             new Vector3(1, -1, 0)
         };
         int[] elts = new int[] { 0, 1, 2, 0, 2, 3 };
         PlaneMesh.vertices = verts;
         PlaneMesh.triangles = elts;
         PlaneMesh.RecalculateBounds();
         
         if(FadeMaterial != null)
             FadeMaterialInstance = new Material(FadeMaterial);
     }
 
     void Update()
     {
         Matrix4x4 local = Matrix4x4.TRS(Vector3.forward * 0.33f, Quaternion.identity, Vector3.one); // Has to be in front of the clipping plan of the camera
 
         Graphics.DrawMesh(PlaneMesh, HeadTransform.localToWorldMatrix*local, FadeMaterialInstance,0);
         FadeMaterialInstance.SetPass(0);
     }
 }

Then in the shader attached to the Material FADEMATERIAL try to do what I want. But no color appears at my screen ... I don't understand.

The shader :

 Shader "Custom/FadeBlack"
 {        
         SubShader
         {
             Tags{ "Queue" = "Geometry+5" "IgnoreProjector" = "True" }
 
             ZTest Always    
 
             Pass
             {
                 ZWrite On
 
                 CGPROGRAM
                 #pragma vertex vert
                 #pragma fragment frag
 
                 #include "UnityCG.cginc"
 
                 uniform float _fovSize;
 
                 struct appdata
                 {
                     float4 vertex : POSITION; 
                 };
 
                 struct v2f
                 {
                     float4 vertex : SV_POSITION; 
                     float4 uv : TEXCOORD0;
                 };
     
                 v2f vert(appdata v)
                 {
                     v2f o;
                     o.vertex = v.vertex;
                     o.uv = v.vertex; 
                     return o;
                 }
 
                 struct fragmentOutput 
                 {
                     float zvalue : SV_Depth;
                     float4 color : SV_Target;
                 };
 
                 fragmentOutput frag(v2f i)
                 {
                     fragmentOutput o;
                     o.zvalue = 0.0;
                     o.color = (1,1,1,1);
                     _fovSize = 0.5;
 
                     if ((i.vertex.y <(-_fovSize) || i.vertex.y >(_fovSize))   ||
                          (i.vertex.x <(-_fovSize) || i.vertex.x >(_fovSize)))
                     {
                         o.zvalue = 1.0;
                                                 o.color = (0,0,0.5,1);
                     }
                     return o;
                 }
                 ENDCG
             }
         }
 }
 

Any idea of what is it happening ?