How can I draw a X*X pixels rect to fill a X*X RenderTexture?

Hello. I'm trying to fill a X*X RenderTexture with output of a shader. My scene has a Camera (EffectCamera script), a Plane (EffectScreenQuad script) and a RenderTexture. In EffectScreenQuad.Start() I create the plane by scratch, moving the vertices. cam is equal to the Camera which have EffectCamera script attached.

 public class EffectScreenQuad : MonoBehaviour
 {
     public Vector2 dimensions;
     public Camera cam;
     
     // Use this for initialization
     void Start ()
     {
         Vector3[] vertices  = new Vector3[4];
         Vector3[] normals   = new Vector3[4];
         Vector2[] uv        = new Vector2[4];
         int[] triangles     = new int[6];
         
         //Create plane vertices that fill a dimensions.x*dimension.y pixels rect
         Vector3 lowerLeftPlaneScreenPos    = new Vector3(0,0,0);
         Vector3 lowerRightPlaneScreenPos= new Vector3(dimensions.x , 0 , 0);
         Vector3 upperLeftPlaneScreenPos = new Vector3(0 , dimensions.y , 0);
         
         Vector3 lowerLeftPlaneWorldPos  = cam.ScreenToWorldPoint(lowerLeftPlaneScreenPos);
         Vector3 lowerRightPlaneWorldPos = cam.ScreenToWorldPoint(lowerRightPlaneScreenPos);
         Vector3 upperLeftPlaneWorldPos = cam.ScreenToWorldPoint(upperLeftPlaneScreenPos);
         
         vertices[0] = lowerLeftPlaneWorldPos;
         vertices[1] = lowerRightPlaneWorldPos;
         vertices[2] = upperLeftPlaneWorldPos;
         vertices[3] = upperLeftPlaneWorldPos + lowerRightPlaneWorldPos;
 
         for (int i = 0; i < 4; i++)
             normals[i] = Vector3.back;
 
         uv[0] = Vector2.zero;
         uv[1] = Vector2.right;
         uv[2] = Vector2.up;
         uv[3] = Vector2.right + Vector2.up;
         
         triangles[0] = 0;
         triangles[1] = 1;
         triangles[2] = 2;
         triangles[3] = 0;
         triangles[4] = 2;
         triangles[5] = 3;
 
         MeshFilter mesh_filter        = gameObject.GetComponent("MeshFilter") as MeshFilter;
         mesh_filter.mesh = new Mesh ();
         mesh_filter.mesh.vertices     = vertices;
         mesh_filter.mesh.normals       = normals;
         mesh_filter.mesh.uv            = uv;
         mesh_filter.mesh.triangles     = triangles;
     }
     
     // Update is called once per frame
     void Update () {
     
     }
 }

The next snippet is the shader. The commented code is what I need to return, but for now I'm just returning the uv coords, since I couldn't debug the returned value using floating point TextureFormats.

 Shader "clothInitPos" {
     Properties {
         _posTexSize("_posTexSize",Float)=16
         _posOffset("_posOffset",Float)=5
     }
     SubShader {
         Tags { "RenderType"="Opaque" }
 
         Pass
         {
             //First pass initializes position texture. To ensure all vertices being generated, a quad must be
             //drawn that fills _posTexSize*_posTexSize pixels.
             
             CGPROGRAM
             #pragma target 3.0
             #pragma vertex vert
             #pragma fragment frag
         
             uniform float _posTexSize;
             uniform float _posOffset;
             
             struct v2f {
                 float4 pos:POSITION;
                 float2 uv:TEXCOORD0;
             };
             
             v2f vert(v2f IN)
             {
                 v2f toFrag = IN;
                 return toFrag;
             }
             
             float4 frag(v2f fromVert):COLOR
             {
                 //Generate a regular grid by scaling uv coords by the number of verts and adding the position
                 //offset
                 //float2 scaledUV = fromVert.uv*_posTexSize*_posOffset;
                 //return float4(scaledUV.x , 0.0f , scaledUV.y , 1.0f);  //Grid in the y=0 plane. Alpha unused.
                 return float4(fromVert.uv.x , 0.0f , fromVert.uv.y , 1.0f);
             }
             ENDCG
         }
     }
     FallBack "Diffuse"
 }
 

I was expecting the result image to be a gradient of red and blue. In EffectCamera.OnRenderImage() I check the returned texture but a lot of pixels has the Camera background color, which makes me confuse.

 public class EffectCamera : MonoBehaviour {
     public Vector2 dimensions;
     public Camera cam;
     
     // Use this for initialization
     void Start () {
     }
     
     // Update is called once per frame
     void Update () {
     
     }
     
     void OnRenderImage(RenderTexture src , RenderTexture dst)
     {
         Texture2D tex2D = new Texture2D((int)dimensions.x , (int)dimensions.y ,
             TextureFormat.ARGB32 , false);
         tex2D.ReadPixels(new Rect(0 , 0 , dimensions.x , dimensions.y) , 0 , 0);
         Color[] colors = tex2D.GetPixels ();
         int dummy = 0;
     }
 }

Any help will be very appreciated.