How to detect all GameObjects within the Camera FOV? Unity3D

Question

I would like to detect all gameobjects tagged by "Wall_[0-24]", which are within the camera FOV. I already tried Raycasting, but as it is only one ray, it doesn't catch multiple objects at the same time. I tried this one:

void Update() {
    GameObject walls = GameObject.FindGameObjectWithTag ("Wall");
    Renderer[] renders = walls.GetComponentsInChildren<Renderer> ();

    for (int i = 1; i < renders.Length; i++) {
        if (walls.GetComponentInChildren<Renderer> ().isVisible) {
            Debug.Log (renders[i] + " is detected!");
        } else {
            Debug.Log ("Nothing's detecetd!");
        }
    }
}

All I get is every wall again and again - doesn't really depend on the camera's position. As my camera follows a certain path, the visible walls should change. In the image the green part is visible and the red one not anymore (because the camera already passed them).

So how could I realize the output of all seen walls by this specific camera?

Thank you for any help!

Answer 1

Using Draco18s' answer, below is how to implement what he said.

Create a new Script and name it CheckWalls and attach the below code

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class CheckWalls : MonoBehaviour
{
    Renderer[] renderers;

    void Awake ()
    {
        GameObject walls = GameObject.FindGameObjectWithTag ("Wall");
        renderers = walls.GetComponentsInChildren<Renderer> ();
    }

    void Update() 
    {
        OutputVisibleRenderers(renderers);
    }

    void OutputVisibleRenderers (Renderer[] renderers)
    {
        foreach (var renderer in renderers)
        {
            // output only the visible renderers' name
            if (IsVisible(renderer)) 
            {
                Debug.Log (renderer.name + " is detected!");    
            }           
        }

        Debug.Log ("--------------------------------------------------");
    }

    private bool IsVisible(Renderer renderer) 
    {
        Plane[] planes = GeometryUtility.CalculateFrustumPlanes(Camera.main);

        if (GeometryUtility.TestPlanesAABB(planes , renderer.bounds))
            return true;
        else
            return false;
    }
}