Make sure the player does not walk up and down

Question

Developing a 3D FPS game in Unity.

The code below is set up so that the player can walk in the direction of the mouse.

However, if you point the mouse cursor up, the player will walk towards the sky, and if you point the mouse cursor down, the player will push through the ground and go underground. In other words, it is in a state of walking in all directions.

I would like to fix this.

I want the player to be able to look up and down, but not be able to advance up and down.

How can I modify the code?

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

public class FPSController : MonoBehaviour
{
    
    float x, z;

    float speed = 0.1f;

　　public GameObject cam;
    Quaternion cameraRot, characterRot;
    float Xsensityvity = 3f, Ysensityvity = 3f;
    
    bool cursorLock = true;

　　float minX = -90f, maxX = 90f;

   public Animator animator;

   public GameObject meinCamera,subCamera;

    // Start is called before the first frame update
    void Start()
    {
        cameraRot = cam.transform.localRotation;
        characterRot = transform.localRotation;

        GameState.canShoot = true;
    }

    // Update is called once per frame
    void Update()
    {
       float xRot = Input.GetAxis("Mouse X") * Ysensityvity; 
       float yRot = Input.GetAxis("Mouse Y") * Xsensityvity; 

       cameraRot *= Quaternion.Euler(-yRot, 0, 0); 
       characterRot *= Quaternion.Euler(0, xRot, 0);

       cameraRot = ClampRotation(cameraRot);
   
       cam.transform.localRotation = cameraRot;
       transform.localRotation = characterRot;

       UpdateCursorLock();
 
       }

       if(Mathf.Abs(x) > 0 || Mathf.Abs(z) > 0)
       {
           if(!animator.GetBool("Walk"))
            {
              animator.SetBool("Walk", true);

              PlayerWalkFootStep(WalkFootStepSE);
            }
    
        }
        else if(animator.GetBool("Walk"))
        {
            animator.SetBool("Walk", false);

            StopFootStep();
        }


         if(z > 0 && Input.GetKey(KeyCode.LeftShift))
       {
           if(!animator.GetBool("Run"))
            {
              animator.SetBool("Run", true);
              speed = 0.25f;

              PlayerRunFootStep(RunFootStepSE);
            }
    
        }
        else if(animator.GetBool("Run"))
        {
            animator.SetBool("Run", false);
            speed = 0.1f;

            StopFootStep();
        }  

        if(Input.GetMouseButton(1))
        {
            subCamera.SetActive(true);
            meinCamera.GetComponent<Camera>().enabled = false;
        }
        else if(subCamera.activeSelf)
        {
            subCamera.SetActive(false);
            meinCamera.GetComponent<Camera>().enabled = true;
        }

    }
    

    private void FixedUpdate()
    {
        x = 0;
        z = 0;

        x = Input.GetAxisRaw("Horizontal") * speed;
        z = Input.GetAxisRaw("Vertical") * speed;

        //transform.position += new Vector3(x,0,z);

        transform.position += cam.transform.forward * z + cam.transform.right * x;
    }

    public void UpdateCursorLock()
    {
        if(Input.GetKeyDown(KeyCode.Escape))
        {
            cursorLock = false;
        }
        else if(Input.GetMouseButton(0))
            {
             cursorLock = true;
            }

        if(cursorLock)
        {
            Cursor.lockState = CursorLockMode.Locked;
        
        }
        else if(!cursorLock)
        {
            Cursor.lockState = CursorLockMode.None;
        }
        
    }

    public Quaternion ClampRotation(Quaternion q)
    {
        q.x /= q.w;
        q.y /= q.w;
        q.z /= q.w;
        q.w = 1f;

        float angleX = Mathf.Atan(q.x) * Mathf.Rad2Deg * 2f;

        angleX = Mathf.Clamp(angleX, minX, maxX);

        q.x = Mathf.Tan(angleX * Mathf.Deg2Rad * 0.5f);

        return q;
    }
}

Answer 1

You could use Vector3.ProjectOnPlane in order to map any movement input into a flat direction on the XZ plane like

transform.position += Vector3.ProjectOnPlane(cam.transform.forward * z + cam.transform.right * x, Vector3.up);

---

I see though that you are using the FixedUpdate which looks like you are using Rigidbody and physics. If this is the case you shouldn't do anything via transform but rather through the Rigidbody component.