3D Rigidbody 물리 기반 이동과 SphereCast 지면·경사로 투영(ProjectOnPlane)

using UnityEngine;

[RequireComponent(typeof(Rigidbody), typeof(CapsuleCollider))]
public class RigidbodyPlayerMovement : MonoBehaviour
{
    [Header("이동")]
    [SerializeField] private float moveSpeed = 8f;
    [SerializeField] private float maxAcceleration = 60f;

    [Header("지면 감지 (SphereCast)")]
    [SerializeField] private float groundCheckRadius = 0.35f;
    [SerializeField] private float groundCheckDistance = 0.5f;
    [SerializeField] private LayerMask groundLayer;
    [SerializeField] private float maxSlopeAngle = 45f;

    private Rigidbody _rb;
    private CapsuleCollider _col;
    private Vector3 _moveInput;
    private Vector3 _groundNormal = Vector3.up;
    private bool _isGrounded;

    private void Awake()
    {
        _rb = GetComponent<Rigidbody>();
        _col = GetComponent<CapsuleCollider>();

        // 물리 회전 잠금
        _rb.freezeRotation = true;
        // 충돌 보간으로 화면 흔들림 억제
        _rb.interpolation = RigidbodyInterpolation.Interpolate;
    }

    private void Update()
    {
        float h = Input.GetAxisRaw("Horizontal");
        float v = Input.GetAxisRaw("Vertical");
        _moveInput = new Vector3(h, 0f, v).normalized;
    }

    private void FixedUpdate()
    {
        CheckGrounded();
        MoveCharacter();
    }

    private void CheckGrounded()
    {
        // 캡슐 하단 구체 위치 계산
        Vector3 origin = transform.position + Vector3.up * (groundCheckRadius + 0.05f);

        if (Physics.SphereCast(origin, groundCheckRadius, Vector3.down, out RaycastHit hit, groundCheckDistance, groundLayer))
        {
            float slopeAngle = Vector3.Angle(Vector3.up, hit.normal);
            if (slopeAngle <= maxSlopeAngle)
            {
                _isGrounded = true;
                _groundNormal = hit.normal;
                return;
            }
        }

        _isGrounded = false;
        _groundNormal = Vector3.up;
    }

    private void MoveCharacter()
    {
        // 경사면에 맞게 입력 벡터 투영
        Vector3 targetDirection = Vector3.ProjectOnPlane(_moveInput, _groundNormal).normalized;
        Vector3 targetVelocity = targetDirection * moveSpeed;

        Vector3 currentVel = _rb.linearVelocity;
        Vector3 currentHorizontalVel = new Vector3(currentVel.x, 0f, currentVel.z);
        Vector3 targetHorizontalVel = new Vector3(targetVelocity.x, 0f, targetVelocity.z);

        // 목표 속도와 현재 속도 차이 계산 및 가속도 제한
        Vector3 velocityChange = targetHorizontalVel - currentHorizontalVel;
        velocityChange = Vector3.ClampMagnitude(velocityChange, maxAcceleration * Time.fixedDeltaTime);

        // Y축 속도는 중력 보존, XZ만 가속
        _rb.linearVelocity = new Vector3(currentVel.x + velocityChange.x, currentVel.y, currentVel.z + velocityChange.z);

        // 경사로 정지 시 아래로 미끄러짐 방지
        if (_isGrounded && _moveInput.sqrMagnitude < 0.001f)
        {
            _rb.useGravity = false;
            _rb.linearVelocity = new Vector3(0f, 0f, 0f);
        }
        else
        {
            _rb.useGravity = true;
        }
    }
}